cleaning spec

I am looking for an industry standard �stock� note to place on our printed circuit board assembly drawings that will capture a level of cleanliness for an IPC-A-610 class 2 board assembly. I�m looking for some quantitative spec for cleanliness that we can call out which would meet our requirements. I have found an Ionic contamination spec for a class 3 which reads 1.56micrograms/cm2 NaCl but none for class 2. I have virtually no experience defining the level of cleanliness of boards and what the industry uses as a standard practice for this requirement. I�m not even sure if ionic contamination is the appropriate quantitative measurement. I�d like to get your recommendation on this and any information or insight on this subject matter would be greatly appreciated.

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