Category Archives: Working Surface Mats and Accessories
We have learnt in a previous post that within an ESD Protected Area (EPA) all surfaces, objects, people and ESD Sensitive Devices (ESDs) are kept at the same electrical potential. We achieve this by using only ‘groundable’ materials. But what do you do if you absolutely need an item in your EPA and it cannot be grounded? Don’t sweat, not all hope is lost! There are a couple of options which will allow you to use the item in question. Let us explain…
Conductors and Insulators
In ESD Control, we differentiate conductors and insulators.
Materials that easily transfer electrons are called conductors. Some examples of conductors are metals, carbon and the human body’s sweat layer.
A charged conductor can transfer electrons which allows it to be grounded
Materials that do not easily transfer electrons are called insulators and are by definition non-conductors. Some well-known insulators are common plastics and glass.
Insulators will hold the charge and cannot be grounded and “conduct” the charge away
Both, conductors and insulators, may become charged with static electricity and discharge.
Electrostatic charges can effectively be removed from conductors by grounding them. However, the item grounded must be conductive or dissipative. An insulator on the other hand, will hold the charge and cannot be grounded and “conduct” the charge away.
Conductors and Insulators in an EPA
The first two fundamental principles of ESD Control are:
- Ground all conductors including people.
- Remove all insulators.
To achieve #1, all surfaces, products and people are bonded to Ground. Bonding means linking, usually through a resistance of between 1 and 10 megohms. Wrist straps and work surface mats are some of the most common devices used to remove static charges. Wrist straps drain charges from operators and a properly grounded mat will provide path-to-ground for exposed ESD susceptible devices. Movable items (such as containers and tools) are bonded by virtue of standing on a bonded surface or being held by a bonded person.
However, what if the static charge in question is on something that cannot be grounded, i.e. an insulator? Then #2 of our ESD Control principles will kick in. Per the ESD Standard, “All non-essential insulators and items (plastics and paper), such as coffee cups, food wrappers and personal items shall be removed from the workstation or any operation where unprotected ESDS are handled.
The ESD threat associated with process essential insulators or electrostatic field sources shall be evaluated to ensure that:
- the electrostatic field at the position where the ESDS are handled shall not exceed 5 000 V/m;
- if the electrostatic potential measured at the surface of the process required insulator exceeds 2 000 V, the item shall be kept a minimum of 30 cm from the ESDS; and
- if the electrostatic potential measured at the surface of the process required insulator exceeds 125 V, the item shall be kept a minimum of 2,5 cm from the ESDS.”
[IEC 61340-5-1:2016 clause 184.108.40.206 Insulators]
Always keep insulators a minimum of 31cm from ESDS items
Well, we all know that nothing in life is black and white. It would be easy to just follow the above ‘rules’ and Bob’s your uncle – but unfortunately that’s not always possible. There are situations where said insulator is an item used at the workstation such as a hand tools. They are essential – you cannot just throw them out of the EPA. If you do, the job won’t get done.
So, the question is – how do you ‘remove’ these vital insulators without actually ‘removing’ them from your EPA? There are 2 options you should try first:
1. Replace regular insulative items with an ESD protective version
There are numerous tools and accessories available that are ESD safe – from document handling to cups & dispensers and brushes and waste bins. They are either conductive or dissipative and replace the standard insulative varieties that are generally used at a workbench. For more information on using ESD safe tools and accessories, check this post.
2. Periodically apply a coat of Topical Antistat
The Reztore® Topical Antistat (or similar solution) is for use on non-ESD surfaces. After it has been applied and the surface dries, an antistatic and protective static dissipative coating is left behind. The static dissipative coating will allow charges to drain off when grounded. The antistatic properties will reduce triboelectric voltage to under 200 volts. It therefore gives non-ESD surfaces electrical properties until the hard coat is worn away.
If these two options are not feasible for your application, the insulator is termed “process-essential” and therefore neutralisation using an ioniser should become a necessary part of your ESD control programme.
Most ESD workstations will have some insulators or isolated conductors that cannot be removed or replaced. These should be addressed with ionisation.
Examples of some common process essential insulators are a PC board substrate, insulative test fixtures and product plastic housings.
