ESD Systems - Questions And Answers

I have been taught that when repairing (working on the inside of) a computer, it is sufficient to connect my ground strap to the metal frame or power supply of the unplugged computer, in order to protect the components from ESD because in doing so, I will be at the same potential as the computer. However, what about the components that are removed from the computer? How are they protected? - Daniel, Lacey, WA

Very good question. Typically, field service work and bench repairs/upgrades should consist of a groundable dissipative work surface, common point ground, ground cord, and wrist strap kit. All ESD sensitive components should be stored in a shielded closed container (sealed shielding bag). It is this work surface that gives temporary protection when handling ESD sensitive devices in or out of its protective environment (packaging). For field service work, the dissipative mat can be connected to just the unplugged device (computer chassis) via the common point as well as the wrist strap to the field technician. This ensures (as you noted earlier) that all conductors (human skin, work surface, chassis and devices grounded to chassis) are at the same potential, minimizing the chance of an ESD event (you can still bring an ungrounded conductor into this protected area and have an ESD event). For bench work, the common point ground would go to the bench (power) ground ensuring that all grounded items on the bench are at the same potential as your sensitive work area. When components are removed from the chassis (equipment) by the operator, they will be grounded only if the operator is touching the ground plane of the device and the operator is grounded via the wrist strap (there may still be exposed floating conductors on the board). ESD sensitive items should immediately be placed into protective packaging such as an enclosed dissipative tote, shielding bag, etc. The dissipative packaging should be at the same working potential (ground) before handling by placing it on the grounded work mat. Or, when components are added to the ESD sensitive work area, they should first be adjusted to the working ground, by being placed on the grounded dissipative working mat. Then opened and the ESD sensitive items removed by a grounded operator. Again, for field service work, you don't necessarily need to connect to ground, but all conductors need to be tied together to the common point ground (mat, operator, chassis, packaging, etc.) to be held at the same potential. It is the difference in potential that enables an ESD event to occur.

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