We have previously learnt that wrist straps are considered the first line of ESD Control. They are used to link people to ground which ensures that that the operator is kept at the same potential as surfaces, objects and ESD sensitive devices. We’ve also discovered that wrist straps need to be visually inspected and checked (while worn) on a daily basis – BEFORE handling any ESD sensitive item. This will alert the operator if their wrist strap has developed a fault and as a result does not ground them any longer.
An alternative to periodic testing is the use of continuous monitors. Per ESD Handbook TR 20.20 paragraph 126.96.36.199.4 Test Frequency, “Because wrist straps have a finite life, it is important to develop a test frequency that will guarantee integrity of the system. Typical test programs recommend that wrist straps that are used daily should be tested daily. However, if the products that are being produced are of such value that knowledge of a continuous, reliable ground is needed, then continuous monitoring should be considered or even required.”
In today’s post we will highlight 4 benefits of continuous monitoring which may help you decide to move away from daily wrist strap checks.
But first a little reminder of what continuous monitors actually are: Continuous monitors come in different styles and sizes but are intended to be kept on your workstation. Some units just ‘sit’ on your bench; others are attached to your working surface matting; some can even be attached underneath the workbench so they don’t take away valuable workspace. Operators connect their wrist strap to the unit to allow for real-time continuous monitoring. If the wrist strap fails, the unit will alarm. Many continuous monitors also feature a parking stud providing a means for the operator to disconnect when leaving their workstation.
There are two different types of continuous monitors available:
- Single-wire continuous monitors allow the use of any standard, single-wire wrist strap and coil cord. The monitor / wrist strap system life-cycle costs are significantly lower than dual-wire systems. While they would not be suitable for the most critical applications, single-wire continuous monitors are an economical way to monitor both the operator’s wrist strap and/or workstation surface.
- Dual-wire constant monitors provide true continuous monitoring of wrist strap functionality and operator safety according to accepted industry standards. Dual-wire continuous monitors provide redundancy because even if one dual-wire wrist strap conductor is severed, the operator still has a reliable path-to-ground with the other conductor.
1. Instant Feedback
Imagine this scenario: you come to work in the morning, you test your wrist strap, it passes and you start work on your ESD sensitive devices. 3 hours later, when you come back from your tea break, you test your wrist strap again and it fails. What to do? You don’t know if the wrist strap only just failed or if it failed right after your first test in the morning. How do you know if the devices you worked on all morning have been damaged? You don’t – after all, latent defects are not visible and failures may only occur at a later time. Using continuous monitoring while working on those ESD sensitive devices will alert the operator as soon as their wrist strap fails. The faulty wrist strap can be replaced with a new model from stock and everyone is happy – no ESD sensitive devices damaged and no unhappy customers.
The EMIT Zero Volt Monitor (50579) in Use
Continuous monitors provide operators with instant feedback on the status and functionality of their wrist strap. The instant an operator’s wrist strap or cord fails, the monitor will issue audible and visual (LEDs) alarms alerting the user and supervisor of the problem. Full time continuous monitoring is superior to periodic or pulsed testing, and can save a significant amount of money in testing costs and rejected product. Periodic testing only detects wrist strap failures after ESD susceptible products have been manufactured. The costs of dealing with the resulting catastrophic failures or latent defects can be considerable. “A properly grounded wrist strap will keep a person’s body voltage to approximately + 10 V. The main advantage to a constant [or continuous] monitor is the immediate indication that the employee receives if the wrist strap falls open. With an unmonitored system, the employee will not be aware of a wrist strap failure until the start of the next shift. This has reliability benefits for an ESD program as it might help reduce or eliminate ESD damage.” [CLC/TR 61340-5-2:2008 User guide Annex B.1.3 Constant monitors].
2. Monitor Operator AND Workstation
An option available with most continuous or constant monitors is the ability to monitor working surface ground connections. “Some continuous monitors can monitor worksurface ground connections. A test signal is passed through the worksurface and ground connections. Discontinuity or over limit resistance changes cause the monitor to alarm. Worksurface monitors test the electrical connection between the monitor, the worksurface, and the ground point. The monitor however, will not detect insulative contamination on the worksurface.” [ESD TR 12-01 Technical Report Survey of Constant (Continuous) Monitors for Wrist Straps]
When the monitor is connected to an ESD Mat working surface, the amount of current that flows is a function of the total resistance between the monitor and through the working surface to ground. When the resistance of the working surface is below a pre-set threshold*, the monitor will indicate good. Conversely, if the resistance level is high when compared to the monitor’s reference*, the unit will alarm. This is an integrating resistance measuring circuit, therefore it is relatively insensitive to externally induced electromagnetic fields.
Installing the Vermason Multi-Mount Monitor (222608) to ground the worksurface
“For units that also monitor the connection of a worksurface to protective earth, it is also possible to reduce or eliminate the checking of the worksurface as part of the periodic audit of the process.” [CLC/TR 61340-5-2:2008 User guide Annex B.1.3 Constant monitors].
*The resistance threshold limits can vary between brands and models (and can sometimes also be adjusted by the user) so make sure you do your homework before committing to a particular unit and check the limit meets your individual requirements.
3. Detect Initial Flex Fatigue
Unlike wrist strap testers, continuous monitors detect split-second failures when the wrist strap is still in the “intermittent” stage. This is prior to a permanent “open” which could result in damage to ESD sensitive components.
The Jewel Mini Workstation Monitor (222603) in Use
“Wrist strap checkers are usually placed in a central location for all to use. Wrist straps are stressed and flexed to their limits at a workstation. While a wrist strap is being checked, it is not stressed, as it would be under working conditions. Opens in the wire at the coiled cord’s strain relief are sometimes only detected under stress.” [ESD TR 12-01 Technical Report Survey of Constant (Continuous) Monitors for Wrist Straps]
4. Eliminate Need for Periodic Testing
Many customers are eliminating periodic touch testing of wrist straps and are utilising continuous monitoring to better ensure that their products were manufactured in an ESD protected environment. Continuous monitors eliminate the need for users to test wrist straps and log the results; by their function, these monitors satisfy the EN 61340-5-1 test logging requirements. “There are also other process benefits from using constant monitors such as the elimination of the need to maintain daily test logs and a reduction in the time for employees to make the daily test.” [CLC/TR 61340-5-2:2008 User guide Annex B.1.3 Constant monitors].