Electronic enclosures are process-essential insulators
An example of isolated conductors can be conductive traces or components loaded on a PC board that is not in contact with the ESD worksurface.
An ioniser creates great numbers of positively and negatively charged ions. Fans help the ions flow over the work area. Ionisation can neutralise static charges on an insulator in a matter of seconds, thereby reducing their potential to cause ESD damage.
The charged ions created by an ioniser will:
- neutralise charges on process required insulators,
- neutralise charges on non- essential insulators,
- neutralise isolated conductors and
- minimise triboelectric charging.
Insulators and isolated conductors are common in ESD Sensitive (ESDS) Devices – Ionisers can help
For more information on ionisers and how to choose the right type of ioniser for your application, read this post.
Insulators, by definition, are non-conductors and therefore cannot be grounded. Insulators can be controlled by doing the following within an EPA:
- Keep insulators a minimum of 31cm from ESDS items at all times or
- Replace regular insulative items with an ESD protective version or
- Periodically apply a coat of Topical Antistat
When none of the above is possible, the insulator is termed “process-essential” and therefore neutralisation using an ioniser should become a necessary part of your ESD control programme.
The control of electrostatic discharge is an important aspect when manufacturing, assembling and repairing devices that employ electronics. Electrostatic discharges can damage an electronic component at any stage of its production or application if not controlled. The primary method of control is to ground (or bring to the same potential) all conductors that come in contact or near proximity to the electronic device(s). These conductors include humans, tools, ESD mats, other electronic devices, boards, connectors, packaging, etc.
There are other components to a good ESD Control programme including removal of unnecessary insulators, shielding, ionisation, environmental controls, training, education and top-down compliance. This post will talk about controlling discharges to a grounded ESD mat on a workstation. Watch out: it’s about to get technical!
Of specific interest in controlling an electrostatic discharge is the time rate of the discharge. A discharge will occur much quicker in/on a conductor with a surface resistance of 102 ohms than in a conductor with a surface resistance of 109 ohms. How fast or slow should the controlled discharge be? Understanding the importance of discharge times will help you choose the right ESD control materials in building, maintaining or auditing your own ESD Safe workbench(es).
The upper and lower boundaries of an ESD safe discharge rate are determined by the application and materials used. To limit the discussion, the potential energy sourced from the Human Body Model (HBM), [refer to EN 61340-5-1], is applied to an ESD sensitive work area or ESD mat.
Body and Movement
You should be familiar with the timing of the human body’s movements relative to handling or working near ESD sensitive devices to have a handle on the upper limit of the controlled discharge. To reduce the likelihood of an operator discharging onto an ESD sensitive device, they should drain any charges before bringing an ESD sensitive device in contact with themselves or another conductor, whether floating or grounded.
Table 1 – Movement times (averaged) from typical operations:
Table 1 shows averaged times (in milliseconds) for the handling of tools or devices at a work bench with a corresponding standard deviation in milliseconds. The shortest time of 153ms, or worst case, is the time that we will design our ESD sensitive workbench tabletop with. You want to be sure that your device is fully discharged well before the 153ms landing time. A good rule of thumb would be to engineer a x2 safety factor. Therefore, your device should be fully discharged before reaching 76.5ms (76.5ms x 2 = 153ms). The time constraint of 76.5ms for body movement defines the upper boundary of the controlled discharge rate (not including the standard deviation of 11ms).
Table 2 – Typical Discharge times [t = R x C x ln(V/V0)] for an RC circuit where C = 200pF and V0 = 249 Volts:
Table 2 shows calculated discharge rates for the human body model (HBM) onto an ESD grounded mat with surface-to-ground (RG) resistances from 102 to 1011 ohms. The more conductive the ESD mat on the workbench is, the faster the discharge, but there is another consideration too.
How fast is too fast? When does the discharge energy at any given time reach a critical level that can damage a semiconductor? The answer depends on several variables relative to the semiconductor’s construction such as line spacing, composition, density, packaging, etc., all leading to an ESD component classification [refer to ESD STM5.1-2007 and the manufactures’ device specifications].
For simplicity’s sake assume the worst case, class 0, which has a 0 to 249 Volt tolerance. Applying the HBM, a conservative worst case capacitance would be 200pF, twice that of the HBM and resistance of 10 kilohms. Therefore, the maximum power (P) level based on Ohm’s Law is P = V2/R (J/s) and the worst case HBM is ((249)2/10K) = 6.2 Watts or Joules per second (Js-1).
