ESD Systems - Questions And Answers

Can a piece of your 1/4" black cushion grade dissipative foam be grounded using your Universal Snap Kit and a standard ground cord?

Our circuit boards are getting damaged (they have discrete components on the underside) because of "rough" worksurface and movement on that surface. We want to use a piece of foam, place board on foam, and move board by moving the foam (not the board). This should hopefully reduce damage. We tried it out on one station with an ungrounded piece of foam but had problems. First of all, the foam was conductive, not dissipative. Most important, the foam failed our connectivity (i.e. "path to ground") test. So we would like to try with a dissipative piece of foam grounded via a snap connection.

If you could provide an answer to our question, it would be appreciated.

Thanks,

Dave

If you do want to ground the foam, the method you describe is fine. Make a hole in the foam to accept the screw of the Universal Snap Kit.

Attach a ground cord to the snap and to ground.

However, if you can reliably get an acceptable RTG (resistance to ground) measurement with the foam placed on the ESD mat, you would not have to ground via a cord. For intimate contact with ESD sensitive items, the foam should be dissipative, that is from 1 x 10E4 to less than 1 x 10E9 ohms.

You are using the foam as a worksurface, so I would think this would be measured using a Surface Resistance Test Kit in accordance with ANSI/ESD S4.1.

If you are requiring a path-to-ground of < 1 x 10E9 ohms, we would think that conductive foam placed on a grounded ESD worksurface, would pass a resistance to ground test. The test method would be as detailed in ANSI/ESD S4.1 worksurfaces or Compliance Verification ESD TR53 by placing a 5# electrode on the conductive foam surface and the sensing lead of the Surface Resistance Test Kit to ground.

Per ANSI/ESD S20.20-2007 the upper limited requirement for worksurfaces is < 1 x 10E9 ohms or 1,000 megohms. S4.1 recommends a lower limit of 1 x 10E6 ohms or one megohm. You can do this measuring the ESD mat, and or with the foam placed on top of the ESD mat.

To measure the surface resistance of the foam, RTT resistance point to point, use two 5# electrodes, and place about 10” apart.

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