No more Paper Logs!
So when using constant monitoring, operators:
- Don’t have to waste time queuing at a wrist strap test station before each shift.
- Don’t have to remember to complete their daily test logs.
It’s also harder to ‘cheat’ with continuous monitors. We’re not saying, your employees would do naughty things like that but we’ve seen it all before: operators ‘pretending’ to perform a wrist strap check, operators failing a wrist strap test and still recording a pass etc. There are always options to bypass a system, but it’s definitely harder when continuous monitors are used.
So should you now run-out and equip all your users with continuous monitors? As with most things in life, the answer is not that simple: it depends! If your company manufactures products containing ESD sensitive items, you need to ask yourself “how important is the reliability of our products”? Sooner or later a wrist strap is going to fail. If your products are of such high value that you need to be 100% sure your operators are grounded at all times, then you should consider a continuous monitoring system.
The purpose of an ESD protective working surface is to aid in the prevention of damage to ESD sensitive items (ESDS) and assemblies from electrostatic discharge. An ESD protective working surface provides protection in the following two ways:
- Providing a low charging (antistatic) working surface area that will limit static electricity to be generated below potentially damaging levels.
- Removing the electrostatic charge from conductive objects placed on the working surface.
1. Types of ESD working surfaces
ESD protective working surfaces are categorised into two general categories: conductive and dissipative.
A conductive working surface is defined by most documents as a material that has a surface resistance of less than 1 x 104 ohms. Conductive materials are the quickest to ground a charge but they can also cause damage by discharging too rapidly. Conductive materials are usually used as floor mats or flooring products.
A dissipative working surface is defined as being materials having a surface resistance of at least 1 x 104, but less than 1 x 109 ohms. Dissipative materials will dissipate a charge slower and are recommended for handling electronic components. Dissipative materials are usually the preferred choice for bench top working surfaces.
Most people in the industry consider working surfaces to be the second most important part of an ESD Control Programme, with personnel grounding being most important.
2. Grounding Methods for working surfaces
Method 1: Grounding via ground cords
- Vermason recommends using an earth bonding point cord when grounding via ground cords. Most earth bonding point cords will ground an ESD protective working surface and provide banana jacks for two wrist strap grounds.
Earth Bonding Point for each workstation
- An earth bonding point should be installed at each workstation and should be connected directly to a verified electrical system ground or to a verified grounding bus which is connected to the protective earth ground. Only one groundable point should be installed on a working surface.
- Wrist straps should never be grounded through a working surface, as the added resistance of the working surface material will prevent the wrist strap from operating properly.
Proper Grounding of Wrist Straps
Method 2: Grounding via a grounded conductive surface
- This alternate form of grounding should only be employed when using a homogeneous dissipative material with a volume resistance of less than 1 x 108
- The dissipative working surface may be placed on a properly grounded laminate, metal or other conductive surface. The working surface will electrically couple to the grounded surface and may not require a separate ground cord.
- When using this type of grounding method be sure to test that the working surface Rg is less than 1 x 109 ohms, tested per IEC 61340-2-3. Also consider increasing Compliance Verification test frequency.
Alternate Grounding Method via a Grounded Conductive Surface
3. Groundable Point Installation
Before installing a groundable point on your work surface you must first determine whether you will need a male stud or female socket, the type of snap hardware and the desired location.
There are generally 3 types of groundable points available for working surface mats: screw-on snap kits, push & clinch snaps (with prongs) or stud & posts sets (requiring installation using a punch and an anvil).
Snap Kits and Tools
- Determine the position of the grounding snap (one only per mat). Punch a hole through the material with a small Phillips screwdriver or awl.
- Insert the screw through the bottom on the snap fastener, the washer and the material. Affix the assembly with the conical nut supplied with the kit and tighten down the screws.
Installing a screw-on Mat Grounding Snap
Push & clinch snaps:
This snap is designed for use with any type of soft mat material: dissipative, conductive or multi-layered. It is recommended for use with three-layered material, because it provides better contact with the internal conductive layer. It is recommended that before inserting this snap, the mat be punctured with a sharp tool where the snap will be placed.
Centre the prongs on the snap assembly. Apply pressure to the snap until the prongs come through the back of the mat, then clinch over prongs making flat to the mat’s bottom side to secure snap as shown in the below picture.
Installing Push & Clinch Mat Grounding Snap
Stud & post sets:
This type of groundable point must be riveted through bench and floor mats to connect ground cords. A punch and anvil are simple but effective tools to achieve a neat finish with firm materials no more than 4mm thick.
- Punch a 5mm diameter hole at the desired location of the mat.
- Insert the post from underneath and apply the stud over the protruding post on the top side.
- Fit the anvil under the post and place the punch inside the stud and hammer the post (or use an arbor press) until it rolls and a tight assembly is achieved.
Using a Punch and Anvil to install Stud & Post Sets
4. Selection of Common Point & Floor Mat Grounding Systems
- Determine the type of common point grounding system you will use: barrier strip, bus bar, grounding block or common point ground cord. Vermason recommends the use of common point ground cords and earth bonding bars.
- If you determine that you will use ground cords, you must now determine the type of ground cord you will use for your workstation grounds. It is the user’s preference to use a ground cord with or without a current limiting 1 megohm resistor to ground working surfaces or floor mats. Selection of the ground cord is determined by user needs and specifications; the resistor is not for ESD control.
Examples of Grounding Cords
- Earth bonding point bars allow the grounding of multiple operators at one common ground point. They also mount easily under the front edge of a workstation benchtop.
Earth Bonding Point Installation
5. Mat Installation
- For best results, allow the mats to lay flat for about four hours at room temperature before installing. This will give the material time to flatten out from being rolled for shipment.