The maximum energy (E) stored in a worst case HBM capacitance (C) of 200 pF and at a maximum voltage (V) of 249 Volts, (using E = 1/2 CV2), yields 6.2mJ.
The next concern is to relate energy to time. The time constant (t) is the measure of the length in time, in a natural response system, for the discharge current to die down to a negligible value (assume 1% of the original signal). For an RC circuit, the time constant (t) is equivalent to the multiple of the equivalent resistance and capacitance. In this case, the time constant (t) of our RC circuit is (10 kilohms) x (200pF) or t = 2ms. Discharging this energy upon touching a conductor at zero volts yields a current, (using I = P/V), of (6.2Js-1)/(249V) or 24.8mA. To avoid damaging a class 0 ESD sensitive device, the discharge current must be below 24.8mA. Engineering in a “2x” safety factor, the maximum discharge current would be 12.4mA. To maintain a discharge current below 12.4mA, we need to look at our grounding equipment on the ESD sensitive workbench.
Table 3 – Discharge currents from a 6.2 mJ lossless energy source (with C = 200pF & V = 249V) dependent on the discharge time.
The rate at which 6.2mJ of energy discharges is very important. Too fast a discharge will lead to an ESD Event, which can electromagnetically be measured using a simple loop antenna attached to a high impedance input of a high-speed storage scope. The faster the discharge the greater the discharge current becomes as well as the emf (electromotive force) on the loop antenna from the EMI (ElectroMagnetic Interference). Table 3 depicts the discharge current for 6.2mJ at varying discharge times. We are assuming lossless conditions during the discharge for the worst case. For our example, to keep the discharge current below 12.4mA, the discharge rate [from Table 3] must be no quicker than 2.01ms. This energy-based-time constraint forms the lower boundary of the controlled discharge rate.
Choosing your Matting
The upper (76.5ms) and lower (2.01ms) boundary of our controlled discharge rate are now defined and can be used to help in choosing the correct ESD mat for an ESD sensitive workstation. ESD mat materials come in many variations. In general, mats are either made from vinyl or rubber material and can be homogeneous or multi-layered. Rubber mats, in general, have good chemical and heat resistance but vinyl tends to be more cost effective. The electrical properties of an ESD mat are important to know in controlling the electrostatic discharge.
An ESD mat will be either electrically conductive or dissipative. Both terms mean that the mat will conduct a charge when grounded. The difference in the terms is defined by the materials resistance, which affects the speed of the discharge. A conductive material has a surface resistivity of less than 1 x 105 ohms/sq and a dissipative material is greater than 1 x 105 ohms/sq but less than 1 x 1012 ohms/sq. Anything with a surface resistivity greater than 1 x 1012 ohms/sq is considered insulative and will essentially not conduct charges.
Back to our example: If the maximum discharge current of our ESD sensitive device is 12.4mA, then the discharge time based on energy must be slower than 2.01ms and based on body movement must be faster than 76.5ms. Using the discharge times from Table 2 and assuming that the mat has a negligible capacitance relative to the HBM, then the mat resistance must be greater than 2.2 x 103 ohms or 2.2 x 104 ohms/sq and less than 8.3 x 107 ohms or 8.3 x 108 ohms/sq. In other words, a very conductive mat for some applications may be too quick to discharge and yield more dangerous ESD events whether properly grounded or not.
Graph 1 shows the natural response of a 249 Volt discharge in an RC circuit using a capacitance of 200 pF (HBM) into resistances (mat) of 104, 105, and 106 ohms. The natural response of the104 ohms curve is below 1% of its’ initial voltage in less than 10ms where the 106 ohms curve takes less than 1ms to discharge to less than 1% (V < 2.49V) of its initial value (V0 = 249V).
The role of Wrist Straps
Another defence, and the most common method, to reduce the risk of creating an ESD event is wearing a grounded wrist strap at the workstation. The wrist strap connects the skin (a large conductor) to a common potential (usually power ground). Properly worn, the wrist strap should fit snugly, making proper contact with the skin, to reduce contact resistance.
