Via in pad - tenting the bottom side

BGA via in pad bottom via capSpeaking of vias in pads, here's a reason we're not terribly fond of the technique of using solder mask to tent the bottom side of a via in a BGA pad.

As I mentioned in my last post, capping the bottom of the via with soldermask is sometimes an acceptable method for dealing with a via in pad. It's not a desirable method, but it does sometimes work. However, with BGAs, the top-open via can still wick solder down to fill the space. At the very least, you will end up with a hefty void in the BGA solder ball. At worst, you will end up with an open, as illustrated here.

Best option: Fill and plate over the via.

Duane Benson
Does cheap mass production = this perfect day?

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Comments

Hi Tomothy;

You are correct. Filling a via would have to be specified in a fab drawing or special instructions. This is another area where the venerable Gerber format hasn't kept up with the technology.

So, how does one specify a plated via versus a standard via? This isn't something done in software, but rather a fabrication drawing, right?

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