Category Archives: How-To

Are your Ionisers working correctly?

The best way to keep electrostatic sensitive devices (ESDs) from damage is to ground all conductive objects and remove insulators from your ESD Protected Area (EPA). This is not always possible because some insulators are “process-essential” and are necessary to build or assemble the finished product. The only way to control charges on these necessary non-conductive items is the use of ionisation systems.
However, if an ioniser is out of balance, instead of neutralising charges, it will produce primarily positive or negative ions. This results in placing an electrostatic charge on items that are not grounded, potentially discharging and causing ESD damage to nearby sensitive items.
It is therefore essential to regularly clean ionisers and verify their functionality. Below we have put together a list of tasks that need to be performed with ionisers on a regular basis.

Maintenance
All ionization devices will require periodic maintenance for proper operation. Maintenance intervals for ionizers vary widely depending on the type of ionization equipment and use environment. Critical clean room uses will generally require more frequent attention. It is important to set-up a routine schedule for ionizer service. Routine service is typically required to meet quality audit requirements.” (User Guide CLC/TR 61340-5-2 clause 4.7.6.7 Maintenance and cleaning)
EIA-625, recommends checking ionisers every 6 months, but this may not be suitable for many programs particularly since an out-of-balance may exist for months before it is checked again. EN 61340-5-1 clause 5.2.4 Compliance Verification Plan  states: “Process monitoring (measurements) shall be conducted in accordance with a compliance verification plan that identifies the technical requirements to be verified, the measurement limits and the frequency at which those verifications shall occur.
Under normal conditions, an ioniser will attract dirt and dust (especially on the emitter points). To maintain optimum neutralisation efficiency and operation, cleaning should be performed on a regular basis.

1. Case
Wipe the case with a soft cloth and deionised water. Fully squeeze the wiping cloth or sponge to remove any excess liquid. If a stronger cleaning solution is required, dab a soft cloth with mixture of isopropyl alcohol and deionised water (70% IPA and 30% DI water).

2. Emitter Points
The emitter points should be cleaned using specific emitter point cleaners or a swab dampened with Isopropyl alcohol. Below are general instructions on how to clean emitter points. However, each unit is slightly different so always refer to the ioniser’s manual.

  • Turn the unit OFF and unplug the power cord.
  • Open the top screen by loosening the screw and swinging the grill to one side.
  • Clean the emitter points using an emitter point cleaner or a swab dampened with Isopropyl alcohol.
  • Re-attach the top screen.
  • Plug in the power cord and turn the unit ON.

Verify the performance of the ioniser by using a charged plate monitor or ionisation test kit (see below).

Cleaning of Emitter Points using SCS 9110-NO as an exampleCleaning of Emitter Points using SCS 9110-NO as an example

With normal handling, the emitter points should not require replacement during the life of the unit.

Verification
EN 61340-4-7 provides test methods and procedures for evaluating and selecting air ionisation equipment. It is recommended to measure the offset voltage and discharge times, clean the unit, including emitter points and air filters if present, offset the voltage to zero (if adjustable), and then repeat offset voltage and discharge time testing. Should the unit not meet offset voltage specifications or minimum established discharge time limits, further service is required. Manufacturers should provide details on service procedures and typical service intervals.
Most companies will assign a number or otherwise identify each ioniser and setup a Compliance Verification / Maintenance / Calibration schedule. If the ionisers all test good, the data can justify lengthening the calibration period. If ionisers require adjustment, the calibration period should be shortened.

Verification should be performed in accordance with EN 61340-4-7.
Below are general instructions on how to verify your ioniser’s offset voltage and discharge time. Always refer to the User Guide accompanying your charge plate monitor or ionisation test kit for proper operation and setup.

1. Testing Ioniser Offset Voltage:
The required limit per EN 61340-5-1 is less than ± 35 volts. Check your ioniser’s operating manual or consult with the ioniser manufacturer to determine what the offset voltage should be for your ioniser.

Charge Plate Monitor (CPM)

  • Position the ioniser and charge plate monitor as shown below.
  • Set the CPM to Decay/Offset mode.
  • Set the CPM to decay and offset voltage mode with a starting charge at either + or – 1 KV and a stopping charge at either + or -100 Volts.
  • Start the decay/offset test sequence on the CPM. This will take a few seconds.
  • Record the decay time, and offset voltage as displayed on the CPM.

Positioning your Charge Plate Monitor for Overhead and Benchtop Ionisers

Ionisation Test Kit

  • Zero the charge plate by touching it with a grounded object. This can either be the finger of a grounded person or some other item which is connected to electrical ground. In either case, zeroing the charge plate should make the display on the field meter read zero.
  • Hold the meter approximately one foot (30.5 cm) in front of the ioniser.
  • Monitor the display. The value displayed is the offset balance of the ioniser, which is the difference between the number of positive and negative ions being emitted.

Auditing ionisation equipment with the Digital Static Field Meter and Conductive PlateAuditing ionisation equipment with the Digital Static Field Meter and Conductive Plate

2. Testing Ioniser Discharge Time:
The required limit per EN 61340-5-1 is “(1 000 V to 100 V and –1 000 V to –100 V) < 20 s or user defined”. Please refer to the ioniser’s operating manual or consult with the ioniser manufacturer to determine what this discharge time should be.

Charge Plate Monitor (CPM)

  • Set the CPM to Decay/Offset mode.
  • Set the CPM to decay and offset voltage mode with a starting charge at either + or – 1 KV and a stopping charge at either + or -100 Volts.
  • Start the decay/offset test sequence on the CPM. This will take a few seconds.
  • Record the decay time, and offset voltage as displayed on the CPM.

Ionisation Test Kit

  • After charging the plate of the ionisation test kit, hold the field meter approximately one foot (30.5 cm) away from the ioniser.
  • Monitor the display of the meter to see how quickly the 1.1 kV charge is dissipated to 0.1 kV.
  • The speed at which this occurs (the discharge time) indicates how well the ioniser is operating.
  • Repeat this procedure for both a positively and a negatively charged plate.

Some ionisers offer adjustment options (e.g. trim pots) which allow modification of the offset voltage.
However, if your ioniser is out of balance (and cannot be adjusted) or if the discharge time is out of specification, the ioniser will require service/repair by an authorised company.

Conclusion
Ionisation is one of the best methods of removing charges from insulators and as a result plays an important role in controlling ESD.
Remember though: ionisers require periodic cleaning of emitter pins and verifying of the offset voltage and discharge time. Otherwise, instead of neutralising charges, the ioniser will primarily produce positive or negative ions. The ioniser will therefore place an electrostatic charge on items that are not grounded, potentially discharging and causing ESD damage to nearby sensitive items.

Controlling Static Charge Generation with ESD Flooring

Static discharges can be noticed when you touch an object of different electrical potential such as a door knob, and a bolt of electricity flows from your charged body to the door knob. This flow of electricity is actually a result of the stored static charge that is being rapidly transferred to the knob. This discharge that can be felt as well as seen, is commonly referred to as an electrostatic discharge, or “ESD”.
The generated static charges are a potentially costly occurrence for office and factory employers. You will learn in today’s post how they can easily be controlled with different types of floor material.

Static Charge Generation from Flooring
When a person walks across a floor, a triboelectric charge builds up in the body due to the friction between the shoes and floor material. The simple separation of two surfaces (such as a person walking across a floor with soles contracting and separating from the floor) can cause a transfer of electrons resulting in one surface being positively and the other one negatively charged, resulting in static charges.

Generating Charges by walking across a CarpetGenerating Charges by walking across carpet

It is not necessarily the static charge generated in the body that does the damage as much as it is the difference in potential that creates an electrostatic discharge.

