ESD and Plastic Enclosures
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). Lets 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.
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 dont
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.
There are basically two aspects of blocking discharge, involving
the material and the penetrations. Nonconductive plastics
wont accept much charge, and what little charge is accepted
wont 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 cant reach into it. If you
cant 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 arent
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 wont 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 wont 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 isnt 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
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
thats another story to be covered in a later article.
If you cant 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 dont 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 cant 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
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
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 cant
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.