Technos,

I have sort of rookie questions on the following situation:

when reflowing a DPAK, let's say, and the pad thermal pad has vias, the solder will flow inside the via. If the holes are covered with solder mask on the other side, will flux residue build up at the bottom of the hole, like a void between the solder and the solder mask? Or it will rather stay on the surface?

Or, if the DPAK is printed with a nicely shaped stencil, so that no solder goes into the hole, but this time there is no solder mask covering the hole and I fill the vias from the bottom, manually, can I generate a flux pocket somewhere, or the flux only stays on the surface?

And if the flux, water soluble, is entrapped therefore not washable, will it be corrosive? 

You must have guessed the application, RF boards, with all these components that have tons of vias under them, vias that must be filled for max thermal transfer, but the solder shall not go on the other side, because the other side is an exposed plane, etc.

Thanks,
Ioan

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