The problem with ESD
The generation of a static charge can pose quite a problem for environments that contain sensitive equipment or components that are vulnerable to static damage, such as electronics manufacturing, repair facilities and medical facilities, including computer rooms and clean rooms.
Controlling the damage and costs caused by ESD is usually the main concern that drives a company to implement a static control programme. The costs involved with static damage not only include the immediate cost of the damaged component, but the contributing cost of diagnostic, repair and labour that is needed to replace or fix the component. In many cases the labour involved can far exceed the component cost.

Flooring Materials
There are several options available on the market ranging from coatings (floor finish or paint) to coverings (vinyl or rubber). The choice of material depends on the mechanical and optical properties required as well as the available budget.
In general, floor coverings will last longer (10 years or more) than a floor coating. They are more durable and have a specific resistance to ground that remains constant over time.

Types of Floor Coverings – click here for more information

Coatings are easier to apply and repair and their initial cost is considerably lower. Coatings are usually applied to existing floors and often serve to convert a conventional floor into an ESD floor. However, regular maintenance is required as coatings will lose their ESD properties over time.

1. Floor Coatings

  • Antistatic Coating:
    Conventional carpets can be treated with an Antistatic Coating or other treatment. It is required that the treatment be replenished on the carpet as it wears away due to foot traffic.
    ESD carpet is available but proper maintenance is very important.
  • ESD Floor Finish:
    Existing hard surfaces (e.g. concrete, sealed or painted wood, linoleum, asphalt) can be treated with ESD Floor Finish to eliminate the need for ESD control flooring. Repeat applications are required periodically to keep ESD properties within specification.
  • ESD Paint:
    Paint is ideal for providing a cost effective static-free environment and is very effective as a static control floor coating for electronics manufacturing, assembly and storage. It controls dissipation of static electricity and provides path to ground.

2. Floor Coverings:
Floor coverings will have either “conductive” or “dissipative” electrical properties:

  • Conductive materials have a resistance to ground (RG) of greater than 1 x 103 ohms but less than 1 x 105
  • Dissipative materials have a resistance to ground (RG) of greater than 1 x 105 ohms but less than 1 x 1012

It is recommended to use conductive flooring material; EN 61340-5-1 requires ESD flooring to be less than 1 x 109 ohms (RG). The same standard requires a person/footwear/flooring to be less than 3.5 x 107 ohms (resistance in series of operator plus footwear plus floor). Remember that floors get dirty which can raise floor resistance. Therefore, it is good to start off with a floor that is conductive (less than 1 x 106 ohms). So even if the resistance increases, you’re within the required limits of the ESD Standard.

  • Carpet:
    ESD control carpets are made with static dissipative yarn and only require that the yarn be kept clean and free of insulative dirt, dust and spray cleaners.
  • Matting:
    Types of matting range from vinyl to rubber and anti-fatigue matting.
    Vinyl is generally cheaper and provides high resistance to many chemicals. Rubber on the other hand is more durable and can withstand extreme hot and cold temperatures. Anti-fatigue matting (AFM Series) is designed to provide comfort for personnel that must stand or walk for long periods.

Considerations when using Flooring Materials
1. Grounding
EN 61340-5-1 requires that all conductors in an ESD protected area, including personnel, must be grounded. This includes ESD flooring; it must be electrically connected and attached to a known ground. The Desco Europe floor mat ground cord 231265 is just one option for grounding floor matting.

2. Periodic Verification
All ESD control items (including ESD flooring) have to be tested:

  • Prior to installation to qualify product for listing in user’s ESD control plan.
  • During initial installation.
  • For periodic checks of installed products as part of EN 61340-5-1 clause 5.2.4 Compliance verification plan.

19290_RtgMeasuring Surface Resistance of ESD Floor Matting – click here for more information

A surface resistance meter can be used to verify compliance of the ESD floor with the ESD standard.

3. Person/Footwear/Flooring System
ESD flooring does not ensure protection from ESD damage unless operators walking across the ESD floor wear ESD footwear, either ESD shoes or ESD foot grounders.
ESD foot grounders are designed to reliably contact grounded ESD flooring and provide a continuous path-to-ground by removing electrostatic charges from personnel. They are easy to install and can be used on standard shoes by placing the grounding tab in the shoe under the foot.
Foot grounders must be worn on both feet to maintain the integrity of the body-to-ground connection Wearing a foot grounder on each foot ensures contact with ground via the ESD floor even when one foot is lifted off the floor. This will more reliably remove static charges generated by human movement.
Desco Europe offer a number of different foot grounder types for your requirements.

Conclusion
Static charges can easily be controlled with different types of floor material which vary in their properties, cost and durability. The best static control systems are not only the ones that protect sensitive components and equipment but are: A) at hand and readily available, B) easily maintained. Floor coverings are long lasting and maintain their ESD properties over time, while existing floors can be economically converted for use in an ESD control program using various types of coatings.
Remember that all ESD control items such as flooring, personnel grounding and specialty equipment should be grounded and tested periodically to verify all components are within specification.

Not sure which ESD flooring is right for you? Request a free ESD Survey at your facility by one of our knowledgeable representatives to evaluate your ESD programme and answer any ESD questions!

 

 

 

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“Product Qualification” is a new Requirement of IEC 61340-5-1

In a previous blog post we discussed the “Walking Test”, a new requirement that was introduced with the latest ESD Standard published in 2016. There was another significant update to the ESD Standard that most people are not aware of: product qualification. Today’s post will explain in detail what this requirement entails and how you can ensure you’re compliant with the current ESD Standard.

Introduction
A second edition of IEC 61340-5-1 was published in 2016 cancelling and replacing the first edition issued in 2007. The current document added section 5.2.3 on “product qualification”.
Product qualification “is the process of certifying that a certain product has passed performance tests and quality assurance tests, and meets qualification criteria stipulated in contracts, regulations, or specifications (typically called “certification schemes” in the product certification industry).” [Source]

So, you’re now probably wondering what exactly this change means for you – simple: if you want to be compliant with IEC 61340-5-1, you have to ensure that any ESD control items you are using in your ESD Control Programme meet the Product Qualifications outlined in the Standard.

Tables 2 and 3 of the Standard outline:

  • Test methods and
  • Associated limits

for the product qualification of each ESD control item.

Requirement for 61340-5-1 ComplianceProduct Qualification is a Requirement for 61340-5-1 Compliance

Acceptable proofs of Product Qualification
So, now you know WHAT is required, the next question is HOW to verify if your ESD control items comply with the ESD Standard 61340-5-1. First things first: for any products you have acquired before adopting IEC 61340-5-1:2016, you can use ongoing compliance verification records as proof of product qualification. For more information on compliance verification, we suggest reading this post.

Once you have adopted IEC 61340-5-1:2016, any new products you chose to purchase need to be qualified to ensure they comply with the ESD Standard. There are a number of things you can use:

  1. The manufacturer’s datasheets.
  2. Test reports from independent laboratories or
  3. Your own internal test reports

Whatever option you choose, the datasheet or report of each product needs to fulfil the following requirements:

  • Display the IEC test method and
  • Mentioned test limits needed to comply with the ESD Standard.

If you are using ESD control items that are not listed in tables 2 and 3 of the ESD Standard, you need to qualify those items before starting to use them. The test method and acceptable limits need to be documented in your ESD control programme. If you need help setting up an ESD control plan, have a look at this post.

Summary
The latest ESD Standard 61340-5-1 published in 2016 added section 5.2.3 on “product qualification”. If your company wants to be compliant with IEC 61340-5-1, you have to ensure that any ESD control items you are using in your ESD Protected Area (EPA) meet the requirements of the ESD Standard.

Acceptable proofs of product qualification are:

  1. The manufacturer’s datasheets.
  2. Test reports from independent laboratories or
  3. Your own internal test reports
  4. Ongoing compliance verification records (only for items acquired before adopting the ESD Standard)

Desco Europe has a comprehensive range of technical information online, including product specification to the required Standards. Contact Desco Europe today for assistance with your product qualification plan.

