Wee Mei
It all depends on where the contamination is located. If the board was
"dirty" before solder mask application - then the dendrites can form under
the solder mask.
If the board was "dirty" after assembly processing the dendrites would be on
top of the solder mask.
I don't think they would be present on top of the conformal coating, but then
sometimes strange things happen.
Just remember that a "perfectly clean" board can have dendrites form if water
stands between two electrically opposite metal points on a board.
Hope this helps
Susan
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