The wrist strap, since it is connected to ground, will quickly discharge any charge the body either generates through tribocharging or becomes exposed to through induction. Any time the body directly touches a charged conductor, a discharge will occur because the body is at a different potential (0 Volts). Controlling this discharge is important if the conductor is an ESD sensitive device and in minimising induced charges through EMI onto nearby ungrounded ESD sensitive devices.
The electrical properties of the skin of an operator can have a wide range in both resistance and capacitance depending on several variables. An operator’s hand touching a charged device will initiate a discharge at the rate of the time constant of the skin before including the RL properties of the wrist strap. To reduce the potential of an unsafe discharge from a device to a very conductive operator, adding resistance to the operator at the interface from skin to device may be necessary. Some solutions are static dissipative gloves or finger cots, which if worn properly, may add from 1 to 10 megohms to the RC circuit of the skin. This, in turn, slows down the discharge rate to well over 2ms.
The upper and lower boundaries of a safe discharge rate are determined by the application and materials used. The movements of the operator define our upper boundary and the max energy, as defined by the ESD sensitive component classification, dictates our lower boundary. We want to design an ESD sensitive workbench to control the discharge rate (via the circuit’s time constant) of our grounded or conductive materials within these limits.
For the HBM and a class 0 device, the materials chosen for a safe ESD workbench should have electrical properties to support discharge rates between 2ms and 76.5ms. These discharge rates, using worst case assumptions, equate to an ESD mat surface with a Resistance-To-Ground (RG) between 2.2 x 103 ohms and 8.3 x 107 ohms. This controlled discharge rate window will vary depending on the class of semiconductor components used (class 0 to class 3B per ESD STM5.1-2007) and the properties of production resources used (human vs. automated).
Please note that the numbers calculated were based on assumptions used to simplify the explanation of the material. Real-world applications are much more complex and require a more detailed analysis, which was beyond the scope of this blog post.
The best-equipped service bench in your shop can be a real money-maker when set-up properly. It can also be a source of frustration and lost revenue if the threat of ElectroStatic Discharge (ESD) is ignored. Static electricity is nothing new; it’s all around us and always has been. What has changed is the proliferation of semiconductors in almost every consumer product we buy. Couple that with the fact that as device complexity becomes greater, often its static sensitivity increases. Some semiconductor devices may be damaged by as little as 20-30 volts!
A typical scenario might be where an electronic product is brought in for service, properly diagnosed, repaired, only to find a new and perhaps different symptom, necessitating additional repair. Damage from static electricity cannot be ruled out unless the technician understands the ESD problem and has developed methods to keep it in check.
It is important to note that we are addressing the issue of ESD in terms of control, and not elimination. The potential for an ESD event to occur cannot be totally eliminated outside of a laboratory environment, but we can greatly reduce the risk with proper training and equipment. By implementing a good static control program and developing some simple habits, the problem can be effectively controlled.
The Source of the Problem
As mentioned earlier, static is all around us. We occasionally will see or feel it by walking on carpet, touching something or someone and feeling the “zap” of a static discharge. The perception level varies but the static charge is typically 2000-3000 volts before we can feel it. Remembering that the sensitivity of some parts is under 100 volts, it’s easy to see that we might never know that an ESD event has occurred.
Even though carpet may not be used around the service bench, there are many other – subtler – static “generators” frequently found around or on a service bench. The innocent-looking Styrofoam coffee cup can be a tremendous source of static. The simple act of pulling several inches of adhesive tape from a roll can generate several thousand volts of static! Many insulative materials will develop a charge by rubbing them or separating them from another material. This phenomenon is known as “tribocharging” and it occurs often where there are insulative materials present.
Sources of Charge Generation: Unwinding a Roll of Tape
People are often a major factor in the generation of static charges. Studies have shown that personnel in a manufacturing environment frequently develop 5000 volts or more just by walking across the floor. Again, this is “tribocharging” produced by the separation of their shoes and the flooring as they walk.
A technician seated at a non-ESD workbench could easily have a 400-500 volt charge on his or her body caused not only by friction or tribocharging but additionally by the constant change in body capacitance that occurs from natural movements. The simple act of lifting both feet off the floor can raise the measured voltage on a person as much as 500-1000 volts.