Organising your ESD Workstation using 5S

Although not strictly related to ElectroStatic Discharge (ESD), we all know that a tidy workstation is essential when it comes to ‘getting the job done’. Having a cluttered desk and not being able to find the tools you need makes everything take twice as long. Ever heard of the 5S methodology? In today’s post, we will show how this approach can be applied to an ESD workstation. We’ll also introduce a few ESD products that can help in becoming more efficient and productive when handling ESD sensitive devices.

5S Methodology
5S is the name of a workplace organization method that uses a list of five Japanese words: seiri, seiton, seiso, seiketsu, and shitsuke. Transliterated into Roman Script, they all start with the letter “S”.[1] The list describes how to organize a work space for efficiency and effectiveness by identifying and storing the items used, maintaining the area and items, and sustaining the new order.” [Source]
Generally speaking, 5S incorporates 5 phases:

  1. Sort
  2. Arrange
  3. Clean
  4. Standardise
  5. Sustain

This post is going to focus on the first two steps of 5S.

Sorting an ESD Workstation
Items at an ESD protective workstation should be either dissipative or conductive so that electrostatic charges are removed to ground when in contact with a grounded operator or grounded ESD mat.
The protection of ESDS is accomplished by providing a ground path to bring ESD protective materials and personnel to the same electrical potential. All conductor and dissipative items in the environment, including personnel, shall be bonded or electrically connected to a known ground or common connection point. This connection results in sharing of charge which equalizes the voltage across all items and personnel and eliminates the chances of an ESD event to ESD sensitive devices. Electrostatic protection can be maintained at a potential different from a “zero” voltage ground reference as long as all items in the system are at the same potential.“ [CLC/TR 61340-5-2 Clause 4.4 Grounding/bonding systems]

So, before moving any further operators need to take a good look at their workstation and eliminate any items that are not essential to their workflow.
All non-essential insulators and items (plastics and paper), such as coffee cups, food wrappers and personal items shall be removed from the workstation or any operation where unprotected ESDS are handled. The ESD threat associated with process essential insulators or electrostatic field sources shall be evaluated to ensure that:

  • the electrostatic field at the position where the ESDS are handled shall not exceed 5 000 V/m;

or

  • if the electrostatic potential measured at the surface of the process required insulator exceeds 2 000 V, the item shall be kept a minimum of 30 cm from the ESDS; and
  • if the electrostatic potential measured at the surface of the process required insulator exceeds 125 V, the item shall be kept a minimum of 2,5 cm from the ESDS.

If the measured electrostatic field or surface potential exceeds the stated limits, ionization or other charge mitigating techniques shall be used.” [IEC 61340-5-1 Clause 5.3.4.2 Insulators]

Arranging an ESD Workstation
Once the essential items required to do the job have been identified, the next step is to arrange them in a way that is suitable to the operator’s workflow. Here are a few tools and items that can be useful:

1. Use Colour and Labels
Having a proper colour and labelling system in place will help arrange a workstation and put items in the right places. This in return will ensure operators can find tools and accessories quickly when required.

Dispensing Bottles
Operators working with solder irons or performing various cleaning tasks at an ESD workstation will likely be using water or some sort of cleaning agent. ESD dispensing bottles can store these liquids. They come in all sorts of sizes and with various pumps or spouts. Using different colours will help identify the many liquids needed at an ESD workstation.

ESD Dispensing BottlesExamples of Dispensing Bottles – more information

Waste Bin Liners
Bin liners come in different sizes and colours and can be useful when it comes to separating waste. They are non-tribocharging and are designed for use in ESD protected areas where electrostatic sensitive devices are present. Even at low humidity they do not become charged with static electricity. They are made from high quality polyethylene and are as strong as conventional refuse sacks.

BinLinersExamples of Waste Bin Liners – more information

Document Holders
Document holders are designed for use within ESD Protected Areas in accordance with EN 61340-5-1. They are static dissipative which means charges are removed to ground when placed on a grounded working surface or handled by a grounded operator. Applying them to ESD safe containers will help finding tools, components and accessories.

Document WalletsExamples of Document Holders – more information

2. Use Boxes and Containers
Everything is tidier when using boxes, right? The workstation looks clean and using document wallets (see above) will instantly tell the operator what is inside of each container or box. Everyone’s a winner!

Letter Trays
Generally conductive, any electrostatic charges on letter trays are removed to ground when the tray is placed on a grounded working surface or contacted by a grounded operator. They are helpful when organising documents (e.g. production orders) at an ESD workstation.

Letter TrayExample of a Letter Tray

Workstation Organisers
Workstation Organisers are ideal for improving the organisation of a workstation and standardising the placement of tools which is a key concept of the 5S methodology. They can be used for various items which are used on the workstation:

– dispensing bottles,
lotions,
flux bottles,
– solder spools,
– cutters,
– tweezers,
– wash bottles and
– various other workbench accessories.

Workstation OrganiserExample of a Worstation Organiser – more information

Ideally, Workstation Organisers should be the exact size required for your work area and have the tool openings cut for the tools you have “sorted” and determined need to be kept at the workstation.

Rack Holders, Containers and Hanging Bins
These types of storage solutions are perfect for PCB boards and components. They are generally made of a conductive material so that when placed on a grounded surface, any charges will dissipate to ground.

PCB ContainersExamples of a PCB Containers – more information

Maintaining an ESD Workstation
The hard part of ‘change’ is sticking with it and not falling back into old habits – this is where the last 3 steps of 5S come into play: clean, standardise and sustain. It’s essential that:

  1. An ESD Workstation is cleaned on a regular basis. Ensure all tools, accessories etc. are in the correct place and ESD precautions are followed.
  2. Procedures and processes are in place so every operator is aware of their responsibilities and how to perform their jobs correctly.
  3. A regular training and audit schedule is created. They are part of any ESD Programme and will not only ensure that ESD sensitive items are handled properly, but that ESD workstations are maintained.

Conclusion
The 5S methodology can be applied to a wide range of industries including an ESD workstation. There are numerous ESD tools and accessories available that can support companies with the implementation of 5S. The results will be increased efficiency, productivity and output.

References:
Huffington Post: 7 Tips to Organize Your Work Space and Stay Productive
Wikipedia: 5S (methodology)

Creating an ESD Workstation

When referring to an “ESD Protected Area” or “EPA”, a lot of people imagine rooms or even whole factory floors with numerous workstations. This very common misconception leads to nervousness and even fear when it comes to implementing an ESD Control Programme. There is a concern regarding the cost and time implications to establish an EPA. However, most often, a simple ESD workstation is completely sufficient to fulfil a company’s needs to protect their ESD sensitive products. Today’s post will provide a step-by-step guide on:

  • how to create an EPA at your workstation,
  • what ESD control products are required and
  • how to correctly set them up.

Introduction
An EPA is an area that has been established to effectively control Electrostatic Discharge (ESD) and its purpose is therefore to avoid all problems resulting from ESD damage, e.g. catastrophic failures or latent defects. It is a defined space within which all surfaces, objects, people and ESD Sensitive Devices (ESDs) are kept at the same electrical potential. This is achieved by simply using only ‘groundable’ materials for covering of surfaces and for the manufacture of containers and tools. All surfaces, products and people are bonded to Ground. Bonding means linking, usually through a resistance of between 1 and 10 megohms. Movable items (such as containers and tools) are bonded by virtue of standing on a bonded surface or being held by a bonded person. Everything that does not readily dissipate charge must be excluded from the EPA.
An EPA can be just one workstation or it could be a room containing a number of different workstations. “The size of an EPA can vary greatly. A protected area may be a permanent workstation within a room or an entire factory floor encompassing thousands of workstations. A protected area may also be a portable worksurface or mat used in a field service situation.” [CLC/TR 61340-5-2:2008 Use guide clause 4.6 Protected areas (EPA)]

Converting your Workstation into an EPA
Creating an EPA at your existing workstation does not need to be complicated or expensive. There are just a few things you will need:

ESD Workstation

1. Working Surface Mat
ESD protective working surfaces aid in the prevention of damage to ESD sensitive items (ESDS) and assemblies from electrostatic discharge.
ESD working surfaces, such as mats, are typically an integral part of the ESD workstation, particularly in areas where hand assembly occurs. The purpose of the ESD working surface is two-fold:

  • To provide a surface with little to no charge on it.
  • To provide a surface that will remove ElectroStatic charges from conductors (including ESDS devices and assemblies) that are placed on the surface.

