2. PWAs:  Conformal Coating Non-Wetting

During the conformal coating process, numerous non-wetting areas were observed. The adhesion problem with the coating appeared to be related to the presence of a silicone based adhesive that was used on the board prior to the coating process. The adhesive apparently was not completely removed completely by the cleaning process.

Reference :
NASA-STD-8739.1,
Cleanliness Requirements
7.2 Cleanliness Testing - Cleanliness testing is used to monitor the effectiveness of PWA cleaning processes. All PWA's shall be tested prior to conformal coating.

1. Two basic test methods are recommended.
a. Resistivity of solvent extract (paragraph 7.5).
b. Sodium chloride (NaCl) salt equivalent ionic contamination test (paragraph 7.6).

2. Other test methods must be approved by the procuring supplier before use.