Consistency as the True Measure of Cleanroom Wiper Quality
This article is based on an original Texwipe publication by Sandeep Kalekar and Jay Postlewaite.
The modern cleanroom is engineered to maintain low levels of contamination within while also preventing impurities from entering the controlled environment. "Contamination" may have different meanings in various controlled areas - while a pharmaceutical company may consider only large fibers a threat, trace levels of elemental contamination could devastate semiconductor production.
Widely recognized as a cleanroom essential, wipers are used in multiple industries as an element of cleanroom protocol for proper maintenance. Purity is critical as wipers themselves can serve as a source of contamination if improperly evaluated.
Measuring Wiper Quality
Cleanroom wiper quality is measured using a range of performance characteristics such as fabric substrate, micro-structure or sorption capacity using standard test protocols. The three main types of contamination being assessed are particles and fibers, ions, and non-volatile extractable matter.
Particles and Fibers
Particles and fibers are measured by first extracting particles into a solution and then counting them. This solution can be pure water, or water based with an additive that lowers surface tension. The motion used to move particles from the wiper to the solution will vary the quantity of particles being counted, with a more vigorous motion resulting in an increased number of particles.
Even once wiper fabric has been processed, extractable or leachable ions often remain. Typical cations include sodium, potassium, calcium, magnesium and ammonium, while typical anions are chloride, fluoride, nitrate, sulfide and phosphate. Wipers are soaked at a specific temperature and for a given time to extract the ions for measurements. Varying temperature and time periods offer different information about the wiper. Extractions performed at elevated temperatures for a short time estimate the maximum ion contamination the wiper contains.
Additives and oils used in fabric manufacturing can linger on wipers once the fabric is processed. To measure these contaminants, the wiper is soaked in solvent of a given temperature for a specified time. Again, the time and temperature of the extraction will impact the amount of matter extracted from the wiper. Extracting the wiper at or near the solvent's boiling point will remove more material.