- Test all workstation grounds for proper resistance to ground.
- Lay the mat in position and snap the ground cord to it. Bring the other end of the ground cord to the common ground point (or earth bonding bar) and attach it using the ring terminal (or other termination device). The electrical systems junction box and connecting conduit should also connect to earth protective ground. Tie the ground wire to the bench to keep it out of the way and neat. You may cut and strip the ground wire to a shorter length and attach it with an extra ring terminal if required.
Note: DO NOT DAISY CHAIN. Because of the high resistances inherent to many types of protective surfaces, daisy chaining of these materials can cause the overall resistance to exceed the required limit of EN 61340-5-1.
ESD working surface should never be grounded in series, i.e. daisy chained
- If your kit includes a floor mat, you should duplicate step 2 and attach the floor mat ground to the same ground point as the working surface ground.
- Measure the resistance from the ground snap on the mat to the common ground point. It should read 1 megohm ±20 percent if you are using a ground cord with a resistor, and less than 10 ohms if you are using a ground cord without a resistor.
- If you have a surface resistance or resistance to ground tester available, you may wish to test the resistance to ground from the mat surface. Note: depending upon the accuracy of the instrument you are using, you may get a wide range of results in resistance to ground tests. In order to get the electrical readings specified per EN 61340-2-3, two 2.2kg electrodes are to be used. This will require a megohmmeter with 100 volt open test circuit voltage and two 2.2kg electrodes.
- If you are using a mat kit that includes the wrist strap, install the wrist strap directly to the common point mat ground cord. Again, test the resistance from the backplate of the wrist strap to the common ground point. It should read 1 megohm ± 20 percent.
Adding a Wrist Strap
- Your completed installation of an ESD workstation should comply with one of the electrical diagrams illustrated below.
Proper wiring diagrams for conductive and dissipative ESD workstations
6. Maintenance and Cleaning
For optimum performance, periodic cleaning is required following the manufacturer’s recommendations.
BE SURE YOU TEST ALL GROUNDS AND THE WRIST STRAP FREQUENTLY
We get a lot of customers asking us if they should use ESD foot grounders or ESD shoes in their EPA. And our answer is always the same: it depends! There really is no right or wrong when it comes to choosing but there are obviously a few things you need to consider before investing in one or the other.
In some cases, protective footwear (shoes, boots, etc.) is required to prevent foot injuries due to falling or rolling objects or from objects piercing the sole. Safety of the operator takes priority over ESD control at all times. If protective footwear with reliable ESD properties is not available or ESD foot grounders cannot be worn with the protective footwear in the ESD Protected Area, other personnel grounding devices such as wrist straps should be used. For more information on using wrist straps, check out this post.
Introduction to 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.
Example of ESD Foot Grounder – more info
Guidelines for ESD foot grounders:
- It is recommended that ESD foot grounders are worn on both feet in order to ensure that a continuous path to ground is maintained at all times (even when lifting one foot).
- Contact strips should be tucked inside the shoe with as much contact area as possible to the bottom of the stockinged foot. ESD foot grounders rely upon the perspiration layer inside of the shoe to make contact through the stocking.
- A current limiting one or two megohm resistor in series with the contact strip is recommended but not required.
- ESD foot grounders should be tested independently at least daily while being worn.
Advantages of ESD Foot Grounders
ESD foot grounders are often preferred over shoes because one size fits many foot sizes, thereby reducing stock holdings and simplifying operations.
ESD foot grounders also usually pass the mandatory resistance test as soon as worn, whereas some ESD shoes require a ‘warm-up period’ in order for the operator’s RG to drop below 35 megohms.
They are easily replaceable, quick to put on and less bulky than ESD shoes. They can also be easily taken off when leaving an EPA.
Disadvantages of ESD Foot Grounders
A common complaint with ESD foot grounders is that they don’t last very long. However, there are a few simple tricks to avoid a quick ‘burn-out’:
- The useful life of an ESD foot grounder will depend a lot on the floor and its surface roughness: the rougher the floor the greater the wear. We recommend ESD foot grounders only to be used indoors where floors are usually smoother (and where the ESD foot grounder is less likely to become wet, thereby short circuiting the resistor).
- The manner in which the wearer walks can also affect the life span of the grounder.
In summary, with reasonable care and if used only indoors, ESD heel and toe grounders can last several weeks.
Introduction to ESD Shoes
Conductive additives are blended into the sole (inside to outside) of ESD shoes and connect to the operator’s feet. ESD shoes provide an electrically conductive path from the wearer to the floor – from the operator’s socks (through the sweat layer), to the insole and then to the other outer sole.
Examples of ESD Shoes
There are a number of considerations when selecting ESD shoes:
- Does the ESD shoe meet the ESD Association (ESDA) standards?
Many manufactures of ESD shoes often reference ASTM standards for their ESD specification but state nothing about ESDA standards. The ESDA standards are written specifically for electronics manufacturing and handling. The walking test defined in the ANSI/ESD STM97.2-2006 is one of the most important methods for qualifying ESD shoes for use in ESD Protected Areas.
- Not all ESD shoes is created equal.
There are different styles of ESD shoes. In most cases the specifications of each style will vary. While one style of ESD shoes may retain its ESD properties for 6 months or longer, another will start failing within 90 days. The performance of all styles of ESD shoes should be verified at least daily on an on-going basis and records should be kept for quality control purposes.
Advantages of ESD Shoes
The major advantage of ESD shoes is that they do not require a tab to connect to the operator. There won’t be any issues with the tab not staying inside the shoe – as soon as ESD shoes are put on, the operator is grounded.
ESD shoes are unlikely to be put on incorrectly and have a lower chance of breaking compared to ESD foot grounders. They are generally more reliable and durable.
Disadvantages of ESD Shoes
A big drawback with ESD shoes is obviously the larger initial investment cost. Especially, if you have a large number of operators working in your EPA, it will be costlier to equip everyone with ESD shoes.
