Posted on 23 Jun, 2015
Q: What’s so special about a "Static Safe Chair”?
A: A lot more than most people think …
There’s more to having a chair in an EPA than just sitting on it. They can not only generate and retain an electrostatic charge, but also subversively act as an inappropriate shelf or work surface …
- By simply getting off a chair, it is possible to generate & retain an electrostatic field well in excess of 3kV; which is the approximate threshold of when you start to feel an ESD. The amount of charge will depend on many variables, e.g. seat material, clothing material, humidity, surface contact & speed of separation, etc.
- The fabric & plastic backing used on some chairs are excellent at generating & retaining electrostatic field / charge. If an electrostatic field is close enough to an ESDS Device and induces sufficient charge to cause an ESD, there is a risk of latent or catastrophic damage.
- Finding free space on the workbench is sometimes easier said than found. It is surprisingly common to “just be quick” and place a PCB on the seat of a chair “while I just …”. As soon as you place the PCB, or any other ESDS Device for that matter, on the seat, it becomes a “shelf / work surface” within an EPA and as such needs to meet grounding requirements.
There are different solutions available:
- Antistatic chairs that generate and retain little or no electrostatic charge when alighting; there is no guarantee of grounding antistatic materials.
- Conductive / dissipative chairs that can be grounded; grounding via conductive glides, castors or metal chains*/balls*.
relying purly on grounding via metal chains or balls can be effective, but the size / weight / contact surface area of the chain / ball will make a huge difference to the grounding efficiency, along with the surface finish & resistance of the floor.
To compound all of that, dust & dirt that gets caught between the chain / ball and the floor can increase the path-to-ground resistance to a point of open circuit. - Fabric covers made with a carbon fibre grid can be fitted over the existing chair seat / back & grounded if necessary.
- Fabric chairs can be sprayed with a topical antistat in order to make them more antistatic