Setting up a “static safe” Programme
Perhaps the most important factor in a successful static control programme is developing an awareness of the “unseen” problem. One of the best ways to demonstrate the hazard is by using a “static field meter”. Although this is not something a service centre would typically purchase, it often can be borrowed from a local static control product distributor. The visual impact of locating and measuring static charges in excess of 1000 volts will surely get the attention of the sceptics.
Static Field Meter – find more information here
Education of Personnel
This is an essential basic ingredient in any effective static control programme. A high level of static awareness must be created and maintained in and around the protected area. Once personnel understand the potential problem, it might help to reinforce this understanding by hanging up a few static control posters in strategic locations. The technician doesn’t need an unprotected person wandering over and touching things on the service bench.
Information on static control and setting up a static-safe workstation is readily available from a variety of sources. Your local electronic parts distributor will often have a variety of ESD Control products and may also have literature from manufacturers on setting up a static-safe area.
To minimise the threat of an ESD event, we need to bring all components of the system to the same relative potential and keep them that way.
- Establish an ESD Common Grounding Point, an electrical junction where all ESD grounds are connected to. Usually, a common ground point is connected to ground, preferably equipment ground. If you need help with grounding your workstation, this post might help to clarify a few things.
- The Service Bench Surface should be covered with a dissipative material. This can be either an ESD-type high-pressure laminate formed as the benchtop surface, or it may be one of the many types of dissipative mats placed upon the benchtop surface. The mats are available in different colours, with different surface textures, and with various cushioning effects. Whichever type is chosen, look for a material with a surface resistivity of 1 x 109 or less, as these materials are sufficiently conductive to discharge objects in less than one second. The ESD laminate or mat must be grounded to the ESD common grounding point to work properly. Frequently, a one Megohm current limiting safety resistor is used in series with the work surface ground. This blog post will provide more information on how to choose and install your ESD working surface.
Types of Bench Matting – click here for more information
- A Dissipative Floor Mat may also be used, especially if the technician intends to wear foot-grounding devices. The selection of the floor mat should take into consideration several factors. If anything is to roll on the mat, then a soft, cushion-type mat will probably not work well. If the tech does a lot of standing, then the soft, anti-fatigue type will be much appreciated. Again, the mat should be grounded to the common ground point, with or without the safety resistor as desired. If you require more information as to how you can manage charge generation from flooring, have a look at this previous post.
- Workstation Tools and Supplies should be selected with ESD in mind. Avoid insulators and plastics where possible on and around the bench. Poly bags and normal adhesive tapes can generate substantial charges, as can plastic cups and glasses. If charge-generating plastics and the like cannot be eliminated, consider using one of the small, low-cost air ionisers available from some manufacturers. It can usually be mounted on the bench to conserve work area, and then aimed at the area where most of the work is being done. The ioniser does not eliminate the need for grounding the working surface or the operator, but it does drain static charges from insulators, which do not lend themselves to grounding. Not sure what tools and accessories to replace? Check out this blog post.
As was mentioned previously, people are great static generators. Simple movements at the bench can easily build up charges as high as 500-1000 volts. Therefore, controlling this charge build-up on the technician is essential. The two best-known methods for draining the charge on a person are wrist straps with ground cords and foot or heel grounds.
- Wrist Straps are probably the most common item used for personnel grounding. They are comprised of a conductive band or strap that fits snugly on the wrist. The wrist strap is frequently made of an elastic material with a conductive inner surface, or it may be a metallic expandable band similar to that found on a watch. Need more information on wrist straps? We’ve created a Q&A post to answer all questions you may have.
- Ground Cords are typically made of a highly flexible wire and often are made retractable for additional freedom of movement. There are two safety features that are usually built into the cord, and the user should not attempt to bypass them. The first, and most important, is a current limiting resistor (typically 1 Megohm) which prevents hazardous current from flowing through the cord in the event the wearer inadvertently contacts line voltage. The line voltage may find another path to ground, but the cord is designed to neither increase or reduce shock hazard for voltages under 250 volts. The second safety feature built into most cords is a breakaway connection to allow the user to exit rapidly in an emergency. This is usually accomplished by using a snap connector at the wrist strap end.