Now you know all the ins-and-outs of ESD foot grounders and ESD shoes, you’re probably more confused than ever. So what should you go for? Well, as we said right at the beginning of this post: it depends! It depends, e.g.
- On your budget: are you prepared to initially invest a larger amount of money or would you prefer to spread the costs evenly over time? Also consider the cost over a longer period of time (i.e. 2 or 5 years). Whilst ESD shoes are more expensive initially, ESD foot grounders have to be replaced regularly which adds up, as well.
- On your operators: When selecting your ESD grounding device, it is a good idea to consider the opinion of the operators. They may not find the style of ESD shoe being considered to be comfortable or they may become frustrated that the ESD foot grounder that has been selected does not stay secured properly. In some facilities, many operators are temporary or on a flexible schedule that would not justify certain types of ESD footwear and it is never recommended that operators share footwear due to hygiene issues.
So have a hard look at the numbers and an honest conversation with your employees and then take it from there!
So we’ve talked about various components in an EPA before (e.g. wrist straps, bags, tape and so on). But there is one important part we have not covered yet: ESD protective working surfaces. So let’s change that right now!
Definition of an ESD protective working surface
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.
Types of ESD protective working surface
When deciding to invest in ESD protective working surface, you have the choice of ESD matting (laid-out on a standard non-ESD bench) or ESD benches. Performance-wise there is no difference so what option you go for depends solely on your personal preference.
Examples of ESD protective working surface matting – for more details click here
Generally speaking, ESD matting offers a lower initial investment and is easier to replace. On the other hand, some people prefer the robust and consistent approach of ESD benches.
Grounding your ESD protective working surface
Whatever your choice, your ESD protective working surface needs to be grounded.
A ground wire from the surface should connect to the common point ground which is connected to ground, preferably equipment ground. For electronics manufacturing a working surface resistance to ground (Rg) of 1 x 104 to less than 1 x 109 ohms is recommended. 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.
Example of an ESD Protective Working Surface – click here for more grounding products
Using a current limiting resistor in the ground cord is the user’s choice. However, the resistor is not for ESD control purposes. The ESD Association standard for grounding is ANSI/ESD S6.1 which recommends a hard ground (no resistor) but allows the use of a current limiting resistor in the mat’s ground cord. “The grounding conductors (wires) from wrist straps, working surfaces, flooring or floor mats, tools, fixtures, storage units, carts, chairs, garments and other ESD technical elements may or may not contain added resistance. Where added resistance is not present, a direct connection from the ESD technical element to the common point ground or common connection point is acceptable and recommended.
Note: Manufacturers may add resistance to the grounding conductors for purposes other than ESD (e.g. current limiting). Added resistance is acceptable for the purposes of controlling ESD provided electrostatic accumulation does not exceed specific EPA requirements. The typical added resistance in grounding conductors is 1 megohm, although other values may be specified.” [ANSI/ESD S6.1 section 5.3.3 ESD Technical Element Conductors]
Working at an ESD protective working surface
Operators should ensure that the working surface is organised to perform work and that all unnecessary insulators and personal items are removed. Regular plastics, polystyrene foam drink cups and packaging materials etc. are typically high charging and have no place at an ESD protective workstation.
An operator installing an ESD protective working surface mat
Insulators can be a considerable threat to your products. Remember that an insulator cannot be grounded so it will retain its charge for a long time. Removing all non-essential insulators from the ESD protective workstation is an important rule. If not, your company’s investment in the grounded ESD working surface may be wasted.
“The biggest threat is Field Induced Discharges, which can occur even at a properly grounded ESD working surface. If an ESDS is grounded in the presence of an ElectroStatic charge, instead of the ESDS having charges removed from it, the ESDS may become charged with a voltage induced on it. Then, when placed on the grounded ESD work surface, a discharge occurs. If the ESDS is removed from the presence of the ElectroStatic charge and grounded again, a second discharge may occur.“ (Ref. ESD Handbook, ESD TR20.20, section 2.7.5).
Maintaining your ESD protective working surface
The ESD working surface must be maintained and should be cleaned with an ESD cleaner. Regular cleaners typically contain silicone and should never be used on an ESD working surface.
Example of an ESD cleaner – click here for more information
The ESD control plan should require testing of the resistance to ground periodically. For more information on testing your ESD working surface, check this post. However, the operator should be on guard every day and check visually that the ground wire is attached.
So you’ve identified ESD sensitive items in your factory and you realise that you need to implement ESD Control measures. But where do you start? There is so much information out there and it can be completely overwhelming. Don’t panic – today’s blog post will provide you with a step-by-step guide on how to set-up a suitable ESD Control Plan.
“The Organization shall prepare an ESD Control Program Plan that addresses each of the requirements of the Program. Those requirements include:
• compliance verification
• grounding / equipotential bonding systems
• personnel grounding
• EPA requirements
• packaging systems
• marking” [EN 61340-5-1 Edition 1.0 2007-08 clause 5.2.1 ESD control program plan]
“Each company has different processes, and so will require a different blend of ESD prevention measures for an optimum ESD control program. It is vital that these measures are selected, based on technical necessity and carefully documented in an ESD control program plan, so that all concerned can be sure of the program requirements.” [EN 61340-5-1 Edition 1.0 2007-08 Introduction]
1. Define what you are trying to protect
A prerequisite of ESD control is the accurate and consistent identification of ESD susceptible items. Some companies assume that all electronic components are ESD susceptible. However, others write their ESD control plan based on the device and item susceptibility or withstand voltage of the most sensitive components used in the facility. A general rule is to treat any device or component that is received in ESD packaging as an ESD susceptible item.
An operator handling an ESD susceptible item
2. Become familiar with the industry standards for ESD control
A copy of EN 61340-5-1:2007 can be purchased from British Standards: “BS EN 61340-5-1:2007 applies to 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). BS EN 61340-5-1 provides the requirements for an ESD control program. The user should refer to IEC 61340-5-2 for guidance on the implementation of this standard.
