ESD Systems - Questions And Answers

We are facing a problem with the handling of IC's. We are producing semiconductor equipment and we are using a vacuum cup to hold the device for transfer. Once we let go of the device the ESD level goes way above the spec of 100v. We are told this is normal to happen but the problem we are told is when we drop the device directly to grounded metal. Is the metal actually causing the spike we are seeing? We are told the static discharge must be a slow process and by placing the device on to grounded metal that we are doing the discharge too fast. We have two ionizer fans but they don't seem to be helping the situation. Any suggestions? - Roger Wong, Singapore

Yes, a metal-to-metal or device-to-metal discharge will usually result in an ESD event. You do have to control the rate of the energy transfer, and one way is using a resistive surface to equalize the charged item(s). The ionizers you are using are either not balanced and actually charging your devices/surfaces or are not directed at the critical surfaces to minimize discharges. When you let go of the cup, you are drastically changing the capacitance and causing a re-distribution of charges on both bodies, the cup will inadvertently go to a higher potential (same charge redistributed over a smaller area). Make sure all conductive items are grounded, including personnel handling any ESD sensitive devices. You can slow the process down so the cup/device is subjected to balanced ionized air for several seconds (2x the decay rate of the ionizer) to sufficiently remove any ungroundable charge. Check your ionizers with a charged plate analyzer or field meter ionization test kit.

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