Category Archives: Question & Answers
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,
– 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
– 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 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 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.
- Jaakko Paasi, VTT Industrial Systems: “Surface resistance or surface resistivity?”
- David E. Swenson, Affinity Static Control Consulting: “Electrical Resistance and Resistivity”
- ESD Association, Inc.: ESD Fundamentals – Part 3: Basic ESD Control Procedures and Materials
Today’s post is going to be a bit shorter than usual. BUT: that doesn’t mean it’s going to be any less interesting. Quite the opposite! So, let’s jump right in.
A little while ago we were approached by a customer with the following statement:
“Generally speaking, most IC’s these days already have adequate protection on their pins, the notable exception being discrete J-FETs, and MOSFETs, especially for RF applications.
It’s difficult to advise when these might be in use on an assembly without giving everyone in-depth training on circuit design, so to avoid trouble in the 1% of cases that matter, it’s a good idea to play safe and keep applying our procedures for the other 99% of parts too.
I am of the opinion that a PCCU in its housing does not need special treatment though. It has ESD protection, and has passed testing for this, so I am not worried about someone touching its pins without wearing a grounded wristband, etc …“
So, is this statement true? Is ESD Control obsolete? Let’s find out!
Types of ESD Damage
Remember that there are two types of ESD damage:
1) catastrophic failure and
2) latent defects.
While catastrophic failures cause an ESD sensitive item to be damaged permanently, latent defects only partially degrade an ESD sensitive item that is exposed to an ESD event. It may continue to perform its intended function and may not be detected by normal inspection. However, intermittent or permanent failures may occur later.
Bottom line: Even if an ESD sensitive component has quite a high withstand voltage and no catastrophic failure has been caused, latent defects may still make your life miserable.
Continued Requirement for ESD Control
Here is a link to the ESD Association’s ESD Technology Roadmap. The document illustrates what future thresholds are expected for ESD sensitive devices and how they impact on ESD Control. The thresholds are determined by current trends in the semiconductor industry and are displayed as “roadmaps”. The aim is to predict future limitations of device protection which are driven by performance requirements and technology scaling.
You should head-over now and read through the document. But in case you haven’t got time, here are the main take-away notes:
- “Finally, these trends point to the need for continued improvements in ESD control procedures and compliance.” [section 1.0 Synopsis]
- “Therefore, the prevailing trend will be circuit performance at the expense of ESD protection levels.” [section 2.1 Overview]
- “Therefore, implementation of advanced HBM controls using the limits and qualifications requirements in ANSI/ESD S20.20, IEC 61340-5-1, or JESD625, would become necessary within the next 3 years.” [section 2.2 Device ESD Threshold Roadmaps]
Bottom line: 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. Therefore, the need for proper ESD protection increases every day.
ESD control procedures and compliance continue to be required
For more information on the ESD damage and the costly effects of ESD, check-out this post.
Protect your sensitive devices from ESD Damage
Every company should document the most ESD sensitive device that they are handling.
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.
This post provides further information on how to set-up an ESD Control Plan.
So, tell us: are there instances in your company where you forego standard ESD Control practices? If so, let us know in the comments – we’d like to hear from you.
Today’s blog post is a continuation of last week when we covered the basics of wrist straps – what they are, what different styles there are etc. Missed the first post? Catch-up here!
Today we’ll be taking a closer look at queries we receive in connection with using wrist straps. So without further ado, let’s jump right in!
What is the 1 megohm Resistor for?
The purpose of the 1 megohm resistor found in series with wrist straps is solely to provide safety to the human body by limiting the amount of current that could be conducted through the body. The 1 megohm resistor is designed to limit the current to 250 microamps at 250 Volts rms AC. This is just below the perception level (and a bit before the nervous system goes awry) of most people. Physical perception of current traveling in/on the body varies depending on size, weight, water content, skin conditions, etc. Remember that the termination of the coil cord with the 1 megohm resistor must always be connected to the operator.
Can I connect to ground via a bench mat?
Many wrist strap users connect the wrist cord to a stud on their ESD protective mat. This process is not recommended as it can increase the total system resistance to ground to over the 35 megohm limit required by EN 61340-5-1 table 1.
It is recommended to connect a wrist strap to an Earth Bonding Point (EBP)
Can I buy a wireless wrist strap?