3. Select a grounding / equipotential bonding system
The elimination of differences in potential “can be achieved in three different ways:
- grounding using protective earth:
the first and preferred ESD ground is protective earth if available. In this case, the ESD control elements and grounded personnel are connected to protective earth;
- grounding using functional ground:
the second acceptable ESD ground is achieved through the use of a functional ground. This conductor can be a ground rod or stake that is used for grounding the ESD control items in use at a facility. In order to eliminate differences in potential between protective earth and the functional ground system it is highly recommended that the two systems be electrically bonded together;
- equipotential bonding:
in the event that a ground facility is not available, ESD protection can be achieved by connecting all of the ESD control items together at a common connection point.” [EN 61340-5-1 Edition 1.0 2007-08 clause 5.3.1 Grounding/equipotential bonding systems]
Example of a grounding/equipotential bonding system
4. Determine the grounding method for operators (Personnel Grounding)
The two options for grounding an operator are:
- a wrist strap or
- foot grounders/footwear.
In some cases, both (wrist strap and foot grounders) will be used.
5. Establish and identify your ESD Protected Area (EPA)
ESD Control Plans must evolve to keep pace with costs, device sensitivities and the way devices are manufactured. Define the departments and areas to be considered part of the ESD Protected Area. Consider if customers and/or subcontractors should be included. Implement access control devices, signs and floor marking tape to identify and control access to the ESD Protected Area.
Example of an ESD Protected Area including signs and floor marking tape
6. Select ESD control items to be used in the EPA based on your manufacturing process
Elements that should be considered include: working surfaces, flooring, seating, ionisation, shelving, mobile equipment (carts) and garments. Check-out this post for more information.
7. Develop a Packaging (Materials Handling & Storage) Plan
When moving ESD susceptible devices outside an ESD protected area, it is necessary for the product to be packaged in an enclosed ESD Shielding Packaging.
8. Use proper markings for ESD susceptible items, system or packaging
From EN 61340-5-1 Edition 1.0 2007-08 clause 5.2.1: “The Organization shall prepare an ESD Control Program Plan that addresses each of the requirements of the Program. Those requirements include: …marking”.
If you are handling ESD sensitive devices, there are 3 symbols you need to know.
The ESD Susceptibility (left) and ESD Protective Symbol (right)
9. Implement a Compliance Verification Plan
Developing and implementing an ESD control programme is only the first step. The second step is to continually review, verify, analyse, evaluate and improve your ESD programme.
“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 must occur. The compliance verification plan must document the test methods used for process monitoring and measurements… Compliance verification records shall be established and maintained to provide evidence of conformity to the technical requirements. The test equipment selected shall be capable of making the measurements defined in the compliance verification plan.” [EN 61340-5-1 Edition 1 2007-08 clause 5.2.3 Compliance verification plan]
Regular programme compliance verification and auditing is a key part of a successful ESD control programme.
10. Develop a Training Plan
“The training plan shall define all personnel that are required to have ESD awareness and prevention training. At a minimum, initial and recurrent ESD awareness and prevention training shall be provided to all personnel who handle or otherwise come into contact with any ESDS [ESD sensitive] items. Initial training shall be provided before personnel handle ESD sensitive devices. The type and frequency of ESD training for personnel shall be defined in the training plan. The training plan shall include a requirement for maintaining employee training records and shall document where the records are stored. Training methods and the use of specific techniques are at the organization’s discretion. The training plan shall include methods used by the organization to ensure trainee comprehension and training adequacy.” [EN 61340-5-1 Edition 1.0 2007-08 clause 5.2.2 Training Plan]
11. Make the ESD Control Plan part of your internal quality system requirements
A written ESD Control Plan provides the “rules and regulations”, the technical requirements for your ESD Control Programme. This should be a controlled document, approved by upper management initially and over time when revisions are made. The written plan should include following:
- Qualified Products List (QPL): a list of EPA ESD control items is used in the ESD control Plan
- Compliance Verification Plan: includes periodic checking of EPA ESD control items and calibration of test equipment per manufacturer and industry recommendations.
- Training Plan: an ESD Programme is only as good as the use of the products by personnel. When personnel understand the concepts of ESD, the importance to the company of the ESD control programme and the proper use of ESD products, they will implement a better ESD control programme improving quality, productivity and reliability.
A video making the rounds on Social Media this week highlights a very common problem in ESD Protection: dry air.
The report shows energy minister Simon Bridges walking around his office spraying the floor with water, applying tape to door handles and using an antistatic mat at his desk. The reason? Simple: a new carpet which was installed at the Beehive and the “little tickles” ministers are getting as a result.
Simon Bridges spraying the floor in his office with water (Source)
Jim Robb who is in charge of maintaining the parliament building provides the following explanation: “We’re in the middle of winter, we’ve got cold polar air, there’s no moisture in it and it’s a very common problem, I’ve been dealing with this for thirty years.”
While the ministers at the Beehive seem to have figured out solutions to avoid the “zap”, we want to show you how you can solve the issue of dry air in your EPA.
Impact of relative humidity on ESD
Just like the ministers in the above report, many people will notice a difference in the ability to generate static electricity when the air gets dryer (the % RH decreases).
Relative humidity (RH) directly affects the ability of a surface to store an electrostatic charge. As RH increases, the time a surface will hold a charge will decrease and the dissipation rate will increase.
An example: walking across a carpet can yield a charge of 35kV at 10% RH (very dry air) but will drop significantly to 7.5kV at 55% RH. In an electronics manufacturing environment lower humidity may result in lower output from production due to an increase in ESD events during manufacturing processes.
A normal range for humidity in an electronics manufacturing environment is be between 30% RH and 70% RH. Some facilities try to maintain a constant moderate RH (~50%), whereas other environments may want lower % RH due to corrosion susceptibility to humidity sensitive parts.
The recommended humidity range is usually determined by the specifications of the devices and components being assembled. Increasing the humidity in an electronics manufacturing facility can help to reduce ESD events but increased humidity can lead to other unwanted quality issues in sn electronics manufacturing environment such as corrosion, soldering defects and the popcorn effect on moisture sensitive devices.
