Antistatic wrist straps or ESD wrist straps are worn by people working in static sensitive devices and assemblies to prevent them building up charge on themselves which could be transferred to the static sensitive item.
Typically wrist straps are used with other anti-static precautions so that the item being worked upon is in a static free environment where charge which might damages the components is safely dissipated.
As movement by people generates static charge, this charge needs to be given a safe route to ground, and the antistatic strap enables this to be achieved.
ESD wrist strap basics
The ESD wrist strap is worn by the operator working on an item of electronics, providing a high resistance path to ground and in this way prevents static build up which could harm the electronic circuit.
The antistatic wrist strap must not be directly connected to ground. Instead the connecting lead has an inbuilt resistor. This serves two functions:
- Dissipates power: One reason for incorporating the series resistor from the antistatic wrist strap to ground is to provide a leakage path for any static build up. The slow discharge prevents damage from sudden static discharges.
- Provides safety: More importantly, the series resistor in the lead, provides safety for the user. In the unlikely case of there being a mistaken connection directly to a high voltage, the resistance, typically 1MΩ prevents a major electric shock from being obtained. With the wrist strap being around the users wrist, this could present a serious hazard.
Beware: It is possible that some coiled leads may not have an inbuilt resistor. When using new leads that may not be originally intended for ESD applications, or may be from an unknown source, check that the lead has an inbuilt resistor. If a wrist strap tester is used, this should flag the issue.
Normally the wrist strap is connected via a lead to the bench or other suitable grounding point. Also the lead is coiled helically to enable the user to mover freely. The coiling prevents the lead from trailing one the work and also enables it to be used more conveniently.
Although there are many varieties of antistatic wrist strap they often have an outer surface made from a non-conductive material and an inner surface that is conductive, often using silver yarn.
The lead can typically be obtained in two varieties:
- Using 10mm snap connector: One version uses the 10mm snap connectors at both ends of the lead. One snap connector fits onto the wrist strap, and the other with the identical connector connects to the bench top / mat / mains ESD connection point using another 10mm snap connector. This makes the lead identical at both ends.
- Using crocodile / alligator clip : On some versions a snap connector is used at one end of the and a crocodile clip at the other. This enables the antistatic wrist strap to be connected to a variety of earth points.
Antistatic wrist straps are an integral component in any antistatic work station or work area. They prevent the user from accumulating static charge that could damage any electronic assembly or component that are using.