There is a popular misconception that ESD is not a concern after parts have been installed onto printed circuit boards (PCBs). The thinking is that multiple parts on the PCB share the ESD current so they withstand higher energy than they would individually. Another notion maintains that all ESD threats are introduced through the edge connectors of the PCB.
The truth is that static is ubiquitous. Even though most people in the electronics industry are fairly responsible wrist-strap wearers, people are not the only source of static. In fact, most static is generated through the contact and separation of materials (tribocharging), and there is no guarantee that this will only happen at the edge connectors.
In his article in the May, 1996, issue of EE-Evaluation Engineering concerning ESD at the PCB level, John Kolyer of Rockwell International made some excellent points.1 One of his key points was: “Shunting the edge connectors is not sufficient to prevent ESD damage.”
Parts on an assembled PCB can be damaged with a perfect shunt on the edge connector, particularly when the ESD threat is through contact to one of the components on the PCB. The threat is not only from personnel.
Potentially, ESD can be created by any rubbing or contact and separation of materials with the PCB or the components on it, not to mention field-induced charging from nearby static sources. Here are some examples: people changing settings on DIP switches, inserting chips into unpopulated sockets such as memory upgrades, or installing PCBs into systems and touching components accidentally.
Removing PCBs from packing material also is a common source of ESD, even if they are in a conductive bag. This is not an issue while the PCBs are inside the protective bag. But in many cases when the PCBs are taken out of the bag and placed on or near the static-charged packing material, a potential ESD failure is unknowingly created.
An example of this type of problem involved some liquid crystal display (LCD) drivers damaged when the clear protective adhesive film was removed from the surface of the display. Since the display is made of glass, it is a very good insulator and can generate and hold a high level of electrostatic charge. Static problems relating to this fact are not unusual even in the manufacture of LCDs.2
Because of the glass, scratch prevention is important between the time of manufacture and the point of use. A protective film is placed over the display to shield against mechanical abrasion. This scratch-protection adhesive film is commonly used on VCRs, digital clocks and CRTs; and is intended to be removed when put into use. When removing the film, static levels of up to 3,000 V have been measured on the LCD.
In this case, the static created by the removal of the film was coupled to the PCB traces connecting the display to the LCD drivers. This caused ESD damage to the output transistors of the drivers, resulting in unreadable characters on the display. This damage occurred even though the PCB was in place in the system at the time.
One solution to the problem is to use an antistatic protective film. This prevents the creation of the damaging charge.
Where this solution is not possible or practical, another option is to use ESD foam. A standard ESD foam, such as 3M Velostat™ 2806/2826, can be cut to fit snugly over the display and pressed in place, electrically shorting all the leads together (Figure 1).3
The foam is a cross-linked, electrically conductive, carbon-compounded polyethylene about 0.25″ thick and with a density of 6 lb/ft3. It has a typical volume resistivity of <105 W •cm and is noncorrosive so long-term contact with device leads or board materials is not an issue.4
The foam is fitted with a ground connection snap to accommodate a standard wrist-strap ground cord (Figure 2). This effectively shunts the leads of the display to ground, providing a discharge path for any ESD. The foam is easily taken off the PCB after the protective adhesive film has been removed from the display, just prior to closing the cover or putting the system in a case.
Given the complexities and the sensitivities of today’s leading-edge products, a simple protection scheme like this one can be a useful tool to solve insidious ESD problems. While not every situation can take advantage of this technique, ESD foam may be quite beneficial in many applications.
References
1. Kolyer, J.M., “Shunting for ESD Protection,” Evaluation Engineering, May, 1996, pp. 132-139.
2. Murakami, T., Togari, H. and Steinman, A., “Electrostatic Problems in TFT-LCD Production and Solutions Using Ionization,” 1996 EOS/ESD Symposium, pp. 365-370.
3. U.S. Patent 5,289,336, “Static Electricity Dispersant,” issued Feb. 22, 1994, to George Gagliano, assigned to Harris Corp.
4. 3M Product Bulletin, “Velostat™ 2800 Series Electrically-Conductive Crosslinked Polyethylene Foam.”
About the Authors
Joe Bernier is a senior principal engineer in the Reliability group at Harris Semiconductor. He holds an M.S.E.E. from MIT and an M.B.A. from Florida Institute of Technology. Harris Semiconductor, P.O. Box 883, M/S 58-98, Melbourne, FL 32902-0883, (407) 729-5565.
George Gagliano is a senior failure analyst in the Reliability group at Harris Semiconductor. He holds an A.S.E. degree from Pinellas Vocational Technical Institute, and has worked in the Reliability group for 20 years. (407) 724-7927.
Copyright 1997 Nelson Publishing Inc.
March 1997
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