The popcorn effect (Source)
Dealing with dry air in an EPA
While not a replacement for grounding, shielding or ESD working surfaces, ionisation can mitigate ESD events in areas where dry air is normal. For more information on ionisation and the different types of ionisers, we recommend reading these two posts:
In summary, worksurface ionisers (Bench Top and Overhead Ionisers) produce positively and negatively charged ions that are moved to the controlled area with fan driven airflow. Point-of-use air ionisers use compressed gas to combat electrostatic attraction neutralising charges on particles causing contamination or visual defects on products.
Using a point-of-use air ioniser
The ESD Standard on Relative Humidity
The ESDA’s TR20.20-2008 discusses RH in a few different places. Some significant statements are listed below.
ESD Handbook ESD TR20.20-2008 section 2.3 Nature of Static Electricity:
“The moisture level in the air, or relative humidity in the environment, are important considerations in the liberation and accumulation of static electricity. It is well known that static electricity in the form of static cling and static shocks are more prevalent when the air is dry. Heating interior air in the winter months dries out the already dry air in the higher latitudes. Static charge accumulation is easier on dry materials since moisture on surfaces tends to allow charges to slowly dissipate or recombine.
However, it is impractical to use humidity control alone to provide static control since static charges are developed even at relative humidity levels of 90% and greater. For most situations, 30 to 70% RH is considered the appropriate range. Special areas, such as wafer fabrication, may require lower humidity control for processes that are affected by moisture (e.g., photoresist application). Soldering is known to be affected by high relative humidity conditions (>70%). For areas that have low ambient humidity, ionization is an important consideration to aid in reducing charge accumulation levels and provide neutralization of charges after they are developed but before they can cause difficulties.”
ESD Handbook ESD TR20.20-2008 section 5.3.16 Humidity:
“Humidity is beneficial in all ESD control program plans. Contact and separation of dry materials generates greater electrostatic charges than moist materials because moisture provides conductivity that helps to dissipate charge. For this reason, ESD effects are most noticeable in the winter since heating systems reduce building environment moisture. Geographic location (desert vs. coastland) is also a major contributor to ambient conditions inside buildings. Any circumstance that results in a low relative humidity will permit a greater accumulation of electrostatic charges. Relative humidity above 30% in ESD protective areas is desirable as long as other adverse conditions are not created as a result of humidity levels. Generally speaking an upper limit of 70% is desirable to prevent corrosive effects on the metal portions of electronic devices and assemblies.
Besides the increasing propensity to generate electrostatic charges on dry materials in general, performance of many ESD protective materials degrade. In fact, when exposed to low humidity conditions, some ESD protective materials become totally ineffective or become sources of electrostatic charges. Therefore, evaluation of ESD control materials should include performance testing in controlled environments at the lowest expected operating relative humidity level. Manufacturers of ESD protective materials should be able to provide performance data in regards to relative humidity. Likewise, materials should be tested in moderate humidity conditions as well to ensure they do not become “too conductive” and present a potential safety hazard to personnel working with substantial voltages. See the Personnel Safety section of this handbook for further guidance in this area.
Humidity control in factories or physically large areas or buildings can be difficult and expensive. In smaller rooms or areas, it may be possible to use portable humidifiers to raise the immediate area humidity. However, in large facilities and factories the environmental systems many need to include steam generation and monitoring equipment to control humidity. This type of equipment is expensive to install and purchase especially in pre-existing facilities. To reduce the total cost impact, companies should consider the need for humidification equipment when planning new facility construction.”
ESD TR20.20-2008 can be purchased directly from the ESD Association.
We have mentioned the term “Faraday Cage” many times on this blog before – for example when talking about the transport and storage of ESD sensitive items or the role of ESD lab coats in ESD Protected Areas. When discussing ESD protection, the concept of the “Faraday Cage” will always come into play. But what exactly is it? Read on to find out…
A Faraday Cage or Faraday shield is an enclosure formed by conducting material or by a mesh of conductive material. Such an enclosure blocks external static and non-static electric fields. Faraday Cages are named after the English scientist Michael Faraday, who invented them in 1836.
An impressive demonstration of the Faraday Cage effect is that of an aircraft being struck by lightning. This happens frequently but does not harm the plane or passengers. The metal body of the aircraft protects the interior. For the same reason, a car may be a safe place during a thunderstorm.
Lightning striking an airplane
In ESD Protection, the Faraday Cage effect causes charges to be conducted around the outside surface of the conductor. Since similar charges repel, charges will rest on the exterior and ESD sensitive items on the inside will be ‘safe’.
ESD control products that provide a Faraday Cage or shielding include Statshield® Metal-In and Metal-Out Shielding Bags or Protektive Pak™ impregnated corrugated boxes with shielding layer when using a lid.
ESD shielding packaging is to be used particularly when transporting or storing ESD sensitive items outside an ESD Protected Area. Per Packaging Standard EN 61340-5-3 clause 5.3 Outside an EPA “Transportation of sensitive products outside of an EPA shall require packaging that provides both:
– dissipative or conductive materials for intimate contact;
– a structure that provides electrostatic discharge shielding.“
ESD Smocks create a Faraday Cage effect around the torso and arms of the operator and shields charges from the operator’s clothing from damaging ESD sensitive devices. (Technically, suppressing the electrical field from clothing worn underneath).
There are standard tests measuring the energy penetration of electrostatic discharges to the interior. The Shielding test method per Packaging standard EN 61340-5-3 is ANSI/ESD STM11.31 and the required limit is less than 50 nanoJoules of energy.
Definitions from the ESD Association Glossary ESD ADV1.0 include: Faraday Cage “A conductive enclosure that attenuates a stationary electrostatic field.”
Electrostatic discharge (ESD) shield “A barrier or enclosure that limits the passage of current and attenuates an electromagnetic field resulting from an electrostatic discharge.”
Electrostatic shield “A barrier or enclosure that limits the penetration of an electrostatic field.”
