Nightmare on BGA Street

I seem to be in a bit of a BGA mood lately. I do that sometimes – pick a subject and talk it to death before moving on. Well, maybe not quite talk it to death, but at least talk it to the pain.

9x13 via in pad BGA land Take a look at this land pattern for a bluetooth module. Anyone see anything odd? Yeah. All of those really big open vias. I know what the designer was trying to do. A good number of the vias are ground connections of one sort or another that need to be connected to an internal ground plane layer.

Given that is is a 1 mm pitch BGA, there is plenty of room to put the vias between the pads and not cause any trouble. That would be one recommended approach. The other would be to have the vias filled and plated over at the board house. No matter what you do, though, the vias can’t be left wide open like this. It’s a real bummer.

Duane Benson
B.V.O.U.S.’s? BGA Vias Of Unusual Size. I don’t think they exist.

How to Fill a Via

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.

Duane Benson
Maybe raspberry jelly filling?