Monday, March 09, 2009

If I was within 2 inches of a printed circuit board with no ESD protection would I discharge any voltage to the pcb causing any partial damage? The reason I ask is our design engineer tells us 2 inches is a safe distance, but according to an ESD Trainer on a course I have recently done damage can occur from as far as 12 inches away.

This is a tricky question and the answer depends on various factors, but I will try to answer simply. I will only consider the risks due to your body being possibly at high voltage because it is not grounded. These are usually the most important and damaging ESD risks in manual handling of PCBs, and are completely removed if your body is grounded via wrist strap or ESD footwear and flooring. So,it is most important for all personnel handling PCBs to be grounded at all times. There are other ESD risks which I will not go into, if the PCB itself is at high voltage.

There are two types of ESD risk in this situation. Firstly, there could be a direct ESD from your body to the PCB if you get sufficiently close so that a spark jumps from your body to the PCB. At normal body voltages this can only happen if you get within a few mm of the PCB, as it takes a few thousand volts to jump each mm of air gap. If you are not getting closer than 50 mm (2 inches) then this is unlikely to happen.

The second risk happens because if your body is at high voltage it is surrounded by an invisible electrostatic field. Any isolated (non-grounded) conductor, including PCB tracks or components, which come within this field have a voltage induced on them. If the conductor becomes grounded at this point, ESD will occur and could if great enough, be quite damaging. (There is also another damage mechanism which could happen which would not require the grounding of the PCB, but it is unusual and I won't go into it here.)

The voltage that is induced on the conductor increases as the conductor gets closer to the high voltage source. Above a certain level, it gets to a point where any ESD arising could be damaging to the PCB. However it is very difficult to predict at what level the damage threshold would be passed. This would depend on the voltage on your body and other factors, as well as the closeness of your body to the PCB and the sensitivity of the components you handle.

So, we could say that the "safe distance" is a matter of guesswork and also influenced by your level of concern over possible damage and tolerance of the risk of ESD damage. If your component ESD susceptibility is low and you aren't too worried by the consequences of a possible ESD, you might judge that a closer distance is safe. If the component susceptibility is high and you have an expensive high reliability product you might judge that a greater separation is necessary for safety. In either case it is just based on guesswork unless backed by a considerable research program involving subjecting your PCBs to field induced ESD.

So, you could consider that both your Engineer or your ESD course Trainer could be right, we just don't know. The Trainer is being more careful and risk averse than the Engineer. But neither of them know for sure, and I can't advise you either without a considerable research program involving subjecting your PCBs to field induced ESD.

One thing I can say is that the risk is easily removed completely if you ground your body through a wrist strap or ESD footwear and flooring. So why not just ground yourself and remove the concern?


Anonymous said...

Dear Mr. Sir,

I had been trying to read material on ESD related matters on the internet and noted your name on 1 of the articles. As the e-mail address was also shown, I decided to write to ask your advise, if you do not mind my asking so.

We supply PCB’s to our customers and it is packaged in MBB bags for moisture reasons as well as to protect the PCB’s for a longer period of time, as the surface finishing used on the PCB is OSP.

Whilst kitting parts at their facility, a customer of ours noted had high voltage on the MBB bags used to pack the PCB’s.

As PCB’s are not an active component, there are not packed in shielded bags. Shielded bags I understand are used for actgive components such as IC’s, etc. Basically components affected by electrostatic discharge. PCB does not fall in tis category.

Other than the customer kitting parts separately, do you have any comments or advise related to this matter.

If you have some time, I will appreciate your kind opinions or advise.

Thanking you in advance.

Best Regards

Static Doctor said...

It is not really a question of active/passive, it is a question of ESD susceptible or not. Some passive components could be susceptible to ESD damage, although it is more unusual. Certainly an unpopulated PCB would not be susceptible to ESD damage and therefore wuld not need to be protected in ESD protective packaging.

Against this, you do not want to take materials that could cause electrostatic charge build-up and fields into an ESD Protected Area. So, it is common to package even non ESD susceptible components in ESD protective packaging for kitting. This would not have to be shielding packaging, pink or black polythene might be used.