Passive “wireless” or “cordless” wrist straps have severe limitations. Assuming you were tribocharged to 10 KV and wearing the “wireless” wrist strap, it would take many hours (days even depending on the ambient relative humidity) to get you below 5 KV, nevertheless 10 Volts. Most (if not all) of the charge reduction would be due to natural recombination of the charges on your skin with the air molecules and the natural conductance of the air through water vapour content.
At this point there is no “cordless” wrist strap system on the market that works at all. In fact “cordless” wrist straps will damage ESDS devices because you will not be grounding your body.
Do I need to wear a wrist strap on both hands?
Good question, since the concept of grounding an individual is not very intuitive. The skin is one of the largest organs of the human body. The resistance of the human body is averaged to 1,500,000 ohms (or 1.5 x 103 ohms) from the ESD-STM5.1 [ESDA Standard for Human Body Model]. The resistance of an individual may vary from 1 kilohm to over 1 megohm. In either case, the skin is conductive in the sense it can conduct electrical current. Therefore, since the skin is (for the most part) continuous, i.e., the ball of your foot is electrically connected to your index finger, then grounding the skin at any point will in fact ground all of the skin. So you can in good conscious say that if you properly wear one grounded wrist strap, then both hands are grounded as well as other exposed skin areas.
Do wrist straps need to be tested?
Yes, wrist straps need to be checked regularly to ensure they are faultless and ground the operator properly. Wrist straps should be worn while they are tested. This provides the best way to test all three components: the wrist band, the ground cord (including the resistor) and the interface with the operator’s skin.
Wrist straps need to be checked before each use. Periodic testing is not required if continuous monitors are used. They provide instant feedback should the wrist strap fail while handling ESD sensitive devices.
Example of a Wrist Strap Tester
What should I do if I fail my wrist strap test?
If the wrist strap tester outputs a FAIL test result, stop working. Test the wrist band and cord individually to find out which item is damaged. There are some methods to troubleshoot your wrist straps. First make sure your tester is properly adjusted and calibrated.
If the operator and wrist strap system fails low:
- Make sure that the person is not directly connected to ground via another path, i.e., touching a grounded metal structure.
- The most common cause of a fail low is a shorted resistor in the wrist strap coil cord. Replace the coil cord with a new one and repeat the test.
If the operator and wrist strap system fails high:
- Make sure the coil cord has a secure connection both the banana jack/socket to tester and the stud snap to wrist strap buckle.
- Ensure there is continuity in the coil cord (you can test with an ohmmeter).
- Remove the wrist strap and hold the bottom part of the band tightly between the operator’s thumb and index finger and test. If the test fails high, the band may be soiled and needs cleaning or the buckle to band connection may be suspect. Either replace the band or clean and then retest.
- If the above test is okay, then the skin of the operator’s wrist may be too dry. Apply ESD lotion to the wrist to re-moisturise the skin thereby increasing its conductivity. Retest. Operators with dryer skin should wear metal banded wrist straps to minimise the contact resistance. If their skin is very dry, application of an ESD lotion may be required as part of their donning process.
You need to obtain a PASS test result before beginning work.
Did we miss any question(s)? Let us know in the comments!
In the past we spoke about some types of ESD bags (antistatic bags vs. dissipative bags, moisture barrier bags). Today we want to focus on the whole selection process for ESD bags: from choosing the correct type of material all the way through to determining the right size for your application. Sounds complicated you say? Honestly – it’s not and once you have the right tools (you’re welcome!), you’ll be an expert in no time. So let’s go!
Choose the correct material for your ESD bag
Before you get started, you need to be clear about the purpose of your ESD bag and the environment it’s being used in. Make sure you have the answers to the following questions:
- What do you intend to put inside the ESD bag? ESD sensitive items? Non-ESD sensitive items?
- Is moisture an issue you need to consider?
- Do you need your ESD bags be self-sealable? Or will you be using ESD tape/labels to close your bags?
- Are the items inside your ESD bag sensitive to physical damage?
- Will the ESD bags (and obviously the items inside) be stored on a grounded shelf inside an EPA or are they being transported outside of an EPA, as well?
Once you have answers to ALL of the above questions, you can move on to the below selection chart and choose the right material for your application.
|ESD sensitive items||Moisture protection||Self-sealable||Physical protection||Inside/Outside EPA|
|Pink Antistatic||x||Inside only|
|Pink Antistatic Bubble||x||x||Inside only|
|Conductive Black||Only if used on a grounded surface||Inside only if used with ESD sensitive items|
A few more details on the different types of materials listed in the above chart:
- Metal-In Shielding Bags
ESD bags which protect ESD sensitive items. The ESD shielding limits energy penetration from electrostatic charges and discharge. They offer good see-through clarity. Available with and without dissipative zipper.