We’re talking about Electrostatic Discharge (ESD) on this blog all the time. But what exactly does it mean and why is it so dangerous? Today’s post will answer those questions!
All matter is constructed from atoms. These atoms have negatively charged electrons circling the atom’s nucleus which includes positively charged protons. As the atom has an equal number of electrons and protons, it balances out having no charge. So far, so good!
The problem is that all materials can tribocharge or generate ElectroStatic charges. Most commonly, this happens through contact and separation – examples are:
- 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.
Unwinding a roll of tap can generate an electrostatic charge
The simple separation of two surfaces can cause the transfer of electrons between surfaces resulting in one surface being positively and the other one negatively charged. With that we’ve just generated an ElectroStatic charge!
The amount generated varies and is affected by materials, friction, area of contact and the relative humidity of the environment. At lower relative humidity, charge generation will increase as the environment is drier. Common plastics generally create the greatest static charges.
ELECTROSTATIC DISCHARGE (ESD)
If two items are at the same electrostatic charge or equipotential, no discharge will occur.
However, if two items are at different levels of ElectroStatic charge (i.e. one is positively and the other one negatively charged), they will want to come into balance. If they are in close enough proximity, there can be a rapid, spontaneous transfer of electrostatic charge. This is called discharge or ElectroStatic Discharge (ESD). Examples in daily life:
- Lightning, creating lots of heat and light
- The occasional zap felt when reaching for a door knob
- The occasional zap felt when sliding out of a car and touching the door handle
Have you felt the zap before?
In a normal environment like your home, there are innumerable ESD events occurring, most of which you do not see or feel. It takes a discharge of about 2,000 volts for a person to feel the “zap”. It requires a much larger ESD event to arc and be seen (e.g. lightning). While a discharge may be a nuisance in the home, ESD is the hidden enemy in a high tech manufacturing environment. Modern electronic circuitry can be literally burned or melted from these miniature lightning bolts. ESD control is therefore necessary to reduce and limit these ESD events.
TYPES OF ESD DEVICE DAMAGE
ESD damage to electronic components can lead to:
- Catastrophic Failures
- Latent Defects
Catastrophic failure causes a failure in an ESD sensitive item that is permanent. The ESD event may have caused a metal melt, junction breakdown or oxide failure. Normal inspection is able to detect a catastrophic failure.
A latent defect can occur when an ESD sensitive item is exposed to an ESD event and is partially degraded. It may continue to perform its intended function, so may not be detected by normal inspection. However, intermittent or permanent failures may occur at a later time.
COSTLY EFFECTS OF ESD
A catastrophic failure of an electronic component can be the least costly type of ESD damage as it may be detected and repaired at an early manufacturing stage.
Latent damage caused by ESD is potentially costlier since damage occurs that cannot be felt, seen or detected through normal inspection procedures. Latent defects can be very expensive as the product passes all inspection steps and the product is completed and shipped. Latent defects can severely impact the reputation of a company’s product. Intermittent failures after shipping a product can be frustrating, particularly when the customer returns a product, reporting a problem which the factory again fails to detect. It consequently passes inspection and the product is returned to the customer with the problem unresolved.
The worst event is when the product is installed in a customer’s system, and performs for a while and then performs erratically. It can be very expensive to troubleshoot and provide repairs in this situation.
One study indicated the cost to be:
- £7 Device
- £7 Device in board – £700
- £7 Device in board and in system – £7,000
- £7 Device and system fails – £70,000
Industry experts have estimated average electronics product losses due to static discharge to range from 8 to 33%. Others estimate the actual cost of ESD damage to the electronics industry as running into the billions of dollars annually.
It is critical to be aware of the most sensitive items being handled in your factory. As electronic technology advances, electronic circuitry gets progressively smaller. As the size of components is reduced, so is the microscopic spacing of insulators and circuits within them, increasing their sensitivity to ESD. As you can predict, the need for proper ESD protection increases every day.
If you’re new to ESD and ESD Control, we suggest you read this article for more information on how to protect your ESD sensitive devices.
Last time we talked about what to look out for when using containers to transport or store ESD sensitive items. Have you implemented our 3 tips yet?
Today we thought we’d cover a topic that ties in nicely with last week’s post: Protektive Pak® Impregnated Corrugated Material. Never heard of it? Don’t panic – we’re here to help! Protektive Pak® Material is made from static dissipative impregnated corrugated material with a buried shielding layer – it provides static shielding to protect ESD sensitive items from ElectroStatic charges, and ElectroStatic Discharges [ESD].
Introduction to Protektive Pak® Material
So now you’re probably wondering what’s different about this type of material – loads of companies out there offer similar products, right? That’s true – BUT what makes Protektive Pak® Material so unique is that its ESD properties are manufactured into the liners of the material itself. Many other materials have a coating or paint applied that gives them their ESD properties. This in itself is not a problem. However, it becomes an issue if the outer layer of your ESD container is damaged.
Protektive Pak® Inplant Handler – for more information click here
Have you ever removed tape from your ESD container or accidently pierced the surface with a sharp object? If you have, chances are you’ve found the black coat give way to a lighter brown material. That’s your ESD properties gone potentially damaging your ESD sensitive devices inside the ESD container. This will not happen with Protektive Pak® Material – even if the outer layer is damaged, your ESD sensitive items are still protected. Not convinced? Check-out this video.
Protektive Pak® Circuit Board Shippers – for more information click here
4 Reasons why you should be using Protektive Pak® Material
Independent ESD tests have proven that Protektive Pak® Impregnated Corrugated Material is superior! Click here to see the full test report. The bottom line is:
- Protektive Pak® impregnated corrugated material has a buried shielding layer.
- Protektive Pak® impregnated corrugated material equals or exceeds the discharge shielding capabilities of a coated box.
- Protektive Pak® impregnated corrugated material has discharge shielding capabilities equal to a metal-out shielding bag.
- Protektive Pak® impregnated corrugated material meets the ANSI/ESD S541 recommendation, avoiding rapid discharge when contacting ESD sensitive items – coated boxes DO NOT.
