Testing Wipe Cleanliness - Considerations and Limitations

Most recognized test laboratories that are contracted   to test cleanroom wipes for cleanliness follow test methods detailed in published  versions  of  the Institute of Environmental Sciences and Technology (IEST) Recommended Practice 4 (IEST RP-CC-004.X). Currently, the IEST has an active Working Group (WG-004) that is revising and improving the current version of these wipe test methods. For the first time ever, it is expected that the revised RP004 will include wipe selection guidance for end users in most major critical environment markets.

While the test methods detailed in the IEST Recommended Practice are sometimes used  to  evaluate wipes for various cleanliness and performance characteristics (particle, residue, extractable ions, sorbency, etc.) in comparison to established specifications or requirements, they are often used to compare wipes from different manufacturers relative to a wipe currently used in a facility. Often this comparison is made in the absence of an established specification, but rather the incumbent wipe is delineated as the acceptable benchmark for wipe cleanliness and performance, against which other wipes are compared.

While there is typically a strong desire to compare wipes using manufacturers’ published wipe cleanliness data, or to compare data from some previous wipe testing to more recent testing of the same or different wipes, this is plainly not possible, especially for the characteristic of particles. The particle cleanliness of all cleanroom wipes falls within a certain range for any given wipe. Indeed, the goal of wipe manufacturers is to make that range as narrow and consistent as possible through control of all the contributors to wipe quality – fabric, construction, processing, converting, laundering, and packaging. While these components are all regulated, an inherent and natural range of variation still exists to lesser or greater extent depending upon many factors, including manufacturer.

Compounded with this range of wipe  variation  are  the very real variations and variables that exist in the testing of wipes, and specifically the sampling and enumeration of particles released from wipes. Sample preparation, equipment condition and calibration, and technician skill and diligence all affect wipe particle test results within a given test lab.

Importantly, it is the differences between test labs that provide the greatest disparities in particle data for a given cleanroom wipe. Even when two test labs are following the same published test methods, differences between labs have significant influence on the test results finally reported. These differences include, but are not limited to:

• Particle sampling equipment
• Particle enumeration equipment; brand, type, sensor, calibration, condition,
• Sampling volume and rate
• Laboratory practices
• Test solutions
• Interpretation of test methods
• Specific sampling and enumeration techniques
• Data interpretation, calculations, and reporting
• Test environment
• All of the above at a specific moment in time