Metal-In Shielding Bags
- Metal-Out Shielding Bags
Integral antistatic and low tribocharging bags which will not electrostatically charge contents during movement. Bags have an aluminum metal outer layer of laminated film. Available with and without dissipative zipper.
Metal-Out Shielding Bags
- Moisture Barrier Bags (MBB)
Offer ESD and moisture protection and can be used to pack SMD reels or trays.
Moisture Barrier Bags (MBB)
- Bubble Shielding Bags
These bags combine the “Faraday Cage” and mechanical protection. They shield about twice as well as normal shielding bags of equivalent size.
Bubble Shielding Bags
- Pink Antistatic Bags
Economical bags which are made of polyethylene. They are for use with non-ESD sensitive items destined for use in an EPA. Available with and without zipper.
Pink Antistatic Bags
- Pink Antistatic Bubble Bags
ESD bags which are made of pink-tinted, amine-free, antistatic polyethylene. They provide good mechanical protection and are for use with non-ESD sensitive items. The 25mm flap has a self-adhesive strip to close the bags.
Pink Antistatic Bubble Bags
- Conductive Black Bags
Black conductive film is made of virgin low density materials with black conductive compound to achieve high toughness and strength.
Conductive Black Bag
Calculating the correct size for your ESD bag
Once you have selected the correct type of material, it’s time to choose the right size for your ESD bag. There are different ways to determine this based on the type of material you use:
- Shielding, Black Conductive and Pink Antistatic Bags
A. Bag Width = Item’s Thickness + Item’s Width + 25mm
B. Bag Length = Item’s Thickness + Item’s Length + 50mm
- Moisture Barrier Bags (MBB)
A. Bag Width = Item’s Thickness + Item’s Width + 25mm
B. Bag Length = Item’s Thickness + Item’s Length + 76mm
- Pink Bubble and Bubble Shielding Bags
A. Bag Width = Item’s Thickness + Item’s Width + 76mm
B. Bag Length = Item’s Thickness + Item’s Length + 76mm
Bonus Tip: Measuring a bag
It might seem obvious to some of you but given that we do get these types of queries on quite a regular basis, we thought this would be a good opportunity to include. Imagine you already have ESD bags that you use in your company. Someone has just taken the last one of the shelf and you need to order some more. How do you know what size ESD bag you have in front of you so you can place a new purchase order? No worries – we have the answer:
A. The width is measured from inside seem to inside seem. This is also your opening.
B. The length is measured from the top of the opening to the bottom of the bag.
Bonus Tip 2: Remember your ESDs items
Outside an ESD protected area, the objective of ESD protective packaging is to prevent a direct electrostatic discharge to the ESD sensitive item contained within and allow for dissipation of charge from the exterior surface. In addition, the packaging should minimize charging of the ESD sensitive item in response to an external electrostatic field and triboelectrification. If the user does not know the sensitivity of the items being used, static shielding packaging should be used.
Do you have any more questions or require a quote for ESD bags? Vermason can help so make sure you get in touch!
We’ve previously published a post that explains when you need ionisation. However, following this post, we got a number of questions that prompted us to dive a bit deeper into the whole subject of ionisers. Basically with this post we’re starting right at the beginning so stay tuned…
Before talking about ionisers in more detail, we need to have a little chat about the types of materials that can be found in an EPA – conductors and insulators:
• Electrical current flows easily
• Can be grounded
Materials that easily transfer electrons (or charge) are called conductors and are said to have “free” electrons. Some examples of conductors are metals, carbon and the human body’s sweat layer. Grounding works effectively to remove electrostatic charges from conductors to ground. However, the item grounded must be conductive.
The other term often used in ESD control is dissipative which is 1 x 104 to less than 1 x 1011 ohms and is sufficiently conductive to remove electrostatic charges when grounded.
When a conductor is charged, the ability to transfer electrons gives it the ability to be grounded.
• Electrical current does not flow easily
• Cannot 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. An insulator 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. Grounding is a very effective ESD control tool; however, only conductors (conductive or dissipative) can be grounded.
Insulators like this plastic cup will hold the charge and cannot be grounded and “conduct” the charge away.
