Why specify different static control parameters for tabletops than flooring?
The advice you were given was correct: Table coverings and flooring materials, because they perform different functions, require different electrical parameters.
Table coverings solve CDM (charged device model) problems—that is, they prevent static from building on and discharge static from sensitive electronic parts. The static on sensitive electronics must be discharged slowly—or the rate of discharge could fry the internal circuitry.
Static control flooring solves HBM (human body model) problems—it prevents and eliminates static on people, carts and moving fixtures. Static charges on the human body must be discharged rapidly—before the person comes into contact with static-sensitive parts.
Further information on static control terms CDM and HBM provided below.
Rules of thumb for electronic parts handling:
- Table coverings should be static dissipative – not conductive.
- Static control table coverings should provide static dissipation in any environment, regardless of the relative humidity.
Hard laminates eliminate static by pulling moisture from the air; the moisture collects on their top layer. This reliance on relative humidity inhibits the ability of hard surfaces to control static when the air is dry. For this reason, hard materials are less desirable as table coverings.
Flooring materials should be conductive – not static dissipative.
The floor should discharge static from moving people—regardless of activity level—rapidly, faster than new charges can accumulate. When mobile people wear static control footwear, a good conductive floor will prevent the accumulation of static above 20 volts. Effective floors also exhibit anti static properties—in other words, they prevent excessive static charges from accumulating on people who are wearing standard footwear. (Read my recent article explaining the difference between conductive and dissipative static flooring.)
As with table coverings, the floor should work at all humidity levels.
The difference between CDM and HBM failures
CDM or charged device model failures occur when charged parts – like circuit boards – discharge to conductive objects. Example: an ungrounded person holding a circuit board places the board on a metal plate. The static passed (from the person) to the board will discharge rapidly to the plate. Under the right circumstances this rapid discharge will result in a CDM failure.
Static dissipative table coverings are the first line of defense in the prevention of CDM problems.
HBM or human body model failures occur when charged people touch sensitive electronic parts. Example: A person walks on a static generating floor (or a highly resistive static dissipative floor). The charged person touches a circuit board. If contact occurs between the person’s hand and a sensitive electronic part, the resulting discharge – called a human body model discharge– could harm or destroy the circuit board.
Static conductive flooring is a key element in controlling HBM problems.