Hi Armynski,
I sell for an offshore board house, yes, it is common for offshore board shops to fill vias. The bubble is caused by air entrampment within the hole it self, when heat is applied during Hot air and the via is covered on both sides the entraped air can expand and blow out making the solder mask blister on one side of the board.
Solder mask in via holes is done for two reasons: 1) Board Designer left no solder mask reliefs for the Via's in their design. Which Manufacture would look at as being requested to 100% Via Plug.
2) Via's were requested to be Filled or Tented. Which the Board house would do a second solder mask operation and put a second coat of mask to either fill or cover the Via Pad. This is normally done do to requests by the assembly houses concerned about paste flowing through the Via and puddling on the other side of the board causing shorts. Normally on BGA's.
Typically this operation is not recommended with boards with Hot air due to its extreme temperatures. Recomended would be to leave a blivit or an opening half the size of the drilled hole to allow for no entrapment of air. Or to change surface finish to Immersion Silver or Gold or OSP.
Hope this helped,
Regards, Boardhouse
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