2. Working Surface Mat Grounding Cord
Your ESD working surface needs to be grounded using a ground cord. A ground wire from the surface should connect to the common point ground (in our example an Earth Bonding Point Plug) which is connected to ground, preferably equipment ground. Best practice is that ground connections use firm fitting connecting devices such as metallic crimps, snaps and banana plugs to connect to designated ground points. The use of alligator clips is not recommended.

3. Earth Bonding Point Plug
Earth Bonding Point (EBP) plugs are designed to provide a common ground point for grounding using protective earth in an EPA. The plugs fit into the mains supply socket, making a connection with the earth conductor only. In place of the live and neutral pins are moulded insulating plastic pins to allow positive location in the socket.
Connectors on the front of the plug are available for connection via ground cords to the various elements of the EPA. Thus each element is held at a common potential.

4. Wrist Strap
Wrist straps are the most common personnel grounding device and are used to link people to ground. They are required if the operator is sitting. A wrist strap is made up of two components:

  • a wrist band that is worn comfortably around your wrist and
  • a coiled cord that connects the band to Ground (in our example an Earth Bonding Point (EBP) Bar).

5. Earth Bonding Point Bar
Note: instead of connecting your wrist strap to an Earth Bonding Point (EBP) bar, you can also connect it to the EBP plug described in #3. EBP bars fulfil the same function as EBP. However, they have been designed to be installed underneath bench tops where they are easily accessible to operators and where they are unlikely to be knocked and damaged or hinder the operator. The earthing cord of the bar needs to be connected to a suitable earth.

Where sitting personnel will be grounded via a wrist strap, this method is not feasible for operators moving around in an ESD Protected Area. In those situations, a flooring / footwear system is required.

6. Foot Grounders
Foot grounders are designed to reliably contact grounded ESD flooring and provide a continuous path-to-ground by removing electrostatic charges from personnel. They are easy to install and can be used on standard shoes by placing the grounding tab in the shoe under the foot.
Foot grounders must be worn on both feet to maintain the integrity of the body-to-ground connection Wearing a foot grounder on each foot ensures contact with ground via the ESD floor even when one foot is lifted off the floor.

7. Floor Mat
Floor matting is an essential component in the flooring / footwear system when grounding moving or standing personnel. The path to ground from operators via heel grounders to ground is maintained by using dissipative or conductive flooring.
Floor mats don’t just ground personnel; they are also used to ground ESD control items (e.g. mobile carts or workstations).

8. Floor Mat Grounding Cord
Just like working surface matting, floor matting needs to be connected to ground. This ensures that any charges on the operator are dissipated through their heel grounders and the floor matting to ground. A floor mat grounding cord is used to link the floor mat to ground (in our example an EBP bar).
Alternatively, matting can be earthed via a strip of copper foil.

Installing an ESD Workstation
Below is a step-by-step guide as to who you can create an ESD workstation at your existing workbench:

1.  Working Surface Mat Lay the working surface mat flat on the workbench with the stud(s) facing upwards.
2.  Working Surface Mat Grounding Cord Connect the working surface mat grounding cord to the working surface mat.
3.  EBP Plug Plug the earth bonding point plug into the appropriate socket at the wall. Note: if you are located outside the UK, there are country-specific bonding points available.
4.  Working Surface Mat Grounding Cord Grounding Connect the other end of the working surface mat grounding cord to the earth bonding point plug
5.  Wristband Place the wristband on the wrist.
6.  Coiled Cord Connect the coiled cord to the wristband.
7.  EBP Bar Attach the earth bonding point bar to the bench. Remember that it needs to be connected to a suitable earth.
8.  Coiled Cord Grounding Connect the other end of the coiled cord to the earth bonding point bar.

If your operators are standing or mobile and grounding via a wrist strap is not feasible, follow these steps:

1. Follow steps #1 to #4 above.
2.  Floor Mat Lay the floor mat flat on the floor with the stud(s) facing upwards.
3.  Floor Mat Grounding Cord Connect the floor mat grounding cord to the floor mat.
4.  EBP Bar Attach the earth bonding point bar to the bench. Remember that it needs to be connected to a suitable earth.
5.  Floor Mat Grounding Cord Connect the other end of the floor mat grounding cord to the earth bonding point bar.
6.  Foot Grounders Place the foot grounders on the feet.

Conclusion
To sum-up, in an EPA you:

  • ground all conductors (including people),
  • remove all insulators (or substituting with ESD protective versions) or
  • neutralise process essential insulators with an ioniser.

With a few simple steps, you can convert your existing workstation into an ESD workstation. You will need:

  • Working Surface Mat
  • Working Surface Mat Grounding Cord
  • Earth Bonding Point Plug
  • Wrist Strap
  • Earth Bonding Point Bar

Optional:

  • Foot Grounders
  • Floor Mat
  • Floor Mat Grounding Cord

 

How to measure the effectiveness of your ESD Control Programme

Introduction
It is now well established that electronic devices and systems can be damaged by exposure to high electric fields as well as by direct electrostatic discharges. While good circuit layout and on-board protection may reduce the risk of damage by such events, the only safe action at present is to ensure that devices are not exposed to levels of static electricity above the critical threshold.
This can only be achieved by introducing a static control programme which usually involves setting up an ESD Protected Area (EPA) in which personnel are correctly grounded and all materials e. g. flooring, bench tops etc. meet the of the ESD Standard. However, setting up an EPA does not of itself guarantee a low static environment. Production procedures may change, new materials may be introduced, the performance of older materials may degrade and so on.
To ensure the effectiveness of any static control programme it is important that regular measurements are carried out:

  • to determine the sensitivity to ESD of devices being produced or handled.
  • to confirm that static levels are lower than the critical level, and that new or modified work practices have not introduced high static levels.
  • to ensure that both new and existing materials in the EPA meet the necessary requirements.

Only after an ‘operational baseline’ has been established by regular auditing will it become possible to identify the origin of unexpected problems arising from the presence of static.

1. Determining the sensitivity of ESDs
The bottom line is: you need to know what you’re dealing with before you can create an action plan. Only once you know the sensitivity of the items you are handling, can you work towards ensuring you’re not exceeding those levels.
Part of every ESD control plan is to identify items in your company that are sensitive to ESD. At the same time, you need to recognize the level of their sensitivity. As explained by the ESD Association, how susceptible to ESD a product is depends on the item’s ability to either:

  • dissipate the discharge energy or
  • withstand the levels of current.

For further information, check out this post.

2. Measurements to prove the effectiveness of an ESD Control Programme
Measuring electrostatic quantities poses rather special problems because electrostatic systems are generally characterised by high resistances and small amounts of electrical charge; the latter being true despite the dramatic effects often associated with static. Consequently, conventional electronic instrumentation cannot normally be used.

