soldermask over vias

[rodster]

The fab house has always asked if they can pull back the soldermask on vias holes by .004" to prevent the possibility of chemical entrapment. We've hadn't had any issues with this. Is this a good process or shall I request they cover with soldermask? What are the pros and cons? Thanks.

[Carl]

The fab house has always asked if they can pull back the soldermask on vias holes by .004" to prevent the possibility of chemical entrapment. We've hadn't had any issues with this. Is this a good process or shall I request they cover with soldermask? What are the pros and cons? Thanks.

You certainly don't want the holes closed on both sides. If both sides are tented, contaminants can get trapped inside the barrel, and the via could explode.

I you are refering to just pulling the soldermask away from the annular ring, that is pretty standard. Most fab houses in my experience pull back the solder mask .003" from all surface lands.

If you have a situation where you have a fine pitch BGA, or features that you are forced to make close and your worried about solder bridging or something like that, you could just tent one side, or you could have them apply soldermask over the annular ring and just leave the hole open.

Or .004" open around the hole for that matter.