Using osee for Selecting among alternative surface cleanling processes

Abstract :
Industry faces a dilemma in trying to select the "right" cleaning process. Evaluating alternative cleaning processes requires the answer to two questions: (1) is the new cleaning process as effective as the one being replaced? (2) Does the new cleaning process leave behind any residue that could be detrimental to subsequent processing or performance? This paper discusses reasons for monitoring surface cleanliness, factors affecting the selection of cleanliness monitoring method and a simple-to-use technique for quantifying the level of surface cleanliness to help answer both of these questions. The technique can be used to; Establish, quantitatively, the cleanliness level achieved by an existing cleaning process; Measure the cleanliness level achieved by the alternative cleaning processes to help select the most effective and cost efficient alternative; Monitor the effectiveness of the cleaning process on an on-going basis.

Why Monitor Cleanliness
Presence of contamination can degrade the performance of parts, components and systems, results in non-conformance and, in the worst case, product failure. Molecular contamination of surfaces can drastically affect the performance of the parts. Thin film contamination on surfaces can result from inadequate or incomplete cleaning methods, from oxide growth during the time between cleaning and performing the next operation, or from failure to properly protect cleaned surfaces from oxide growth during the time between clearing and performance of next operation, or from failure to properly protect cleaned surfaces from oils, greases, fingerprints, release agents, or deposition of facility airborne molecules generated by adjacent manufacturing or processing operation.

Cleaning is part of many manufacturing operations. Parts may require cleaning before they can be electroplated or painted, before they can be soldered, or before they can be packaged and shipped for end use. Thus cleaning is necessary for various reasons to assure desirable product appearance or performance.

In most cases, control of cleaning processes is achieved by specifying the operating parameters of the cleaning process, e.g. chemical concentration, temperature, water pressure or the amount of time the parts are washed or rinsed. This approach defines how "clean" a part should be by specifying the process used to do the cleaning (i.e. dip Part A in Cleaning Solution B at Temperature C for X minutes), without regularly checking how clean parts actually are. This approach takes advantage of knowledge gained through experience with the cleaning process or through measurements taken during initial testing of the cleaning process. This method while practical and good most of the time, cannot be consistently relied upon for precision cleaning. This type of procedure generally also specifies the properties of the cleaning solution and replenishment of the chemicals on a periodic basis. This approach does not take into account the number of parts that may go through the process in a given period of time. The more parts go through the cleaning process, the more contamination is removed from the parts, which is mixed in the cleaning solution. It also does not take into account the amount of contamination present on each part. The type and amount of contamination on each part varies from time to time and from vendor to vendor. Thus, this approach to assuring cleanliness works only if the average number of parts and the average level of contamination on each part are consistent during a given period of time. If this condition is not achieved, the part cleanliness level will deteriorate below the acceptable level. Without the use of a surface cleanliness monitoring method, the lower level of contamination will not be detected until there are problems downstream.

Hence, in many cases, it is more effective for a level of cleanliness to be specified and have that level checked by measuring cleanliness on a percentage or all of the parts. This is especially true in precision cleaning applications.