Electrical Field
Wherever electrostatic charges accumulate, they can be detected by the presence of an associated electric field. The magnitude of this field is determined by many factors, e. g. the magnitude and distribution of the charge, the geometry and location of grounded surfaces and also the medium in which the charge is located.
The current general view of experts is that the main source of ESD risk may occur where ESDS can reach high induced voltage due to external fields from the clothing, and subsequently experience a field induced CDM type discharge.” [CLC TR 61340-5-2 User guide Garments clause 4.7.7.1 Introductory remarks]

50597_Use2Using a Digital Static Field Meter to test static fields

A static field meter is often used for ESD testing of static fields. It indicates surface voltage and polarity on objects and is therefore an effective problem solving tool used to identify items that are able to be charged.

A field meter can be used to:

  • verify that automated processes (like auto insertion, tape and reel, etc) are not generating charges above acceptable limits.
  • measure charges generated by causing contact and separation with other materials.
  • demonstrate shielding by measuring a charged object and then covering the charged item with an ESD lab coat or shielding bag. Being shielded the measured charge should be greatly reduced.

ESD Events
ESD events can damage ESD sensitive items and can cause tool lock-ups, erratic behaviour and parametric errors. An ESD Event Detector like the SCS EM Eye will help detect most ESD events. It detects the magnitude of events and using filters built into the unit, it can provide approximate values for some ESD events for models (CDM, MM, HBM) using proprietary algorithms.

CTM048-21_Use2Using the SCS EM Eye ESD Event Meter to detect ESD Events

Solving ESD problems requires data. A tool counting ESD events will help carry out a before-and-after analysis and will prove the effectiveness of implementing ESD control measures.

3. Checking materials in your EPA
When talking about material properties, the measurement you will most frequently come across is “Surface Resistance”. It expresses the ability of a material to conduct electricity. It is therefore related to current and voltage. In fact, the surface resistance of a material is the ratio of the voltage and current that’s flowing between two pre-defined electrodes.
It is important to remember that the surface resistance of a material is dependent on the electrodes used (shape as well as distance). If your company implements an ESD control programme compliant to the ESD Standard EN 61340-5-1, it is therefore vital to carry out surface resistance measurements as described in the Standard itself. For more information on the definition of resistance measurements used in ESD control, check out this post.
A company’s compliance verification plan should include periodic checks of surfaces measuring:

  • Resistance Point-to-Point (Rp-p) and
  • Resistance-to-ground (Rg).

222643UseMeasuring Surface Resistance of worksurface matting using a Digital Surface Resistance Meter Kit

Surface resistance testers can be used to perform these tests in accordance with EN 61340-5-1 and its test method IEC 61340-2-3; if these measurements are within acceptable ranges, the surface and its connections are good. For more information on checking your ESD control products, catch-up with this and this post. This 2-part series goes into depth as to what products you should be checking in your EPA and how they should be checked.

Conclusion
Measurements should form an integral part of any ESD control programme. High quality instruments are available commercially for measuring all the parameters necessary for quantifying the extent of a static problem. We hope the list above has provided an introduction to the techniques most commonly used.

How to neutralise a charge on an object that cannot be grounded

We have learnt in a previous post that within an ESD Protected Area (EPA) all surfaces, objects, people and ESD Sensitive Devices (ESDs) are kept at the same electrical potential. We achieve this by using only ‘groundable’ materials. But what do you do if you absolutely need an item in your EPA and it cannot be grounded? Don’t sweat, not all hope is lost! There are a couple of options which will allow you to use the item in question. Let us explain…

Conductors and Insulators
In ESD Control, we differentiate conductors and insulators.
Materials that easily transfer electrons are called conductors. Some examples of conductors are metals, carbon and the human body’s sweat layer.

ConductorA charged conductor can transfer electrons which allows it to be grounded

Materials that do not easily transfer electrons are called insulators and are by definition non-conductors. Some well-known insulators are common plastics and glass.

InsulatorInsulators will hold the charge and cannot be grounded and “conduct” the charge away

Both, conductors and insulators, may become charged with static electricity and discharge.
Electrostatic charges can effectively be removed from conductors by grounding them. However, the item grounded must be conductive or dissipative. An insulator on the other hand, will hold the charge and cannot be grounded and “conduct” the charge away.

Conductors and Insulators in an EPA
The first two fundamental principles of ESD Control are:

  1. Ground all conductors including people.
  2. Remove all insulators.

To achieve #1, all surfaces, products and people are bonded to Ground. Bonding means linking, usually through a resistance of between 1 and 10 megohms. Wrist straps and work surface mats are some of the most common devices used to remove static charges. Wrist straps drain charges from operators and a properly grounded mat will provide path-to-ground for exposed ESD susceptible devices. Movable items (such as containers and tools) are bonded by virtue of standing on a bonded surface or being held by a bonded person.

However, what if the static charge in question is on something that cannot be grounded, i.e. an insulator? Then #2 of our ESD Control principles will kick in. Per the ESD Standard, “All non-essential insulators and items (plastics and paper), such as coffee cups, food wrappers and personal items shall be removed from the workstation or any operation where unprotected ESDS are handled.
The ESD threat associated with process essential insulators or electrostatic field sources shall be evaluated to ensure that:

  • the electrostatic field at the position where the ESDS are handled shall not exceed 5 000 V/m;

or

  • if the electrostatic potential measured at the surface of the process required insulator exceeds 2 000 V, the item shall be kept a minimum of 30 cm from the ESDS; and
  • if the electrostatic potential measured at the surface of the process required insulator exceeds 125 V, the item shall be kept a minimum of 2,5 cm from the ESDS.”

[IEC 61340-5-1:2016 clause 5.3.4.2 Insulators]

Always keep insulators a minimum of 31cm from ESDS itemsAlways keep insulators a minimum of 31cm from ESDS items

“Process-essential” Insulators
Well, we all know that nothing in life is black and white. It would be easy to just follow the above ‘rules’ and Bob’s your uncle – but unfortunately that’s not always possible. There are situations where said insulator is an item used at the workstation such as a hand tools. They are essential – you cannot just throw them out of the EPA. If you do, the job won’t get done.
So, the question is – how do you ‘remove’ these vital insulators without actually ‘removing’ them from your EPA? There are 2 options you should try first:

1. Replace regular insulative items with an ESD protective version
There are numerous tools and accessories available that are ESD safe – from document handling to cups & dispensers and brushes and waste bins. They are either conductive or dissipative and replace the standard insulative varieties that are generally used at a workbench. For more information on using ESD safe tools and accessories, check this post.

2. Periodically apply a coat of Topical Antistat
The Reztore® Topical Antistat (or similar solution) is for use on non-ESD surfaces. After it has been applied and the surface dries, an antistatic and protective static dissipative coating is left behind. The static dissipative coating will allow charges to drain off when grounded. The antistatic properties will reduce triboelectric voltage to under 200 volts. It therefore gives non-ESD surfaces electrical properties until the hard coat is worn away.

If these two options are not feasible for your application, the insulator is termed “process-essential” and therefore neutralisation using an ioniser should become a necessary part of your ESD control programme.

Neutralisation
Most ESD workstations will have some insulators or isolated conductors that cannot be removed or replaced. These should be addressed with ionisation.
Examples of some common process essential insulators are a PC board substrate, insulative test fixtures and product plastic housings.

Electronic enclosures are process-essential insulators
Electronic enclosures are process-essential insulators

An example of isolated conductors can be conductive traces or components loaded on a PC board that is not in contact with the ESD worksurface.

An ioniser creates great numbers of positively and negatively charged ions. Fans help the ions flow over the work area. Ionisation can neutralise static charges on an insulator in a matter of seconds, thereby reducing their potential to cause ESD damage.
The charged ions created by an ioniser will:

  • neutralise charges on process required insulators,
  • neutralise charges on non- essential insulators,
  • neutralise isolated conductors and
  • minimise triboelectric charging.

Ioniser ExampleInsulators and isolated conductors are common in ESD Sensitive (ESDS) Devices – Ionisers can help

For more information on ionisers and how to choose the right type of ioniser for your application, read this post.

