Desco - Questions And Answers

Hi. I am supposed to train our company on ESD and generally feel well prepared except for one thing: People at our company like to place pink antistatic foam (one-foot square sheets that come in a roll) under PC boards. They use the foam because it helps keep the board from sliding on an ESD workstation and because they believe it will prolong the life of the workstation surface. However, I believe this is poor practice because foam is an insulator. On the other hand, from measurements I've taken with a static meter, objects placed on this foam (resting on the workstation surface) will dissipate their charge. So, is this an acceptable practice? - Anonymous, Brooklyn Park, MN

If the pink foam is dissipative and electrically connected to ground, then you should be all set (OK to use in general). This really depends on the ESD Sensitivity of the products you are trying to protect. To measure the surface resistance to ground (RTG) of the foam you will need to use a megohmeter. Try taking measurements at both 10 and 100 Volts. You should have a surface resistance of less than 10E9 Ohms and greater than 105 Ohms to be safe. You said you took measurements with a static field meter, which is OK, but will not completely ensure that the foam is safe. One property you can check with this meter is the discharge time. It is important that the work surface will remove a charge in less than 100 ms. If it takes a few seconds to remove a charge, this is too long and could put your ESD Sensitive parts at risk of receiving an ESD event. There are other materials to choose from to place your WIP (work in process) on the workstations for ESD protection and non-slip characteristics. Our rubber mat material is an excellent choice for this. Some questions to answer: What is the sensitivity level of the parts I’m trying to protect? Is the foam conductive at a safe dissipative level? The foam will discharge a charge within about 100 ms? The foam is grounded (if conductive)? Operators are wearing ESD wrist straps? If yes to all these questions, then you are employing some safe ESD preventive practices.

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