Random Via-in-Pad Myth #7

Myth #7: In regards to via-in-pad, all PCB finishes are the same


Well, it might seem so, but let’s look a little closer. No. Not that close. Back the camera up a bit.

Here’s a good example: In some cases, it’s okay to seal off the via with soldermask on the opposite side of the board. It’s not the optimal way to do it, but when the geometries aren’t that small, it can work. It needs to be a part where voiding isn’t an issue, because the solder may still go down the via and cause some of those voids. “Void” may be accepted in C code, but it’s usually bad form in a PCB.

Getting back to the subject… Immersion silver gives a nice smooth surface. It’s fairly easy to solder and provided the boards are used promptly or stored properly, it’s a good RoHS choice.

BGA via in pad Silver But, it’s not a good choice for a situation where you cap a via with soldermask on the underside of the PCB. The immersion silver finish will likely out-gas a bit and when contained, as in the sealed off space between the solder on the top and the soldermask on the bottom, that outgassing can be corrosive and lead to reliability issues sometime during the life of the product. So if you do need to have vias that are capped on the bottom side, you should consider a surface finish other than immersion silver.

Duane Benson
No more silver on Walden Pond