First Published in the EOS/ESD Technology April/May 1990

Cutting Through a
Cloud of Static

Static problems 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 be?

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 looking.

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 safety reasons.

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.]