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
“Process essential” Insulators
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 the ESD control programme.
Examples of some common process essential insulators are a PC board substrate, insulative test fixtures and product plastic housings.
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.
Reduction of charges on insulators does occur naturally by a process called neutralisation. Ions are charged particles that are normally present in the air and as opposite charges attract, charges will be neutralised over time.
A common example is a balloon rubbed against clothing and “stuck” on a wall by static charge. The balloon will eventually drop. After a day or so natural ions of the opposite charge that are in the air will be attracted to the balloon and will eventually neutralise the charge. An ioniser greatly speeds up this process.
A balloon “stuck” on a wall by static charge.
What is an ioniser?
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.
An ioniser creates positively and negatively charged ions.
Note: Ionisers require periodic cleaning of emitter pins and the offset voltage must be kept in balance. Otherwise, instead of neutralising charges, if it is producing primarily positive or negative ions, the ioniser will place an electrostatic charge on items that are not grounded.
This citation from the ESD handbook provides an excellent summary:
“The primary method of static charge control is direct connection to ground for conductors, static dissipative materials, and personnel. A complete static control program must also deal with isolated conductors that cannot be grounded, insulating materials (e.g., most common plastics), and moving personnel who cannot use wrist or heel straps or ESD control flooring and footwear. Air ionization is not a replacement for grounding methods. It is one component of a complete static control program.
Ionizers are used when it is not possible to properly ground everything and as backup to other static control methods. In clean rooms, air ionization may be one of the few methods of static control available.” (ESD Handbook ESD TR20.20 Ionization, section 22.214.171.124 Introduction and Purpose / General Information)
Now that you know what conductors and insulators are, how to treat them in an EPA and when to use ionisation, the next step is to learn about the different types of ionisers available. However, as this post is already quite long, we will save that part for next week so stay tuned…. Click here to read the follow-up post.
In previous posts we covered how to ensure your ESD Flooring is working and what the difference is between conductive and dissipative floors. So the obvious question now is: how do I actually create an ESD floor??? Well, no panic – we can help! Vermason has a number of items in the offering that will help you achieve just that: create an ESD floor. And not just that – it’s also incredibly easy!
Unlocks our ESD polymers with the least amount of labour while minimising the pH level needed to rinse and neutralise. The Statguard® Stripper can save one or two labour rinse steps which cuts down labour time and can save you money. Typically 80% of the cost of maintaining a tile floor is labour. Cutting down the labour time is a savings to both the Building Service Contractor and the Customer.
Statguard® Dissipative Floor Finish
Exceeds the required limits of EN 61340-5-1 for Resistance (operator grounding) of < 1.0 x 109 ohms when tested per IEC 61340-4-1 and Operator Charge Generation of < 100 volts when tested per IEC 61340-4-1 when proper ESD footwear is used.
Statguard® Floor Cleaner
Statguard® Dissipative Neutral Floor Cleaner is formulated to clean floors treated with Statguard® Dissipative Floor Finish, as well as other dissipative or conductive floor tiles and polymer type floor finishes. It is formulated with dissipative agents that will rejuvenate and improve the static dissipative properties of the flooring surface that it is used with without leaving behind residue that can impede dissipation or conductive properties of the flooring material or finish.
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. Industry experts estimate that average electronics product losses due to static discharge 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 therefore critical to be aware of the most sensitive items being handled in your factory as the need for proper ESD protection increases every day.
Here are some tips on how to get ahead of the game:
1. Establish and identify an ESD Protected Area (EPA)
An ESD Protected Area (EPA) is a defined space within which all surfaces, objects, people and ESD Sensitive Devices (ESDs) are kept at the same potential:
- All surfaces, products and people are linked to ground.
- Movable items, such as containers and tools, are bonded by virtue of standing on a grounded surface or being held by a grounded operator.
- Everything that does not readily dissipate a charge must be excluded from the EPA.
A good to place to start your fight against ESD is our example of an EPA as it lists all products required in a proper EPA.
Example of an EPA
A few things to remember:
- The ESD protected area should have signage to clearly identify where it is.
- Only trained or escorted people are to be allowed in the EPA.
- All conductors including Personnel must be grounded. Operators must either wear wrist straps or footwear in combination with an ESD floor. ESD worksurfaces (e.g. mats) are to be grounded.
- Wrist straps and footwear are to be tested daily. For wrist straps a conintuous monitor can be used instead.
