ESD and Plastic Enclosures

By William D. Kimmel, P.E.
and Daryl D. Gerke, P.E.

Kimmel Gerke Associates, Ltd.

Plastic is the material of choice for enclosures, led by cell phones and other handheld devices. Unfortunately, such enclosures exhibit increased vulnerability to electrostatic discharge (ESD). Let’s take a look at some of the key issues when designing your equipment. Note we said equipment, not enclosure, because you will need to consider more than just the box.

Direct vs. Indirect Discharge

Both direct and indirect discharge can be a problem with electronics, particularly plastic cases. Direct discharge occurs when you zap directly into some part of the enclosure. Direct discharge upsets normal electronics operation and is often destructive to electronic components.

Indirect discharge is when you zap to a metallic surface near the equipment, and the resulting electric and magnetic fields couple to the inner workings of the equipment. These effects upset normal electronics operation but usually don’t cause damage.

The bottom line is, direct discharge is much more severe than indirect discharge, so if you can eliminate direct discharge, you have greatly reduced the ESD problem. This is not always easy do to, and may well not be possible for a given device.

Eliminating Direct Discharge

There are basically two aspects of blocking discharge, involving the material and the penetrations. Nonconductive plastics won’t accept much charge, and what little charge is accepted won’t migrate far from the contact point. The question is whether the material has adequate breakdown strength to withstand the high voltages present in static charge buildup.

If the plastic lacks adequate breakdown strength, the charge will burn through, especially if there is a metallic member immediately underneath. One of the nasty side effects is that the discharge leaves behind a carbonized path that increases ESD vulnerability. One of the more vulnerable cases is the touch pad – you need to make sure the touch pad will withstand the highest possible static charge voltage, regardless of what the actual regulatory requirement will be. People may be willing to tolerate an occasional upset, but if the device is damaged, you are going to have an unhappy customer.

Fortunately, most plastics will withstand static charge, so the next question is what about the gaps, including the seams and other openings. Any seam will leave a gap through which ESD can penetrate, especially to metallic members in close proximity to the seam, as shown in Figure 1. Note that the discharge path is not necessarily line-of-sight, and even a pinhole path is sufficient.

Your cure is to place metallic members well away from the seam, so that static charge can’t reach into it. If you can’t reposition the member, you might try coating it with a high breakdown dielectric.

We have noticed that arcs seem to travel much farther when following a path adjacent to a dielectric material. We aren’t material experts, but we suspect that surface conditions break down more easily, effectively shortening the path. We have observed arcs traveling surprisingly long distances through circuitous paths due to this phenomena.

Sometimes you get surprised by the path – a shielded die-cut membrane that has adequate breakdown strength for direct discharge, but will be vulnerable to discharge to the metal at the edge.

Other paths are screws, controls and indicators, as shown in Figure 2. The sharp tip of the screw peeks through the plastic in 2a, and the arc goes around the LED in 2b.

One might ask, where does the charge go to if the device is floating? Clearly, it is capacitance to ground. Depending on the device and the packaging, figure on a minimum of 10pF and perhaps as much as 100pF. So, while the actual charge transfer may be diminished, it won’t be eliminated. Further, you may then have a low-loss path with a higher than expected damped ring wave.

We have a lot of problems with LCDs. The screen is normally nonconductive, and won’t accept much charge, but there is considerable capacitance to the panel itself. Bringing the tip of the gun up to direct contact with the surface disrupts the display, perhaps changing color or blanking the screen. Since the LCD works by creating an electric field local to the pixel, it isn’t unreasonable to expect that an external electric field will do something as well. This effect may persist for some time, perhaps even requiring the LCD to be reinitialized or even powered down. As to the fix, we suggest interposing a piece of glass or other transparent material to increase the spacing and thus reduce the e-field. Better, yet, make the media conductive so that the field can be intercepted before it gets to the liquid crystal – this is not effective unless you have a conductive enclosure with which to connect the shield.

If you are successful in blocking direct discharge, you will be left with indirect discharge, a much lower threat. These need to be handled internally, largely by lowering ground impedances, controlling loop areas and local filtering – that’s another story to be covered in a later article.

What About Coatings?

If you can’t block the direct discharge, you have two possible alternatives. The first, and most difficult, is to harden the internal circuitry to tolerate the high discharge current or to go to a metallized coating. In fact, metallized coatings are often employed for RF shielding (either for radiated emissions or immunity), but such coatings often create a whole new problem with ESD. Figure 3 shows the metallization brought almost to the surface, easily reached by a discharge. Where previously you may have had only a few places to which discharge could reach out, now you can discharge into virtually the entire perimeter of the enclosure.

This is not necessarily fatal. A well-shielded enclosure will work as well for ESD as it does for radiation. All cell phones are shielded – the seams are closed with an EMI gasket. That tells the tale – well-shielded enclosures are well protected from ESD effects, whether direct or indirect. The problem occurs where the shield is interrupted. A good case in point is the laptop computer – it may be shielded, but there are holes everywhere, including the big display opening and the keyboard, but also include the myriad of connector penetrations around the entire perimeter.

If you are going with a coated enclosure, you will need to pay particular attention to closing the seams – you need to bring the coating up to the seam and make sure the seams close – tongue and groove makes better contact than butt or lap joints. Better, yet, do as the cell phone people do – use EMI gasketing around the perimeter.

Unfortunately, the packaging people like to mask off the coating well clear of the mating surface – you don’t want the coating to show at the exterior. We have lost count of the number of cases we found the conductive coating masked off to 1/8 inch from the seam. Sadly, you have created the worst of both worlds – you have created an entire perimeter of ESD points while doing nothing to control ESD and little to control RFI.

If you can’t conductively close the seams around the entire perimeter of the enclosure, you may well be better off eliminating the coating completely and looking for internal solutions.

Summary

ESD effects can be caused by direct or indirect discharge. Direct discharge is much more severe, often resulting in damage. So the first line of defense is to design to avoid direct discharge effects.

ESD always heads for a conductive member close to the surface. Such members include recessed screws, and switches and indicators. These should be set back far enough that the arc can’t jump that far, or covered with a dielectric capable of withstanding the high e-field.

Done right, metallized coatings work well, but they also create new ESD problems, so you will need to proceed with care.