Wrist Band and Grounding Cord – more information
- Foot/Heel Grounders or ESD Shoes are frequently used where the technician needs more freedom of movement than the wrist strap and cord allows. The heel grounder is often made of a conductive rubber or vinyl and is worn over a standard shoe. It usually has a strap that passes under the heel for good contact and a strap of some type that is laid inside the shoe for contact to the wearer. Heel grounders must be used with some type of conductive or dissipative floor surface to be effective and should be worn on both feet to ensure continuous contact with the floor. Obviously, lifting both feet from the floor while sitting will cause protection to be lost. If you can’t decide between foot/heel grounders or ESD shoes, this comparison may be of help.
Don’t forget to regularly check your personnel grounding items:
The Personnel Grounding Checklist
An effective static control programme doesn’t have to be expensive or complex. The main concept is to minimise the generation of static and to drain it away when it does occur, thereby lessening the chance for an ESD event to happen. The ingredients for an effective ESD program are:
- Education: to einsure that everyone understands the problem and the proper handling of sensitive devices.
- Workstation Grounding: through the use of a dissipative working surface material and dissipative flooring materials as required.
- Personnel Grounding: using wrist straps with ground cords and/or foot-grounding devices.
- Follow-up to ensure Compliance: all elements of the programme should be checked frequently to determine that they are working effectively.
The ESD “threat” is not likely to go away soon, and it is very likely to become an even greater hazard, as electronic devices continue to increase in complexity and decrease in size. By implementing a static control programme now, you will be prepared for the more sensitive products that will be coming.
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
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.
Today we want to talk about a subject many users forget about when it comes to ESD protection: periodic verification.
Whilst many people understand the basic concepts of ESD and as a result insist on a properly equipped ESD Protected Area (EPA), they then forget all about it. They use the same products day-in, day-out, year after year, without knowing if their products are actually still working properly.
So today we want to look at the most common products in your EPA that you should be checking on a regular basis. And because there are quite a few product groups to discuss, we have split this post in 2 parts – we don’t want to scare you away with a never-ending blog post…
Why periodic verification?
Each component in an ESD protected area (EPA) plays a vital part in the fight against electrostatic discharge (ESD). If just one component is not performing correctly, you could damage your ESD sensitive devices potentially costing your company money. The problem with many ESD protection products (think wrist straps!) is that you can’t always see the damage. Just by looking at a coiled cord that has no visibly damage to the insulation you would not know if the conductor on the inside is damaged. That’s where periodic verification comes into play.
ESD protected area (EPA) products should be tested:
- Prior to installation to qualify product for listing in user’s ESD control plan.
- During initial installation.
- For periodic checks of installed products as part of IEC 61340-5-1 Edition 1 2007-08 clause 5.2.3 Compliance verification plan.
It’s #3 we will be focusing on in this 2-part series.
The purpose of ESD bench matting is to ensure that when charged conductors (conductive or dissipative) are placed upon the surface, a controlled discharge occurs and electrostatic charges are removed to ground. However, this only occurs if the ESD work surface is actually connected to ground. If the matting is out-of-spec, not grounded at all, the stud on the mat has become loose or the ground cord has become disconnected, charges cannot be removed.
Many companies use a daily checklist, which includes the operator having to verify that ground cords are firmly connected.
Remember to regularly clean your bench matting to maintain proper electrical function (e.g. Reztore Surface and Mat Cleaner). Do not use cleaners with silicone as silicone build-up will create an insulative film on the surface.
The company’s compliance verification plan should also include periodic checks of work surfaces measuring:
- Resistance Point-to-Point (Rp-p) and
- Resistance-to-ground (Rg).
Testing a working surface using 222643
Surface resistance testers can be used to perform these tests in accordance with EN 61340-5-1 Electrostatics and its test method IEC 61340-2-3; if these measurements are within acceptable ranges, the worksurface matting and its connections are good.
As discharges from people handling ESD sensitive devices cause significant ESD damage, the wrist strap is considered the first line of ESD control.
Before handling ESD sensitive items, you should visually inspect the wrist strap to see if there are any breakages etc. The wrist strap should then be tested while worn using a wrist strap tester. This ensures all three components are checked: the wrist band, the ground cord (including resistor) and the interface with the wearer’s skin. Records of each test should be kept. Wiggling the resistor strain relief portion of the coil cord during the test will help identify failures sooner. Analysis and corrective action should take place when a wrist strap tester indicates a failure.
