cheapest isn't always lowest cost.

It's extremely difficult to control solder mask, so if you are using it as a via fill, plan for misses.  Some vias will not be filled or tented, so you may still get solder flowing through to the bottom.  Some will be tented but not filled, trapping gas and causing voids and loose solder.  If your design is able to tolerate this, you can save that 1 or 2% of fab cost.  If not, that 1 or 2 or even 5% fab savings will become a 10% increase due to assembly failures - or customer returns.

If this is a lead free assembly, you may find that you get a higher percentage of good boards with no bottom side bumps simply by careful attention to paste deposition patterns and reflow recipe.

Pete

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