Screaming Circuits: A Little Pad In That Via


A Little Pad In That Via

Here's an interesting via-in-pad situation we ran across a while ago. I'm not sure what the chip is. Probably a processor or an FPGA that generates a bit of heat.

The intention with the layout was on the right track. However, the implementation wasn't really workable. All of those big vias were put there to sink heat away from the thermal pad on the bottom of the chip. The vias being so large, heat would be removed by air convection as well as conduction through the via walls. The thought is good.

Padinvia

In practice though, there are a couple of big problems with this approach. First, the pad on the PCB is much larger than the pad on the chip and the extra area is not masked. Even without the vias, that could make for more difficult solder control. The biggest problem is simply the large open vias. It really isn't possible to solder such a layout with standard surface mount process. Since this is a prototype, we found a way to make it work, but for volume manufacturing, as well as for cost reduction in the prototype, a different approach is needed.

Padinvia_alt

In the second image here, we've simulated what a good workable footprint could look like. This isn't the only possible way to do it, but it would work well. Here, the exposed pad matches the size of the heat slug on the chip. The rest of the pad is still there for thermal conduction but it is masked off. There are a generous number of thermal vias in the masked area and a few capped vias in the solder area. Even better would be to plug and copper-plate over the vias in the solder area, but this way works too and is less expensive at the board fab house.

Duane Benson
Flying spaghetti monster says what?

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