- Periodic checks of installed products (e.g. ESD worksurfaces, ESD flooring etc.) are required.
- Remove all non-essential insulators or neutralise essential insulators with ionisers.
- Only handle unpackaged ESDs in an EPA when grounded.
- Wristbands are to be worn snug; the grounding tab of foot grounders must be placed under the foot in the shoe; ESD smocks need to cover all clothing on the torso.
- Use packaging with shielding properties to store or transport ESDS outside the EPA.
2. Identify ESD sensitive items (ESDs)
It is critical to be aware of the most sensitive item being handled in your factory. From goods-in through to dispatch packaging shielding properties should be used to protect ESDs during transport and storage. Any ESD sensitive item should be identified with the ESD sensitivity symbol, either on itself or its container. The ESD Sensitivity Symbol (also called Susceptibility or Warning Symbol) identifies items that can be damaged by ESD and should ONLY be unpackaged and handled while grounded at an ESD protected workstation.
3. Provide ESD control training
This is probably the most important point of all. You can have the best EPA in the world but if your staff don’t know about ESD and the problems it creates, it will be money wasted. People handling ESDs are still a major source of ElectroStatic charges and discharges. Operators need training and need to be vigilant that ESD control procedures are followed. In order for an ESD control programme to be effective, operators must be aware of:
- the threat of ESD,
- understand & adhere to the rules of controlling static electricity
- and know how to properly use ESD control items.
Antistatic is an ESD control property properly referred to as “low charging”. This is an important property for all bags used inside or outside of an ESD protected area (EPA). Specifically, it is important that the inside of the bag is low charging. This ensures that when the ESD Sensitive Device is inserted into the bag or otherwise moves, there is minimum electrostatic charge generation.
Dissipative is the electrical property of resistance so when grounded electrostatic charges will be removed to ground. This is an important ESD control property as the bag can be grounded; being dissipative provides an electrical path for an electrostatic charge to dissipate from the bag.
Shielding Bag used in an ESD Protected Area (EPA)
All bags inside the ESD Protected Area [EPA] should be both low charging and dissipative whether packaging ESD sensitive item or non-ESD sensitive items.
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:
a) dissipative or conductive materials for intimate contact;
b) a structure that provides electrostatic discharge shielding.
NOTE 1: If electrostatic field shielding materials are used to provide discharge shielding, a material that provides a barrier to current flow should be used in combination with the electrostatic field shielding material.
NOTE 2 Dissipative materials are preferred for intimate packaging in situations where charged device model (CDM) damage is a concern.”
View our offering of ESD Bags HERE.
Seeing ElectroStatic Discharge (ESD) damage is basically impossible. Damage to semiconductor device structure is NOT visible at ordinary magnifications of an optical microscope. If the microscope is capable of 1000X-1500X magnifications, you just might be able to “see” something. The method used, only occasionally as there is considerable expense, is by delayering and etch enhancement producing high magnification photographs using a scanning electron micrograph (SEM). See Images of ESD Damage, photos of Human Body Model (HBM) ESD damage provided by Hi-Rel Laboratories, Inc. at 6116 N Freya, Spokane, Washington 99217 (509-325-5800 or www.hrlabs.com). Used with their permission.
To have an ESD control programme conform to EN 61340-5-1 does the programme have to use all the ESD protected area ESD control items listed in Table 3? These are: working surfaces, storage racks, trolleys, flooring, ionization, seating, and garments.
No, you can decide which ESD control items to use.
Per EN 61340-5-1 clause 5.2.1 “ESD control program plan, The organization shall prepare an ESD control program plan that addresses each of the requirements of the program. Those requirements concern:
- compliance verification,
- grounding/bonding systems,
- personnel grounding,
- EPA requirements,
- packaging systems,
Each company has flexibility designing its programme as EN 61340-5-1 Introduction states: “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.”
The ESD control programme plan is to be written. We recommend starting by reviewing Annex A of User guide CLC/TR 61340-5-2:2008 “Example ESD Control Document based on IEC 61340-5-1”. It notes “The following document demonstrates the flow and required sections for an ESD control program as defined by IEC 61340-5-1. This program is based on one of the most basic ESD programs that can be implemented. In most cases, an actual ESD program will utilize more ESD control elements. Personnel are grounded by a wrist strap. Handling operations are performed at a grounded worksurface and ESD sensitive devices are moved from operation to operation inside a metallized shielding bag.”
To ask an ESD Question Click Here