First Published in the EOS/ESD Technology
Through a Cloud of Static
can be subtle, and often demand more than
just a one-step solution.
Garry R. Grzelak
Teradyne Central Inc., Deerfield, IL
Piloting a product through a new
manufacturing process at Teradyne Central helped us realize that
we were also piloting an ESD problem. When testing our pilot boards,
we found a high failure rate in a MOSFET switch and asked our
component analysis group to evaluate the failed devices. It was
found that all 10 of the devices had been destroyed through ESD
damage at the GND pin of the MOSFET switches.
We were surprised by this since we
have an extensive ESD-control program that includes wrist straps,
static-dissipative workstation mats, a conductive-tile floor,
heel straps, and static-shielding bags. Where could our problem
The new product required us to coat
the PCB with conformal coating. Our failure data showed that the
problem occurred between the conformal coating process and final
system test. Thus, this process seemed to be a good place to start
Finding the Problem
Our coating process used aerosol
spray cans to cover the components on the PCBs as they were positioned
in the spray booth; the spray operator wore a wrist strap during
all handling and spraying operations. Even so, when we measured
the charge during this process we found a charge of 12 kV generated
during the aerosol spraying.
We had found our source, so we stopped
the coating process and formed a task force to solve the problem.
Step One: First, we tried conducting
charge to ground from the aerosol can by connecting its metal
rim to ground, but that had no measurable effect on the ESD generation.
[Most likely, there was either a high resistance or an open circuit
somewhere between the aerosol particles and the ground cable,
probably as a result of the design of the spray can.-- Ed.]
Since we did not fully understand all the potential solutions,
we called an ESD consultant.
|Figure 1: Replacing the aerosol
application method with an electric sprayer reduced charge
on the circuit board from 12 kV to 5 kV.
Step Two: The consultant's
first recommendation was to switch from the aerosol can approach
to a liquid -base conformal coating applied using an electric
pain sprayer (Fig 1). This method reduced the charge to 5 kV--
better, but still not up to our requirements.
|Figure 2: Grounding the sprayer's
nozzle cut charge further, this time to only 500 V.
Step Three: We then purchased
an electric spray gun with a metal nozzle, which could be connected
to ground (Fig 2). This grounded-nozzle approach reduced the generated
charge to 500 V. This was a major improvement, especially compared
with the original 12 kV, but still caused some ESD damage.
What we were left with was the "aerosol
effect" of the comformal coating solution breaking up as
it was sprayed on the PCB; a triboelectric charge was created
as the particles of the conformal coating separated upon leaving
the nozzle. The conformal coating, which is insulative by design,
developed a charge in much the same manner that Scotch tape generates
a charge as it is peeled off a roll.
Step Four: Our next idea was to bathe
the PCB in ionized air while in the spray booth to neutralize
the 500-V charge developed by the tribocharging coating (Fig 3).
Since the conformal coating we used had a relatively low flash
point, presenting a fire hazard, we used a nuclear ionizer for
|Figure 3: Adding a nuclear
ionizer virtually nullified charge on the board.
After adding the nuclear ionizer
to the coating process, we recorded no measurable static charge
during application of the conformal coating.
What had Happened?
How did this tie in with the GND
pin being blown on the MOSFET switch? We had been using a titanium
clip mask to keep the PCB's gold edge connectors from getting
coated. A convenient place for the operator to hold the board
while removing the clip was directly over a gold circuit finger
connected to the GND pin of the MOSFET switch.
The damage occurred when the wrist-strap-grounded
operator (at 0 V) made contact with the charged PCB (at 12 kV)
via the titanium clip.
At this point, the process had been
qualified, and we were ready to resume manufacturing. A work instruction
was created to document the conformal coating process and maintenance.
[Editors Note: Smaller companies, or those manufacturing in
small quantities, who much use aerosol cans near ESD-snesitive
parts might also wish to investigate grounders manufactured specifically
for such cans. Several are available; see the 1990 EOS/ESD
Technology Buyers' Guide, June/July 1989.]