When handling ESD-sensitive components, we must protect them from ESD damage. ANSI/ESD S20.20 tells us its purpose is “to provide administrative and technical requirements for establishing, implementing and maintaining an ESD Control Program,” and it “applies to organizations that manufacture, process, assemble, install, package…. or otherwise handle…parts, assemblies, and equipment susceptible to ESD damage…” It all sounds very “big company.”
What if you have three people trying to run a repair shop or make a few products? Or a single servicing engineer visiting customers? Even larger companies sometimes have single workstation electrostatic protected areas (EPA) or activities that do not seem to fit the standard. Not to mention the R&D guys with a single workstation prototyping area… It just does not seem practical to have ESD control according to IEC 61340-5-1 and ANSI/ESD S20.20, and why would they bother?
Let us deal with the last bit first. As an R&D guy, I would not want to waste my time trying to debug a prototype that I had unknowingly damaged through ESD. I would want this even less if my project was a one-off product going to a customer or if I was fitting memory or boards into a client’s equipment. Yes, it does happen.
Joe has a small company and wants a workstation to produce prototypes, one-off, and small quantity products next to his desk in his converted garage. This is otherwise fitted out as an ordinary office which he shares with a couple of sales and admin colleagues. His main customer wants him to comply with S20.20. If all goes well, he might need to hire another electronics guy to work with soon. Fortunately, Joe went on an ESD training course when with his last employer, so he knows what to do.
Joe fits his workstation surface to an ESD control specification and grounds it via a common point ground buss bonded to the electric protective earth. He makes sure he eliminates all unnecessary insulators from this workstation. He wants to sit at the workstation as he works and roll to and from his office desk. He uses a grounded floor mat and a wheeled chair. He wears a wrist strap bonded to the common point ground while working there. Joe decides the ESD risk from electrostatic fields arising from using his office chair is negligible. After all, he will always be sitting in it and grounded when working at the workstation. So, he does not need a grounded floor mat as there is nothing to ground with it. This will reduce his cash outlay and can be added as a refinement later. When he does, he will also be able to use ESD footwear to ground himself through the mat when standing, which will be useful for mobility.
Joe has a lot of test equipment, some of which has insulating and potentially charging enclosures. He puts this equipment on a shelf over 30 cm above the workstation. So, the shelf does not have to be restricted to ESD control equipment or materials, provided he keeps the ESD susceptible items (ESDS) on the work surface and does not move items between workstation and shelf. The ESD risk from fields is minimized by keeping the 30 cm distance. Components go in ESD control specification bins or other packaging, at the back of the workstation. Joe decides that the boundary of the EPA is the boundary of the workstation and floor mat and puts up an EPA sign.
The key requirements of S20.20 are documentation – the ESD Control Plan, ESD Training, Compliance Verification (CV), and Product Qualification Plans. So, Joe writes a one-page ESD Plan that adequately specifies his workstation ESD controls. He specifies the use of protective earth as ESD earth in the Plan. Referring to the standard (which is available complimentary), he makes a short checklist of all the other things that must be covered in his documentation.
Joe’s ESD Training Plan notes that his colleagues need to know there is an ESD risk, can identify the EPA boundary, and they must not enter his EPA or touch anything in it – unless, of course, they have had appropriate ESD Training. He adds some notes on what that training would need to include. For the moment, Joe is testing the EPA equipment himself, but in the future, he will probably get his recruit to do it, so he adds that training on ESD measurements is needed for the person who does the ESD testing.
Joe combines his CV Plan in a simple table that also specifies the measurable ESD control equipment performance requirements, reference to the test procedure, and frequency of testing. He also writes a small test procedure with photographs for each of the test methods and equipment. For a wrist strap test, he will use a small off-the-shelf tester, so he notes the pass/fail limits in his CV plan.
|Joe’s Garage Compliance Verification Plan|
|Item||Test||Pass criteria||Test frequency|
|Work surface||Resistance from surface to ground||1 MΩ to 1 GΩ||monthly|
|Wrist strap ground point||Resistance from connector to ground||1 Ω ± 10%||monthly|
|Wrist strap as worn||Resistance from hand to groundable point (lead end)||0.7 to 35 MΩ||daily|
|Packaging surface||Point to point resistance||10 kΩ to 100 GΩ||Monthly sampling and spot checks|
Table 1: Sample Compliance Verification Plan
The Product Qualification Plan must give criteria by which Joe knows the ESD control items will work in the EPA. For this, he simply specifies the datasheet values needed to give installed ESD control items within the range specified in his CV Plan. He also specifies that correct performance must be verified on installation using the CV Plan methods.
Joe’s full final ESD control documents end up as a few pages, will be compliant with S20.20, and if correctly implemented and maintained, should satisfy his customer. Lastly, he adds a note that he must update the Plan if the ESD Control specification of his EPA is updated!
Some interesting situations come up in small electronics facilities, which can, at first sight, cause a challenge to the ESD control practitioner. An attendee at an ESD control course was setting up a mobile servicing EPA in the back of his van. How, he asked, should he ground it? Must he take a metal rod that can be pushed into the ground somewhere convenient outside? The answer was remarkably simple. Connecting the EPA equipment to a common ground point could be a good ground just by equipotential bonding. If there is no voltage difference between conductors, there can be no ESD – this is recognized as accepted grounding practice under S20.20. Knowledge and understanding are keys to implementing simple and effective ESD control. ESD control does not have to be complicated. It just needs to be well thought out and control the ESD risks.
Suppose I want to take an ESDS memory device out of its ESD protective package and put it into a computer in an uncontrolled area. The ESDS is safe until I take it out of its package. So, I leave it in there until I am safe to insert it into the computer. The main ESD risks will be if the voltage is different between the ungrounded computer and my body. I can control this by connecting myself with a wrist strap to a suitable part of the computer, e.g., chassis or a 0 V rail (equipotential bonding again). A secondary risk is that charged insulators nearby could cause induced voltages on the ESDS, which might give ESD when it touches the computer. I reduce this by removing insulators to over 30 cm from the working area. It can help suppress fields to have an earthed static dissipative mat to place over desk surfaces. Once I have everything grounded (or equipotential bonded), I can take the ESDS from its package and insert it into the computer. Portable service kit solutions are available that provide the wrist strap and cord, as well as a grounding cord and connector for when that can be used. A flexible dissipative mat is often provided that acts as a roll-up container for the kit between work areas.
The IEC 61340-5-1 and ANSI/ESD S20.20 standards allow a great deal of flexibility in how an effective ESD control program can be achieved. They do require that the control program, training, compliance verification, and ESD control product qualification practices are adequately documented. Part of the art is to ensure that this is done in a concise and clear manner that is easily understood by the users, customers, and auditors who will want to understand the controls and check their compliance.