Friday, September 21, 2012

When components in non-ESD packaging are assembled with ESDS in an EPA used in the

  Users of electrononic components do not want static-generating materials in their EPAs, but  suppliers of non-ESDS often use non-ESD packaging. Is there any packaging guidance for non-ESDS?

Users of ESD susceptible components will not want to bring ordinary packaging materials into an EPA workstation where ESDS are handled. There are no requirements in the standards for suppliers of non-ESDS components to supply their product in non static generating packaging. So, many will use ordinary plastics in their packaging which could compromise ESD safety and compliance if brought onto an EPA workstation. It is for the user to decide how to handle this, e.g. by removing non-compliant packaging before bringing the component into the EPA workstation.

This is one reason why the modern standards say that non-essential insulators must not be present on a workstation where ESDS are handled, rather than in the EPA in general. Some people write their ESD control program to allow items in non-ESD packaging to a bench conveniently placed near the one where they handle ESDS, but not near enough to cause ESD risk from the plastic packaging. The packaging can then be removed from the component on a workstation where ESDS are NOT handled and the component transferred to the workstation where they are assembled with ESDS. This approach relies on a high level of understanding by operators to prevent stray packaging arriving on the workstation where ESDS are handled and other ESD risks.

Others write their ESD program to prohibit non-essential insulators from the EPA, and repackage the non-ESDS into ESD packaging before they are brought into the EPA.

Tuesday, August 21, 2012

Is my core conductive garment suitable for use in electronics manufacture?

We are looking for a new ESD garment/jacket. The recent garment we tested reduces in Rp-p with washing, but passed the charge decay test. The supplier stated that this fabric is woven with a core conductive fibre, so therefore the charge decay is the valid test (referring to BS EN 1149-3). BS EN 61340-5-1:2007 only specifies that the garment should pass Rp-p < 1x1012 ohms. So if the garment passes the charge decay test but fails the resistance test where do we stand?

 EN1149-3 is a standard for evaluating garments for use in flammable atmosphere areas, not electronics manufacture. The evaluation criteria are quite different.

The only test currently recognised by 61340-5-1 is the point to point resistance test for which it gives requirements. If it fails the test, it fails the test.

If you want to accept garments on a different basis than this test, and still comply with 61340-5-1, then you would need to devise some  sort of technical evaluation of the garment performance to convince yourself it does the job you intend it to do. You should then document your technical evaluation, including your qualification test methods and results, as a tailoring exercise in the ESD Program Plan. You should of course also instigate a program of suitable compliance verification tests in your Compliance Verification Plan.

Core conductive fabrics often fail the resistance test by the nature of their construction. Whilst that does not necessarily mean they are unsuitable for use in electronics manufacture, it does mean we have no agreed method of demonstrating whether they are suitable or not.

