We here at Screaming Circuits keep telling people to fill and plate over vias in BGA pads. The same goes for small passives too, like 0402s and 0201s. As I closed in this post, best option is to fill and plate over. Bigger parts can use some of the solder mask techniques, but BGAs — especially 0.5 mm and small pitch — and tiny passives really do require filled and plated vias.
The question of the day, however, is: “Just how do I get those vias filled?” Here again is another place where our old Gerber file format falls a little short.
There are quite a number of different methods for filling vias: Copper plugs, copper plate, surface material (i.e. HASL), conductive epoxy, non-conductive epoxy. Maybe others. But, the Gerbers don’t have a method for specifying this.
So here’s what you do.
1) You need to contact your fab house to determine what techniques they have available and what they recommend for your fab project.
2) You have to create a set of fab drawings or special instructions that point out which vias need to be filled and with what material.
Maybe raspberry jelly filling?