Summary
Insulators, by definition, are non-conductors and therefore cannot be grounded. Insulators can be controlled by doing the following within an EPA:

  • Keep insulators a minimum of 31cm from ESDS items at all times or
  • Replace regular insulative items with an ESD protective version or
  • Periodically apply a coat of Topical Antistat

When none of the above is possible, the insulator is termed “process-essential” and therefore neutralisation using an ioniser should become a necessary part of your ESD control programme.

6 Tips for handling “Class 0” Items

When talking about ESD Classifications a little while ago, we identified a “class 0” item as withstanding discharges of less than 250 volts.
The introduction of IEC 64340-5-1 states “This part of IEC 61340 covers the requirements necessary to design, establish, implement and maintain an electrostatic discharge (ESD) control program for activities that: manufacture, process, assemble, install, package, label, service, test, inspect, transport or otherwise handle electrical or electronic parts, assemblies and equipment susceptible to damage by electrostatic discharges greater than or equal to 100 V human body model (HBM), 200 V charged device model (CDM) and 35 V on isolated conductors.

So, the obvious question is: how do you handle items that are susceptible to voltages of less than 100V? That’s what we’re going to answer in today’s blog post.

Introduction
Years ago, it was common for devices to be vulnerable to voltages greater than 100V. As the need for smaller and faster devices increased, so did their sensitivity to ElectroStatic Discharges as circuit-protection schemes were removed to stay ahead of the market. These new extremely sensitive components are now susceptible to discharges nearing 0V. Obviously, this causes problems for companies handling these devices: while their ESD programme may be in compliance with the ESD Standard, extremely sensitive devices require tighter ESD Control to protect them from ESD failures.

Definition of “Class 0”
Before moving any further, we need to qualify the term “class 0” as there is a lot of confusion out there as to what this term actually means. As stated above, the HBM Model refers to any item with a failure voltage of less than 250V as a “class 0” component. However, in recent times, the term has been used more and more to describe ultra-sensitive devices with failure voltages of less than 100V. Whilst the following tips and tricks really work on any “class 0” item, they are specifically designed to protect extremely sensitive items that withstand discharges of less than 100V.

People are often a major factor in the generation of static chargesUltra-sensitive devices are extremely common these days

Do your homework
Imagine someone (a customer, your boss etc.) is approaching you and demands you to update all internal procedures so your company can handle “class 0” components. Do you know how to handle this request? Or would you be pulling out your hair trying to figure out what needs to be done? As explained further above, “class 0” refers to a wide range of items and there are a few things you should remember before making any changes to your existing ESD programme:

  1. Verify what ESD Model your company/engineers/customers etc. are referring to. As we have learnt in the past, there are different ESD models (HBM, CDM, MM) as well as individual classifications for each model. A lot of people get confused when it comes to citing ESD classifications. In reality, there is only one “class 0” which refers to the human body model (HBM) but it’s always best to check.
  2. Check the specific withstand voltage an individual part is susceptible to. “Class 0” refers to all items that withstand discharges of less than 250V. However, there is a big difference between a failure voltage of 240V or 50V. You need to have detailed ESD sensitivity information available before being able to make decisions on how to improve your existing ESD control programme. This step is actually part of creating a compliance verification plan.
  3. A part’s ESD classification is only of importance until it is ‘merged’ into an assembly. So, the ESD classification of a device only refers to the stand-alone component. Once it goes into another construction, the classification of the whole assembly is likely to change.

Below are 6 tips that will help your company to upgrade your ESD control programme so you can effectively and efficiently handle ultra-sensitive items without risking ESD damage.

One thing to note: proactive actions are critical. There is no point in figuring out how to protect your components from ESD damage AFTER you have received them. Trust us: it’s gonna go wrong! Instead, focus on getting things sorted BEFOREHAND. That’s the best approach to stay ahead of the game.

1. Improve Grounding
So, you will already know that inside an EPA, all conductors (including people) are grounded. Now you’re probably thinking: “But I’ve already grounded my operators and worksurfaces. What else is there left to do?”. Firstly, well done for properly grounding the ‘objects’ in your EPA – trust us, that’s not a given! The next step is to tweak things a bit to allow for even better protection. Here are some suggestions:

Personnel:

  • Decrease the wrist strap and ESD footwear upper limit. The ESD Association has test data showing charge on a person is less as the path-to-ground resistance is less.
  • Use continuous monitors and ESD smocks
  • Introduce/increase use of ESD flooring
  • Use sole or full coverage foot grounders (rather than heel grounders)

WorkstationFull coverage foot grounders are recommended when handling ultra-sensitive devices

Worksurfaces:

  • Reduce the required limit for Point-to-Point resistance of 1 x 109 per the ESD Standard to 106 to 108 ohms (see #5). The reason for this reduction is simple: 1 x 109 is too high as it still produces thousands of volts of in electrostatic charges. However, the resistance cannot be too small either as this can lead to a sudden ‘hard discharge’ potentially damaging ESD sensitive components.

Other:

  • Improve grounding of carts, shelves and equipment to Ground
  • Minimise isolated conductors like devices on PCBs

2. Minimise Charge Generation
The best form of control is to minimise charge generation. First of all, you should always use shielding packing products like bags or containers (especially when outside an EPA) as these protect from generating charges in the first place. For more information on choosing the correct type of ESD Packaging, we recommend reading this post.

The next step is to eliminate charges once they are generated – this can be achieved through grounding and ionisation. We’ll cover ionisation in #3 and #4. We’ve already talked about improved grounding in #1. However, for ultra-sensitive components, we also recommend the following:

Both types of ESD products create a low tribocharging coating which allows charges to drain off when grounded. The antistatic properties will reduce triboelectric voltage to under 200 volts.
For more tips on managing charge generation from flooring, check this post.

3. Remove Insulators
When talking about conductors and insulators, we explained that insulators cannot be grounded and can damage nearby sensitive devices with a sudden uncontrolled discharge. It is therefore critical to eliminate ALL insulators that are not required in your EPA: plastic cups, non-ESD brushes, tapes etc. How? Here are a couple of options:

EBP-Bar-for-FlyerUse ESD safe accessories whenever possible

If an insulator is absolutely necessary for production and cannot be removed from the EPA, you could consider a topical treatment which will reduce triboelectric charges.
Is this not an option, then move on to tip #4.

4. Use Ionisation
First of all, ionisation is not a cure-all. We’ve learnt that ionisers neutralise charges on an insulator.
However, that does not mean that you can just have any insulator in your EPA because the ioniser will “just fix it”. No, in this instance, prevention is generally a better option than the cure. So, your priority should ALWAYS be to remove non-process essential insulators from your EPA – see tip #3. If this is not possible – then ionisation becomes essential:

  • Ionisers can be critical to reduce induction charging caused by process necessary insulators
  • Ionisers can be critical in eliminating charges on isolated conductors like devices on PCBs
  • Offset voltage (balance) and discharge times are critical considerations depending on the actual application
  • Ionisation can reduce ElectroStatic Attraction (ESA) and charged particles clinging and contaminating products.

It is recommended to use ionisers with feedback mechanisms so you’re notified if the offset voltage is out of balance.

5. Increase ESD Training and Awareness
ESD Training is a requirement of every ESD Programme. When handling ultra-sensitive devices, it is even more important to remind everyone what pre-cautions are necessary to avoid damage. Regular ‘refreshers’ are a must and it is recommended to verify the effectiveness of the training programme, e.g. through tests. So, who, when and what should be taught? Easy!

Training is an essential part of an ESD Control ProgrammeESD Training is a vital part of every successful ESD Control Programme

  • ESD training needs to be provided to everyone who handles ESD sensitive devices – that includes managers, supervisors, subcontractors, visitors, cleaners and even temporary personnel.
  • Training must be given at the beginning of employment (BEFORE getting anywhere near a sensitive products) and in regular intervals thereafter.
  • Training should be conducted on proper compliance verification procedures and on the proper use of equipment used for verification.