Checking wrist straps using 222566
It is recommended that wrist straps are checked at least daily. An even better solution to daily wrist strap checks is the use of continuous monitors. They will alarm if the person or work surface is not properly grounded.
A note on worksurface matting and wrist straps: if you are using earth bonding points, earth bonding bars etc. to ground the operator and/or bench matting, remember to inspect and test those regularly as well (every 6 months for example).
Make sure you read the follow-up post here.
We thought today we could focus on ESD during storage and transport. If you have read our recent post on Tips to Fight ESD, you will remember how important it is to protect your ESD sensitive items when leaving an EPA. Yet, too often we see customers who have the perfect EPA, but when it comes to transporting and storing their precious components, it’s all falling apart.
1. Packaging required for transporting and storing ESD sensitive items
During storage and transportation outside of an EPA, we recommend that ESD sensitive components and assemblies are enclosed in packaging that possesses the ESD control property of shielding.
- In ‘shielding’ we utilise the fact that electrostatic charges and discharges take the path of least resistance.
- The charge will be either positive or negative; otherwise the charge will balance out and there will be no charge.
- Charges repel so electrostatic charges will reside on the outer surface.
2. The Faraday Cage effect
A Faraday Cage effect can protect ESD sensitive items in a shielding bag or other container with a shielding layer. To complete the enclosure, make sure to place lids on boxes or containers and close shielding bags.
Cover must be in place to create Faraday Cage and shield contents.
3. Types of shielding packaging
The below list gives a few examples of what types of shielding packaging is available on the market. This list is by no means complete; there are many different options out there – just make sure the specifications state “shielding” properties.
- 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-Out Shielding Bags
Integral antistatic and low tribocharging bags which will not electrostatically charge contents during movement. Bags have an aluminium metal outer layer of laminated film. Available with and without dissipative zipper.
- Moisture Barrier Bags
Offer ESD and moisture protection and can be used to pack SMD reels or trays. Check out this post for more information on MBB and ESD Control.
- 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.
- Component/Circuit Boards Shippers
These boxes offer an efficient way of shipping or storing ESD sensitive circuit boards and other items. They provide ESD shielding with the lid closed. The foam cushioning reduces stress from physical shock.
- In-Plant Handlers/Storage Containers
Shield ESD sensitive items from charge and electrostatic discharges (with lid in place). They provide ESD and physical protection for ESD sensitive circuit boards.
4. Additional options for storing ESD sensitive items
Do you have the following in place?
- ESD flooring
- Grounded personnel (using foot grounders). Read this post for more information on how to ground moving personnel.
- Grounded racking
Operator wearing foot grounders
IF (and this is a BIIIG IF) the above requirements are fulfilled, you can use conductive bags or containers to store your ESD sensitive items. Conductive materials have a low electrical resistance so electrons flow easily across the surface. Charges will go to ground if bags or containers are handled by a grounded operator or are stored on a grounded surface.
Conductive materials come in many different shapes and forms:
- Conductive Black Bags
Tough and puncture resistant bags which are made of linear polyethylene with carbon added. The bags are heat sealable.
- Rigid Conductive Boxes
Provide good ESD and mechanical protection. Boxes are supplied with or without high density foam for insertion of component leads or low density foam which acts as a cushioning material.
- PCB Containers
Are flat based and can be stacked. They are made of injection moulded conductive polypropylene.
Again, there are many more options available on the market so make sure you do your research.
Note: we do not recommend using conductive packaging to transport ESD sensitive devices. Also, pink antistatic and pink antistatic bubble bags are not suited for storing or transporting ESD sensitive components.
5. Final thoughts
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.
Also, do not staple ESD bags shut. The metal staple provides a conductive path from the outside of the ESD bag to the inside. The use of a metal staple would undermine the effectiveness of the ESD bag making a conductive path for charges outside the bag to charge or discharge to ESD sensitive components inside the bag. To close an ESD bag, it is recommended to heat seal or use ESD tape or labels after the opening of the bag has been folded over. Alternatively, you can use ESD bags with a zipper.
