The biggest single cause of de-wetting (a lack of adhesion) of conformal coating on a printed circuit board is silicone contamination accidently introduced in the production line. The silicones can come from many areas including RTVs on the bench and lubricants for machines.
“Out of all the Key Material Challenges I see on a regular basis, one of the most prolific is difficulties in adhesion to solder resist or to no clean residues and contaminants.“
“It can be as basic as the coating just will not stick to the PCB or it runs away from areas on the PCB. At other times, it’s interpreting just how well the coating is stuck and determining if it’s good enough for the customer? Normally, this cannot be answered instantly and leads to some serious thinking!”