6. Create an enhanced Compliance Verification Plan
We talked in a previous post about compliance verification, what it is and how to create a plan that complies with the ESD standard. So, if you already followed our steps and have a plan in place, you’re probably wondering how you can possibly improve on that? Here are a few tips:

  • Use a computer data collection system for wrist straps and foot grounders testing, e.g. SmartLog Pro™
  • Increase the testing frequency of personnel grounding devices from once per day to every time the operator enters the EPA
  • Use continuous monitors where operators are grounded via wrist straps. Consider computer based monitor data collection system, e.g. SMP. This should include continuous monitoring of the mat Ground.
  • Use Ground continuous monitors, e.g. SCS Ground Master. At a large facility, the most frequent reoccurring violation is the ESD mat ground cord either becoming disconnected from the mat or grounding point. As Ground continuous monitors will only test the fact that the mat is grounded, it is still imperative that the Resistance to Ground of the mat is regularly tested. Remember that the use of improper mat cleaners can raise the mat surface resistance above the upper recommended level of <109
  • Test ionisers more frequently or consider self-monitoring ionisers. Computer based data collection systems are a good alternative, too.
  • Increase the use of a static field meter and nano coulomb testing to verify that automated processes (like auto insertion, tape and reel, etc) are not generating charges above acceptable limits.

Conclusion
The bottom line is: the only way to protect ultra-sensitive components is to increase ESD protective redundancies and periodic verifications to all ESD Control technical elements.
If you handle ultra-sensitive items, to decrease the probability of ESD damage, additional precautions are required including additional and/or more stringent technical requirements for EPA ESD control products, increasing redundancies, and more frequent periodic verifications or audits. Additionally, ESD control process systems should be evaluated as to their performance as a system. You will need to understand how the technical elements in use perform relative to the sensitivity of the devices being handled. Thus, tailoring the process to handle the more sensitive
parts. For example: If the footwear/flooring allows a person’s body voltage to reach say 80 volts and a 50 withstand voltage item gets introduced into the process, you will either have to allow only handling via wrist straps or would have to find a way to modify the footwear/flooring performance to get peak voltages below the 50 volt threshold.

Remember: it is YOUR responsibility to do protect YOUR devices and YOUR reputation. The ESD Standard can only give recommendations and it’ll always be behind current/future developments. As soon as a Standard is published, technology will have progressed. So, if – in order to protect your devices – your company needs to implement methods/procedures that exceed the recommendations of the ESD Standard, so be it.

References:

Resistivity and Resistance in ESD Control

There is a lot of confusion out there as to what the difference is between resistivity and resistance. We get asked questions on a regular basis so hopefully this post will put an end to any misunderstanding – we’ll explain the difference between the two and will point out the measurements you really need to worry about when it comes to your ESD Control Programme.

The difference between Resistivity and Resistance
Resistance or resistivity measurements help define the material’s ability to provide electrostatic shielding or charge dissipation.“ [Source]
However, resistance and resistivity values are not interchangeable. Let’s get a bit technical here to illustrate the difference between the two:

1. The resistance expresses the ability of a material to conduct electricity. It is therefore related to current and voltage. In fact, the surface resistance of a material is the ratio of the voltage and current that’s flowing between two pre-defined electrodes.
With a pure resistive material,

Resistance Calculation

where:

– R is the resistance (expressed in Ohm Ω),
U is the voltage (expressed in Volt) and
I is the current (expressed in Amp).

The unit of measure for surface resistance is W. It is important to remember that the surface resistance of a material is dependent on the electrodes used (shape as well as distance). If your company implements an ESD Control Programme compliant to the ESD Standard EN 61340-5-1, it is therefore vital to carry out surface resistance measurements as described in the Standard itself.

2. The surface resistivity of a material describes a general physical property. It is not influenced by the shape of the electrodes used or the distance between them. “Surface resistivity, ρ,  can  be  defined  for electric current flowing across a surface as the ratio of DC voltage drop per unit length to the surface current per unit width.” [Dr. Jaakko Paasi, VTT Industrial Systems: “Surface resistance or surface resistivity?”]
As Dr. Jaakko Paasi describes in his research note, surface resistivity can be expressed by using a concentric ring probe as  

Resistivity Calculation

where:

– k is the geometrical coefficient of the electrode assembly,
rcentre is the outside radius of the centre electrode and
router is the inside radius of the outer electrode.

For the electrodes recommended by EN 61340-5-1, the coefficient k = 10.
The unit of measure for surface resistivity is W but in practice you will often see W/square (which technically is not a physical unit).

As previously explained, the surface resistivity does not depend on shape or distance of the electrodes used when performing the test. You can compare results freely – no matter what type of electrode was used to get the measurements in the first place.

Converting from Resistivity to Resistance
Values of surface resistance and surface resistivity become comparable if the measured surface resistance value is multiplied by the geometrical coefficient of the used electrode fixture.” [Dr. Jaakko Paasi, VTT Industrial Systems: “Surface resistance or surface resistivity?”]
If you measure surface resistance according to EN 61340-5-1, then the corresponding surface resistivity can be calculated by multiplying the resistance value by the geometrical coefficient factor k = 10. Likewise, surface resistivities can be converted to surface resistances by dividing the surface resistivity value by 10.

Per User guide EN 61340-5-2:1999 Clause 4.1.1 “Point-to-point resistance has been discussed, rather than the surface and volume resistivity which was found in previous standards and reports. This change has been made to cater for non-homogenous materials, which are becoming increasingly common in these applications, as well as ease of measurement.
Particular care is needed in interpreting results when measuring non-homogeneous materials such as multilayer mats or conductive-backed synthetic fibre carpeting containing a small amount of conductive fibre. Buried conductive layers can provide shunt paths. Be clear when stating what you have measured!

A few notes in regards to measuring surface resistance and resistivity:

  • On large surfaces, such as bench-mats, readings will sometimes vary with increasing time of measurement. This is due to the ‘electrification’ of the mat beyond the area measured. It is therefore important to measure properly and to keep the duration of measurement constant. Fifteen seconds is an arbitrary but practical duration for measurement time.
  • Moreover, the materials needing to be checked in an EPA are most of the time, non-conductive polymers that have been made conductive or antistatic by addition of conductive particles or by special treatments during manufacture. The resistivity of such materials may vary from one point to another or they may be direction dependent (anisotropic).
  • EN 61340-5-1 goes some way to specifying the procedures to be followed and test probes to be used, so that the results can be compared, at least roughly.
  • Also, the resistance of some materials may vary with humidity level and temperature. It is therefore good practice to take a note of these two parameters when measuring.

So now that we’ve identified what the difference is between surface resistance and resistivity, there is one more thing we want to cover in today’s post: the different types of surface resistances you will come across when dealing with ESD and how to measure them:

1. Resistance to Ground (Rg)
Resistance to Ground is a measurement that indicates the capability of an item to conduct an electrical charge (current flow) to an attached ground connection. The higher the resistance in the path, the more slowly the charge will move though that defined path.” [Source]
The Resistance to Ground is measured to ensure that surfaces in an EPA are correctly grounded. This is certainly one of the most useful measurements in an EPA.

Performing a Resistance to Ground TestPerforming a Resistance to Ground Test

To perform the test:

  • One 2.3kg cylindrical probe is required for this measurement.
  • Connect the probe to a megohm meter and place it on the surface to test.
  • Connect the other ohmmeter lead to earth or to an ESD ground point.
  • Measure the resistance at 10V for conductive items and 100V for dissipative items.

2. Resistance Point-To-Point (Rp-p)
A point-to-point measurement used during the qualification process evaluates floor and worksurface materials, garments, chair elements, some packaging items, and many other static-control materials.“ [Source]
Resistance Point-To-Point is used to assess the performance of an item used in an EPA.

