This is interesting topic that I would like to share and discuss with you..
I came across few projects from various SMT factories where its product(Computer) reach field, experience No Post/boot defect. This problem become very obvious when there is heavy rain.
When the affected product returned to SMT factories for verification, they found that this problem cannot be duplicated. However, when they put these affected motherboard into humidity chamber where they set it at 90% humidity level, they can see the problem..When decrease the humidity(Less than 90% humidity level), the problem disappear..
This problem also can disappear if we clean the motherboard bottom side, especially near to the BGA bottom side...This action lead them to believe that this could be flux residue that induce this defect...
However,as far as I know, flux residue can cause this problem when its absorb enough moisture and become conductive which lead to low impedance. This problem should not go away even the humidity drop less than 90%..What I mean is that this problem will stay there unless we heat it up which make the flux residue vaporize..Am I right?
At the same time, we also found that wave flux solid content is out of specification. Its current value is 1.22wt% and spec call for 2.5% +/- 0.5wt%. Is there any relationship between low solid content and No Post issue??
Note: Surface Insulation Resistance (SIR) test for this flux is within specification.