Using many conformal coatings as under fills will eventually result in air pockets beneath the components.

  If they actually get underneath the components solvent borne coatings (most AR, UR and even a few silicones like DC2577) will form a pocket of wet coating which will eventually dry by diffusion of the solvent through the cured coating. If pockets of wet, solvent laden coating are baked the air pocket will break through the dry coating skin.

  Solvent borne conformal coatings significantly shrink in volume as the solvent evaporates.
  What bridges form between board and components is unpredictable.

  It's difficult to displace all of the air beneath components and becomes more difficult as the size of the pockets get smaller. When a PC is dipped in a low viscosity conformal coating using a slow moving machine many tiny bubbles come to the surface. If a board is plopped in and pulled out quickly (as would probably be done if the operator wanted to go on break) there will be no tiny bubbles.

  Placing a wet board in a moderate vacuum will pull out the air pockets. The vacuum must not be too strong or it will cause the solvent in the wet coating to boil.

  The ability of a liquid coating to fill a small space is dependent upon the viscosity of the liquid and it's surface tension.

  Dipping a board in solvent and allowing the excess to drain off will help solvent borne coatings wick into small pockets. It is not absolute though. Very small pockets may fill up with solvent.

  Few PC boards will be effected by sudden changes in pressure. Maybe aeronautical boards should have a different spec. or be governed by special rules defined in IPC documents.

  Ed


Edward Mines BEChE, MEChE
238 Willard Dr.
Hewlett, NY  11557
516 205-6523

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