Static Control in the Cleanroom
Based on an original publication by Larry Levit, Ph.D.
Though often overlooked, the presence of static electricity in the cleanroom can create serious issues resulting in reduced overall profitability. All too often, this effect is simply written off through budget changes citing unforeseen issues. However, with proper understanding the issue of static electricity in the cleanroom is wholly preventable.
What makes a cleanroom unique?
Often maintained at a low relative humidity (RH) to optimize production, the cleanroom is designed to exclude most particulate contamination through the use of high efficiency particulate air (HEPA) filters. Any object brought into the cleanroom is first wiped with special cloths or cleaners, which unfortunately also produce static charge – particularly when coupled with the low relative humidity level.
Usually, static charge is naturally dissipated through conduction to ground via surface contamination or residue. In the cleanroom however, products are handled with gloves and therefore remain clean. Surface charge is also naturally dissipated through ions in the air generated by radioactivity, running water, electrical motors and similar factors. In a cleanroom these ions are unable to pass through the HEPA filter, and they are automatically removed from the air as it enters the room. Use of glass or plastic insulators like Teflon™ initiate further complications with an aggressive hold on static charges.
As a general rule, cleanrooms efficiently generate static without a means of dissipation and the resulting charge far exceeds that present in a conventional environment.
Static charge in the cleanroom
Static charge effectively attracts objects with an opposite charge, repelling objects with the same charge. Not only does electrostatic discharge cause physical damage to electronic devices, but it also radiates high frequency electrical pulses through the room. These pulses can mimic a software bug, corrupting microprocessor-controlled applications, causing further time delays and more potential damage.
Because static charge poses a serious contamination issue, a great deal of effort is placed into establishing airflows to transport any contamination in the cleanroom beyond the product before it has the opportunity to settle. Unfortunately the electrostatic force of a particle in the cleanroom outweighs the aerodynamics of this engineered airflow.