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Question One of our customers manufactures a mobile cart into which we place electronics boards that we have designed and manufacture. Since the cart has plastic wheels, we have had to implement a grounding strap or drag chain. In addition, the customer has had a problem with plastic packaging materials that are placed over the carts for shipment. The top of the cart is a vacuum formed, plastic, non-conductive material that supports charge created when the plastic packaging bag is removed. If the keypad of the product is touched after the plastic bag has been removed, there is a possibility of causing operational problems because of the discharge. We have temporarily put packaging paper over the area of the keypad and taped it securely to the top of the cart. This forces the installer to remove the paper from the top after the plastic bag has been removed and presumably discharge the top of the cart in the process. Are there any alternative methods to preventing charge from building up and dissipating charge after plastic bag removal? I assume using an anti-static type packaging material would probably work, but that is not reasonable at this time. What else might be done or used to protect the keypad and dissipate the charge? The cart has a painted steel chassis that is insulated from ground through the wheels with the drag chain as the only conductive point to ground. - Anonymous, Exton, PA
Answer Well, first off, the cart is a hazard as it is used because the top/shelf surface is a large charge generator and cannot be grounded through any means. For the ground strap to work, you have to have a conductive (ESD) floor and all parts of the cart should be conductive as well. To remedy this, place a dissipative mat on top of the cart and ground it to the chassis of the cart which will provide a path to ground through the ground strap (drag chain). There are also cart covers you can use to cover the cart during storage or when mobile to help secure the parts within against ESD. Plain Plastic material is a very bad choice to use around ESD sensitive devices as it can and will create large electric fields that can charge nearby insulators or isolated conductors. Use of dissipative plastic would eliminate this threat. If you are covering a large area, you could use our trash can liners. Use of antistatic spray may help too, spraying the charge generative surfaces to help minimize the electric fields they generate upon contact and separation.
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