Silver via problems

Here's an interesting via situation that I hadn't heard about until just recently.

In these two cases, the engineer has a QFN with a center pad that is required more for grounding then for cooling. Given the lack of criticality, it would seen that the part would give a lot more layout flexibility.

In the photos you can see the area to be soldered clearly. Surrounding that, there are fill areas, traces and vias, all masked off. Look closely at the vias though and you'll see that they aren't completely masked off. There is exposed metal on all of the vias. This creates a risk of shorts to the center ground pad. It may work, but if the QFN doesn't lay perfectly level with an air gap, its center pad can short to these vias. The fact that the intended solder points have vias in them means that it is likely that the QFN will be sucked down flush to the board.

What happened here? The engineer intended for the vias to be masked?

Well, it turns out that silver, the board finish in this case, can have problems with fully sealed vias. The silver surface can outgass a bit into the void and cause corrosion. The board house does not cap vias on silver boards to prevent that and without perfect registration, areas of the annular ring end up exposed, as in this case.

According to the board house, gold and HASL surfaces do not exhibit the same problem. We discussed a couple of possible solutions, but in the end, the engineer had the board remade with an ENIG finish with completely capped vias.

Duane Benson