Let’s start with a quick story titled “The Mystery of the Talking Plant”.
On a dry, wintry day a son calls out to Mom.
MOM!!! The plant in the living room is talking to me, really!
“No, seriously Mom. I had just finished vacuuming and the plant next to me started making a buzzing noise.”
Bemused, the Mom asked “What was it saying?”
“I don’t know. Just a buzzing noise.”
“Sweetheart, plants don’t talk.”
A few minutes later the son said “I just got zapped”.
Talking plants? Nah. Just ESD (ElectroStatic Discharge) at play in the home. The buzzing noise the son heard when he was near the plant was the sound of air molecules ionizing between his body and the plant. As he edged closer to the plant to hear what it was “saying”, the entire charge that his body had accumulated when vacuuming the carpet, discharged instantly to the plant, zapping him as if he had touched a doorknob.
Objects can build up electrical charge on their surfaces by a process known as triboelectric charging (or triboelectrification – the rubbing of two dissimilar insulative materials against each other (think slippers walking on carpeting). ESD events occur when an object (think human body) retains an electrical charge then releases all of its accumulated charge to another object with little or no charge on it (think plant or doorknob).
Build up of surfaces charges are frequently ignored because the charge usually bleeds off into the surrounding air with little effect. In a semiconductor, disk drive, flat panel or medical device (e.g. pacemaker) manufacturing environment, ESD events can ruin sensitive components. And it doesn’t take 25,000 volts (on our poor lady above) to cause problems. Fractions of a volt can wreak havoc with unprotected (i.e. unshielded) pacemaker circuitry. So manufacturers have learned how to assemble sensitive devices by taking the necessary steps to eliminate undesirable ESD events.
Not quite and not yet. Many of the above manufacturing procedures are conducted in cleanrooms, since the devices must be protected from particle contamination. Virtually every controlled environment must incorporate cleanroom wipers to wipe down critical surfaces to remove particles that have settled (once particles have settled on a surface only wiping will effectively remove them from that surface).
The cleanest cleanroom wipers generally are manufactured from knit polyester (or other synthetic) fabric and packaged in plastic bags to protect the wipers from contamination.
Can you see the problem yet?
When a dry cleanroom wiper is removed from a plastic bag, you have one insulative surface (the wiper) moving against another insulative surface (the plastic bag), setting up triboelectric charging of the wiper. Bring that charged wiper near to an unprotected device and you have the perfect condition for a destructive ESD event.
Fortunately, there is a solution to the problem. Simply dampening the wiper with deionized water, or a mixture of deionized water and isopropyl alcohol (any proportion), changes the wiper’s electrical characteristics from insulative to dissipative, thereby eliminating the possibility of an ESD event. Using pre-packaged pre-wetted wipers is an even better approach, since dissipative wipers are removed from the package, eliminating the triboelectric charging effect. As an added benefit, it is well known that dampened wipers are much more effective in removing surface particles than dry wipers. In this case, dampened wipers are much safer from an ESD perspective, providing the perfect approach to wiping ESD-sensitive surfaces.