When the solution is simple, God is answering. – Albert Einstein
The relationship between facility cleanliness and product cleanliness, between design criteria and operational conditions is one that even today oftentimes remains a mystery. Particle fallout is the source of visible contamination for contamination sensitive products during storage and assembly. While working in clean facilities helps minimize this effect, particle fallout still represents a major source of particulate contamination. While cleanroom air may be continuously monitored, limited information exists to correlate these measurements with expected particle fallout levels.
Particulate cleanliness levels are specified by IEST-SDT-CC1246D and the standard assumes a freshly cleaned surface. The effectiveness of surface cleaning is proportional to size, so larger particles are more easily removed. At the same time, particle fallout rates are proportional to size, meaning that larger particles are more likely to land on exposed surfaces instead of staying in suspension and being removed by the cleanroom air handling system.
For aseptic fill and semiconductor manufacturing applications, the classification of cleanroom required is readily ascertainable, e.g., due to regulatory requirements or line width/“killer defect” analysis. But what about all of the other cleanroom applications such as medical device, aerospace, optics, injection molding, photovoltaic cell manufacture, etc.? We routinely field questions from these general cleanroom users who are finding visible contamination (often labeled as “Foreign Object Debris” or “FOD”) on product, even in Class 100 rooms! Clearly, unless the HEPA filters are damaged (which can be readily diagnosed by leak testing) then the contamination is not coming through the filtration system, but is being generated by activities carried out within the room or may be evolving from the product itself.
We perform diagnosis and testing in these situations by setting out four-inch silicon (optical quality) witness (collection) wafers to measure and analyze the surface particle fallout within the facility/process line. This test enables us to determine the size distribution of particles that are actually landing onto hardware and helps us to identify probable sources. If need be, we can also have the collected contaminants processed for compositional analysis (FTIR and/or GCMS).
With increasing economic pressures—even in the face of commensurately increasing product sensitivity to contamination, few users are willing to rely on overdesigned rooms with excessive numbers of air changes to ensure the necessary product yields. Good tooling and equipment layout practices, appropriate placement of HEPA filters, and even more importantly the positioning of the air returns, robust pressure differentials, attention to work surface height, product orientation, exposure time, and sensitivity to contamination, rigorous cleanroom certification testing, continuous monitoring of operational airborne particle counts, compliance with the latest electrostatic protected area rules, regular cleanroom housekeeping, proper gowning, and strict operational and maintenance procedures (and enforcement!) are all part of the solution.
Most cleanroom providers have little operational experience and users may not understand clearly how to build clean and keep clean versus trying to “inspect quality in at the end.” It is evident that airborne particle counts in and of themselves are not sufficient to characterize a cleanroom, nor are airborne particle counts necessarily related to product surface cleanliness levels. Producing quality product cost-effectively with minimal need for contamination- related rework requires a systems engineering approach to development of a successful clean manufacturing strategy.