Performing a Resistance Point-To-Point Test

To perform the test:

  • Two 2.3kg cylindrical probes are required for this measurement
  • Connect the probes to a megohm meter.
  • Place the material to be tested on an insulative surface such as clean glass and place the probes on the material.
  • Measure the resistance at 10V for conductive items and 100V for dissipative items.
  • Move the probes so as to measure in a cross direction and repeat the test.

Point-to-point measurements are important during the qualification process for proper evaluation of flooring and worksurface materials. After installation, the resistance-to-ground measurement is more applicable since it emulates how the material really behaves in practice.” [Source]

3. Volume Resistance (RV)
Although this is one of the less common measurements when it comes to ESD, it’s still worth to mention the volume resistance here. You would measure the volume resistance when a non-grounded item such as a container is to be placed on a grounded item, such as a mat. The volume resistance will indicate whether the item can be used in the desired manner.

Performing a Volume Resistance TestPerforming a Volume Resistance Test

To perform the test:

  • Two 2.3kg cylindrical probes are required for this measurement
  • Connect the probes to a megohm meter.
  • Put the first probe upside down and ‘sandwich’ the test sample between it and the second probe placed on top.
  • Measure the resistance.

So hopefully we have put an end to any confusion in regards to surface resistivity and resistance and answered all your questions. If there is anything else you’d like to know, let us know in the comments.

References:

How to manage Charge Generation from Flooring

We’ve previously learned that the simple separation of two surfaces can cause a transfer of electrons resulting in one surface being positively and the other negatively charged. A person walking across a floor and soles contracting & separating from the floor is such an example. The resulting static charges that generate are an annoying and costly occurrence for office and factory employees. The thing is, they can easily be controlled with existing carpets and tiled floors. Learn how in today’s post.

What is Static Electricity?

Static electricity is an electrical charge that is at rest – as opposed to electricity in motion or current electricity. Static charges can be generated by either friction or induction. Typical examples are the Wimshurst machine that uses friction and the Van de Graaff generator using electrostatic induction.

How is Static Electricity generated?

The most common generation of static charge is the triboelectric charge or the friction electricity developed when rubbing together and then separating two masses. For example, when two blocks are rubbed together and then separated, a triboelectric charge is developed on each block. The two blocks will have opposite polarities; one will be negatively charged and the other will be positively charged. Other examples include:

  • Unwinding a roll of tape
  • Gas or liquid moving through a hose or pipe
  • A person walking across a floor and soles contacting & separating from the floor.

 

Charge Generation Unwinding a Roll of TapeCharge Generation: Unwinding a Roll of Tape

Static Charge Generation from flooring

When a person walks across a carpeted or tiled floor, a triboelectric charge builds up in the body due to the friction between the shoes and floor material. The more you generate, the greater the voltage potential developing in the body – you are basically acting as a capacitor.

Everyone’s capacitance to hold charges is different. However, a sure sign of static presence is hair standing on end or static discharge sparks. Static discharges can be noticed when you touch an object of lower electrical potential such as a door knob, and a bolt of electricity flows from your charged body to the door knob. This flow of electricity is actually a result of the stored static charge that is being rapidly discharged to the lower potential object.

This discharge that can be felt as well as seen, is commonly referred to as an electrostatic discharge, or “ESD”.

 

Generating Charges by walking across a CarpetGenerating Charges by walking across carpet

It is not necessarily the static charge generated in the body that does the damage as much as it is the difference in potential that creates an electrostatic discharge. The ESD event can be felt at the human sensation threshold of 3000 volts. If one feels or sees the static shock, it is a minimum of 3000 volts. The potential static charge that can develop from walking on tiled floors is greater than 15,000 volts, while carpeted floors can generate in excess of 30,000 volts.

The problem with ESD

The generation of a static charge can pose quite a problem for environments that contain sensitive equipment or components that are vulnerable to static damage, such as electronic manufacturing, repair facilities or medical facilities including computer rooms and clean rooms.

Controlling the damage and costs caused by ESD is usually the main concern that drives a company to implement a static control programme. The costs involved with static damage not only include the immediate cost of the damaged component but the contributing cost of diagnostic and repair labour that is needed to replace or fix the component. In most cases, the labour involved will far exceed the component cost. If the damaged component performs enough to pass Quality Control (QC), it is called a soft failure as opposed to a hard failure when it does not pass the QC. It is far more expensive for a soft failure occurring at the manufacturer which then leads to a hard failure in the field which escalates product returns and field service cost.

 

The Solution

As with any type of control, there are several levels of protection. The method for choosing the necessary degree of ESD protection starts with defining your static sensitivity for electronic components. The ESD Association defines different classes of sensitivity for the HBM (Human Body model) and CDM (Charged Device Model).

ESDS Component Sensitivity Classification

How can you determine the class of sensitivity of the devices within your facility? Look at your product flow through your facility, start at receiving and walk the components or products through until they are at dispatch ready to ship. Chances are, you have several different product flows through your facility. Each flow or loop will have specific components that enter or travel the loop. Make a list of all the sensitive components in each loop and determine the static voltage sensitivity or rating from each of the manufacturers. The lowest voltage sensitivity will dictate the sensitivity class of each loop. The philosophy here is “the chain is only as strong as the weakest link”. Each loop should have the required ESD protection for the most sensitive components that will travel this loop. This will define what class of protection is needed for each loop. You can have different class loops as long as the loops are closed, not allowing other components in. The objective here is to define a static control programme to safeguard your most sensitive component.

Flooring Materials

ESD control carpet and conventional carpet with antistatic treatments can still generate up to 1,500 volts, far exceeding the class 1 limits for the HBM. These carpets, however, when properly maintained, can provide safe grounding and electrostatic propensity below the class 2 and 3 sensitivity range.

Proper maintenance for ESD control carpets is rather simple but very important. For conventional carpets that are treated with a topical antistat or other treatment, it is required that the treatment is replenished on the carpet as it wears away due to foot traffic. The amount of treatment on the carpet can be determined by testing with a surface resistance meter. The higher the resistance readings of the floor, the lower the amount of static control treatment that is present on the carpet. The level of treatment should be monitored by resistance readings and kept between 1 x 106 and 1 x 1010 ohms. Some ESD floor finishes can be used as a carpet treatment. This requires a simple spray bottle filled with 50/50 mix of ESD control floor finishes and water. Always check with the floor finish manufacturer before use. Application of diluted floor finish usually requires a 1 to 2 spray coat on the carpet depending on the level of resistance you want.

 

Topical AntistatReztore® Topical Antistat – for more information click here

ESD control carpets are made with static dissipative yarn and only require that the yarn is kept clean and free of insulative dirt, dust and spray cleaners.

ESD control floor tiles can also generate triboelectric charges depending on the construction of the tile. The tile (dissipative or conductive) may have voids between the impregnated conductive sections which allows triboelectric charges to be generated and then drained. This cyclic voltage can be very harmful to sensitive components.

ESD control floor finishes alone can provide both non-triboelectric charging as well as a path to ground. Such floor finishes can be applied on many surfaces including sealed concrete, vinyl tile and especially ESD control tiles. If the ESD control tile is generating triboelectric charges, ESD control floor finish will complement these tiles with its non-triboelectric properties, as well as enhancing the surface’s electrical properties. The ease of maintenance for an ESD control floor finish is another benefit when used on top of any tile floor, especially carbon impregnated conductive tile that may form streaks of black carbon on the surface.

 

Floor FinishStatguard® Floor Finish – for more information click here

Conclusion

The best static controls are not only the ones that protect sensitive components and equipment but are: A) at hand and readily available, B) easily maintained. For these reasons, carpets and tile floors should not be overlooked as sources for static control. Existing carpet or tile floors can be easily included into an ESD control programme.

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