Sealing ESD Bags the correct way
One final word of warning:
When ESD sensitive items are unpackaged from shielding bags or other containers, they should be handled by a grounded operator at an ESD workstation
The authors of a recent article in InCompliance Magazine titled Early Life Failure of Dissipative Workstation Mats have confirmed that Vermason’s Statfree™ T2 was the top performing worksurface mat (Mat ID#4 in the article) in their study. To read the rest of the article click HERE.
Vermason T2 Mats
Part of any good ESD Control Programme is periodic verification of the specifications of products being used for the ESD Control Plan. The most obvious example of periodic testing is when operators test their foot grounders or wrist straps before entering an ESD Protected Area. The ESD Association’s ESD TR53-01-15 Compliance Verification of ESD Protective Equipment and Materials describes the test methods and instrumentation that can be used to periodically verify the performance of ESD protective equipment and materials. Plexus Manufacturing Solutions in Neenah, WI noticed during periodic verification of their workstation mats, that the mats were drifting out of spec and wanted to determine what was causing the mats to fail. Through their in-house tests, they concluded that:
- A solid ESD control programme including compliance verification periodic testing with data collection provides valuable and useful information.
- Factory fluorescent lighting, likely from ultraviolet (UV) radiation, can have a measurable effect on the electrical properties of ESD dissipative mats over time.
- Different mats tolerate or resist the effects of factory fluorescent lighting.
- Since this experiment did not take into account lot-to-lot variation at the supplier for each mat P/N, future mats purchased will require lot traceability information to be stamped on the bottom of the roll, every 0.9m minimum.
- The ESD Association technical report ESD TR4.0-01-02 Worksurfaces and Grounding Survey should consider adding ultraviolet light resistance as a property to consider in the Worksurface Selection Guide.
Vermason’s Statfree™ T2 mats include Ultraviolet (UV) stabiliser additives in the mat formula to stabilise colour retention, ensure ESD performance over time and eliminate surface layer deterioration resulting in cracking when the product is flexed. Additionally, we mark lot traceability information on the bottom of the roll every 0.9m to provide tracking information for lifespan of the product and to ensure quality control.
Top 3 Benefits of Statfree™ T2 Working Surface Matting:
- Superior wear, heat and chemical resistance.
- Withstands abrasion, stretching and tearing.
- Ultraviolet (UV) additives stabalise colour retention to ensure ESD performance over time and eliminate surface layer deterioration.
Continuous Monitors provide operators with instant feedback on the status and functionality of their wrist strap and/or workstation. They 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. Continuous Monitors verify the ground integrity of both the operator and ESD workstation and eliminate the need for periodic testing (and record keeping).
Single-Wire Monitoring allows 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 workstation surface.
The Multi-Mount Monitor continuously monitors:
• One operator wearing a wrist strap
• One ESD worksurface
Its small package and mounting tabs with holes make it highly suitable to install on most equipment or work bench surface.
The Jewel® Mini Monitor continuously monitors:
• One operator wearing a wrist strap
• One ESD worksurface
This unit is ideal for individual workstations for assembly, test, packaging or other applications.
The Dual-Operator Monitor continuously monitors:
• Two operators wearing a wrist strap
• Two ESD worksurfaces
This unit is ideal for workstations that are shared by two operators.
To determine the correct monitor for your application, make sure you check our Continuous Monitor Selection Chart.
|231726||Long Soft Rubber Nozzle, W/ Mini Crevice Tool||£78.72|
|231727||Long Soft Rubber Nozzle||£66.22|
|231728||Long Soft Rubber Nozzle, W/ Brush Head||£67.47|
|231729||ESD Micromotor Filter, Charged Filtermedia||£13.74|
231700 – Vacuum Cleaner
“The fundamental ESD control principles that form the basis of IEC 61340-5-1 [include]:
- Insulators in the environment cannot lose their electrostatic charge by attachment to ground
- Avoid a discharge from any charged, conductive object (personnel, equipment) into the device … by bonding or electrically connecting all conductors in the environment, including personnel, to a known ground”
[IEC 61340-5-1 Edition 1.0 2007-08 Introduction]
**The prices in the Vermason Online Catalog supersede all pricing on new product announcements and may change without notice