How Clean is Clean?
While washer design and process layout has been proven and accepted to work for different industries and applications, for general cleaning applications, “clean” and “clean enough” have been a bit difficult to quantify for non-critical applications. Different standards have been developed through the years (primarily developed and directed by the electronics industry) and adopted by relevant areas of industry. These standards involve methods for post cleaning checks to verify surface energy, counts for particle size and quantity per square area, as well as lesser-related checks such as visual surface topology. These generally accepted methods and tests for verifying clean surfaces are listed and briefly discussed in the process verification section of this document.
In general, the washer design must maintain an achievable level of cleaning sustainability, provide an accepted level and ease of servicing/maintenance for the industry, and provide a cleaning process capable of generating clean glass beyond the minimum threshold required for the application. Under-cleaning will not achieve the surface standards required for the application, while Over-cleaning involves unnecessary equipment (higher capital investment), and also make such process equipment unnecessarily expensive, difficult, and more resources to maintain the level of cleaning quality.
The level of clean glass processed on the washer should be recorded soon after installation, post-startup, and validated at regular intervals (see post-process verification in later section), to determine potential issues before they cause production downtime or failed assemblies.
For preparing the surface for additional processing, consider not only how “clean” the surfaces are required for presentation to the process, but also the contamination existing on the surface, requiring removal prior to the process. In general, contamination is defined as molecular contamination (residues; non-particulate contaminates) and particulate contamination (particles) on the surfaces.
Examples of molecular contamination :
· Noncorrosive thin films (corrosion or chemically absorbed contaminates are not included in this type)
· Cutting and manufacturing oils and fluids
· Finger/hand prints from human contact with the surface (skin oil)
· Greases (ingress into the washer, or roll printed from an upstream process)
· Suction cup marks
Examples of particulate contamination :
· Packing materials and airborne contaminates deposited onto surfaces by electrostatic charge
· Grinding fines/swarf
· Fine pieces of glass slivers generated from the cutting and breakout process
· Loose/electrostatic chips/particles (lightly adhered), or adhesion chips/particles (bonded chips)
· Interleaving Separators such acrylic (PMMA) beads, paper fibers from paper interleaving sheets, cotton fibers from string interleaving
· Soils from environmental buildup, upstream from the washer
· Suction cup marks
· Dried minerals and salts from upstream surface wetting