Friday, June 03, 2011

Benchmarking ESD best practice across industries

I work across a range of industries and topics covering ESD in electronics manufacture, electrostatic ignition hazards in industrial processes and occasionally explosives handling. I think that static control has developed further in the electronics industry than in the other areas, partly because the sensitivity of components to ESD is often much greater than the sensitivity of (for example) flammable gases or dusts to ignition by ESD. The 100pF capacitor in a 100V HBM test contains only 0.5 uJ of energy, and most of that energy is not delivered to the device under test. In contrast 20uJ is required to ignite hydrogen-air mixture (which is quite sensitive) and 200uJ to ignite most hydrocarbon-air mixtures. So, for handling modern electronic components we have to control static electricity in some ways more carefully than in industrial processes.
One similarity that runs between the 3 areas is that human body ESD (from charged personnel) is very important in manual processes. So, grounding of personnel is a key ESD prevention measure.
One big difference is in the degree of standardisation of ESD control. In the electronics industry we have the option of compliance with one of the ESD control standards such as ESD S20:20 or IEC 61340-5-1. Standards like this do not exist for static control in industrial flammable atmosphere areas. This is deliberate on the part of the experts who write standards as industrial processes can be very different and what is a high risk in one process can be acceptable in another depending on the exact circumstances. It would be very difficult to write standards that do not burdon industry with inappropriate control measures. In contrast, in explosives handling there are well defined key control measures that are enshrined in documents such as the UK Manufacturing and storage of explosives regulations. These requirements depend on the sensitivity of the explosives handled.
In the electronics industry, we have highly automated manufacture of systems in which charged device ESD is a significant source of ESD damage. This occurs when a device itself charges to a high voltage and is the source of ESD. This type of risk is absent from industrial flammable atmosphere areas and explosives handling. However isolated conductors such as metal parts can become charged and be a significant risk in all three fields.
 The basis of static control is similar in all three fields. In manual operations, the voltage built up on personnel is controlled by grounding them. In all 3 industry areas footwear and flooring are used extensively for this. Wrist straps are used especially for grounding seated personnel in the electronics industry.
 The use of insulating materials is carefully controlled in all three industry areas. In electronics this is mainly due to their ability to build up charge and give high electrostatic fields which can induce high voltages on nearby conductors. In the flammable atmosphere areas, and in explosives handling, it is also because brush discharges from insulators can ignite sensitive materials.
 In all three industry areas it is very important to avoid having isolated conductors which can charge to high voltages and provide a source of damaging or incendive ESD. So, all conductors (especially metal items) are normally grounded where possible unless risk evaluation shows this to be unnecessary.
 Modern electronics ESD standards have a requirement that the user write an ESD Control Program Plan, and ESD Training Plan and a Compliance Verification Plan documenting what the organisation is doing for ESD control. This documentation imposes a rigour on the ESD Control Program which is very beneficial. Sadly this approach is often  lacking in the other industry areas, although less so in explosives handling. 
 So in conclusion, I believe the electronics industry leads the way in ESD control compared to other industries. This is partly through necessity due to the particular sensitivity of the devices handled, and partly because the industry lends itself to standardised ESD control measures  and approaches.

Will conformal coating help prevent ESD?

Yes and no. It will help prevent direct ESD to the components which are protected by the coating. However the coating itself can become charged and can induce high voltages on the pcb. This can give ESD when a connector pin or other exposed conductive part makes contact with another conductor. The energy in this type of ESD is quite high and is a risk to any component through which the ESD current may flow on the pcb. Modules in plastic enclosures can also suffer this risk. Also, the conformal coating gives no protection against external electrostatic fields which can have the same effect as charge on the coating.

Of course the overall risk of damage occurring is difficult to predict.

Tuesday, September 28, 2010

Why use an ESD seat?

I have read the IEC 61340-5-1/2 and we are building a new production area. If everyone who works with ESDS, uses wrist straps while seated, do they need to sit on an ESD chair? (The floor, the tables, the garments are ESD protected)
If they connected to ground via wrist strap, why should I connect them to ground via chair?

Firstly, the ESD chair is not for grounding personnel. The seat may have quite high resistance to ground, up to 10^10 ohms. For grounding personnel, we need a much lower resistance <35 Mohms and this cannot be achieved by contact with many seats. Contact between the seat and body is unreliable because there are layers of clothing in between!

The purpose of having an ESD seat is so that the material of the seat does not become charged and cause an electrostatic field and ESD risk.

Tuesday, June 15, 2010

What ESD packaging should I use?

We need to move pcbs with ESD sensitive components on from an EPA to another EPA passing through a non-EPA area. What is the recommend method for tranporting these devices to minimise ESD damage? We don't want to use individual shielding bags.

There is no single recommended method or packaging type. However I reccommend the packaging used has the following characteristics:

1) no part of the packaging should be an exposed insulating materila which could charge up and cause ESD risk in an EPA.
2) the material in contact with the ESD sensitive parts should be dissipative
3) there should be a surrounding conductive material to form an electrostatic field shield
4) an air gap, dissipative material or other barrier should prevent ESD currents being conducted from the outside of the packaging to the ESD sensitive parts.

A shielding bag has all these characteristics, but it is quite possible to design a larger packaging system to contain many items that achieves the same using different packaging materials.

Thursday, October 08, 2009

Why would a capacitor be ESD sensitive?

Why do some capacitors have a dependency on capacitor size vs ESD?

There is good reason for a capacitor to be ESD sensitive. If you push enough charge into it, you will eventually exceed the dielectric strength and breakdown voltage, and the insulation will break down. So the ESD susceptibility is dependent on the capacitance and breakdown voltage. A high capacitance high breakdown voltage device will have low ESD susceptibility, but a low capacitance low voltage capacitor could be easily damaged by ESD.

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?