Comparing Impregnated Corrugated Protektive Pak® and Coated Materials
Now that we have talked about the advantages of Protektive Pak® Material – how exactly does it compare to the more common coated materials out there on the market? The below table provides a summary:
|Impregnated vs. Coated Material|
Manufactured by one paper mill with computerized control, resulting in consistent high quality.
|Manufactured without computer controls and applied at various geographical locations, resulting in quality variations.|
Carbon is added during the paper making process. The paper is a 6-layer process. The top surface layer is static dissipative, measuring 107 to 109 ohms. The conductive layer is in the 5th layer from the surface measuring <104 ohms.
|Material is coated or printed with carbon loaded black ink which is then coated with a clear sealer to help coating stay on. Shielding layer is very close to surface and high carbon content can bleed through. Result is very poor and inconsistent static dissipative effectiveness.|
|3||LOWER SULPHUR CONTENT
Manufactured from 100% recycled paper with consistently low sulphur content.
|Manufactured from either recycled or virgin paper or a combination of both. sulphur content may be low or high which can cause corrosion to leads and circuits.|
1,000 Times Thicker: Abrasion tests have shown no loss in particles at 100 cycles, only 1% loss for 200 cycles and 60% loss for 500 cycles.
|Tests have shown a 50% loss in particles in only 10 cycles and a 100% loss in 100 cycles.|
|5||SLOWS RAPID DISCHARGE
Burying the conductive layer under a dissipative surface reduces the potential for a rapid discharge when contacted by a charged device.
|A very conductive surface that may pose a charged device model (CDM) ESD danger to components stored in open bin boxes, in-plant handlers, shippers, totes, nesting trays, etc.|
|6||BETTER SHIELDING EFFECTIVENESS
Shielding effectiveness is equal to or greater than coated conductive materials.
|Some coated products shield poorly due to inconsistent application procedures by some manufacturers.|
More durable structure, 1,000 times thicker, which consistently shields your product from ESD, is also safer and better for the environment.
|Simple structure which can lack consistency of ESD shielding, durability and safety.|
All microscopic photos are approximately the same scale. A PDF version of the above table is available here.
Do you use containers to store or transport ESD sensitive items? If so, make sure to read on! We’ve compiled a list of 3 tips you should follow to make sure your ESD sensitive devices are fully protected. So let’s get started:
1. Use shielded containers!
ElectroStatic Discharge (ESD) is silent, quick and potentially lethal to electronic parts. When electronic parts are not properly handled during manufacturing, assembly, storage or shipping, damage from ESD can reach into the millions of dollars each year.
For an ESD control container to be effective and meet EN 61340-5-1 Edition 2.0 2016-05, the requirements are:
- Surface resistance 1 x 104 to < 1 x 1011 ohms per IEC 61340-2-3
- Discharge Shielding (energy penetration) < 50 nanoJoules per IEC 61340-4-8
We know what you’re saying now: “But non-shielding containers are so much less expensive than ESD shielding containers.” Unfortunately, it’s not as simple as that.
Non-shielding containers might be cheaper, but they are not less costly when it comes to handling ESD sensitive items. Anytime ESD sensitive parts and assemblies are handled, regular containers are not a sound option, even part of the time, as the risk of ESD damage is always lingering. As a result, costs will be incurred, either via ESD damage or as an additional investment in discharge shielding packaging and material handling containers.
The disadvantages of cross-using shielding and non-shielding containers include:
- Increased cost
- Risk from ESD damage
- Handling inconvenience
The cost of a discharge shielding container is far less than the cost associated with damaged parts or extra handling that result with a “less expensive” non-shielding container. So the bottom line is: ALWAYS go for shielded containers!
2. Put a lid on it!
A Faraday Cage effect can protect ESDS contents in a container with a shielding layer (this is what a shielding bag has). This Faraday Cage effect protects people in real life when a lightning bolt strikes an airplane or automobile with the charge residing on the outer metal fuselage or car body.
The Faraday cage effect causes charges to be conducted around the outside surface of the conductor. Since similar charges repel, charges will rest on the exterior.
To complete the enclosure, make sure to place lids on boxes or containers. Packaging with holes, tears, or gaps should not be used as the contents may be able to extend outside the enclosure and lose their shielding as well as mechanical protection.
|Outside an EPA||Inside an EPA|
Outside the ESD protected area (EPA), the lid needs to be in place to provide the ESD control property electrostatic discharge shielding. Per Packaging Standard EN 61340-5-3 clause 5.3 Outside an EPA: “Transportation of sensitive products outside of an EPA shall require packaging that provides both:
|Inside the EPA, it would still be a good idea to have the lid in place, but it is not a requirement. When using a shielded container, electrostatic charges and discharges take the path of least resistance. Packaging with the discharge shielding property protects ESD sensitive items from the effects of static discharge that are external to the package.|
3. Choose the right foam!
Generally speaking, there are 2 types of foam available with shielded containers: pink static dissipative and black conductive foam. Depending on your application and/or budget you should choose the one best suited for you.
There are resistance differences but the key is that the black foam resistance is inherent and longer lasting:
|Pink Static Dissipative Foam||Black Conductive Foam|
|Static dissipative polyurethane <1011 ohms||Conductive polyurethane 1 x 103 to < 1 x 105 ohms|
|Antistatic low charging – minimising electrostatic charge generation||Permanently conductive|
|Will lose electrical properties over time||Will not lose electrical properties|
|When exposed to the environment, the foam will discolour (turn yellow) over time||Foam will not discolour over time|
|Economical||Higher initial investment; better value for long term applications|
|Ideal for short term use and/or one-time shipments||Ideal for storing or transporting ESDS over a prolong period of time, and reusing the container|
|Not recommend for lead insertion applications||Not recommended in applications where Static Dissipative properties are required|
|Easy to adapt for custom uses by die-cutting, laminating, etc.|
|Non-contaminating, non-corrosive, and non-sloughing|
We hope you found this post helpful and informative – let us know if you have any requests for future blog posts.