Mr. Naisbitt is correct that machine dipping PC boards in diluted conformal coating will get the coating under every component.

  The performance of the machines that he sells compared to dipping boards in by hand surprised me and I had a lot of experience in chemical engineering pilot plants prior to working with conformal coatings. The slow speed of the machines (dipping inches per minute) causes many small bubbles to form that are not formed when dipping by hand. Slow withdrawal keeps these tiny bubbles on the surface of the liquid rather than the surface of the board.

  As mentioned in an earlier message, solvent borne coatings (types AR, UR & even some silicones) loose considerable volume as the solvent evaporates so the cavity beneath BGAs will have a significant gas space.

  I have seen several instances where these "air" spaces blew out between wires when the board was baked (the solvent comes out quickly and the gas is expands). If the board is not baked solvent will slowly diffuse through the cured coating until there is no more solvent.

  Oxygen or moisture is necessary to cure one part polyurethane conformal coatings and most silicone conformal coatings. Diffusion of air through cured conformal coatings is slow so it's likely that it will take a long time for these coatings to completely cure, if ever. Some heat curing silicone coatings rely on the release of a reaction inhibitor when heated; these may not cure completely or even cure at all in thick cross section (the distance between the center of a BGA and fresh air is much greater than the maximum conformal coating thickness called out by IPC-CC-830).

  These issues should be addressed in both IPC-CC-830 and the IPC conformal coating handbook.

  Ed Mines


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