September 2, 2009 AT 12:00 am

Vias under power transistors

Screaming circuits always has handy tips on what not to do

Normally, a power component like this would have a big pad and some heat dissipation space under the heat slug. It might even have thermal vias going to another heat pad on the back side of the board. If it’s being run at anything close to it’s full current capability, that would be a requirement. This one is probably a bit underutilized though, rendering the big pad un necessary. It’s still a good idea for mechanical strength, but in the prototype world, we don’t always follow the rules

The problem here though, is that the open vias under the part can short to the thermal pad. That’s bad mojo. Some board fab houses can put a solid coat of mask over the vias and they’ll be okay. Not all board houses will do that though, and if the board has a silver finish (this one is HASL), you wouldn’t want the vias sealed because silver vias can outgas and corrode if completely sealed.

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  1. Would probably put a thin bar pad at the top of the ground tab for mechanical strength. Don’t need to follow the rules, but if anyone knocks the device, it’ll tear the pads right off. Just my 0b10 cents.

  2. The tab may touch the vias and short them. Or it may not. If that right there doesn’t tell you what’s wrong with this idea, I don’t know what will.

  3. myself, oops we copy&pasted the wrong paragraph. this is an example of what -not- to do! its more clear now 🙂

  4. How about not putting vias under thermal pad in the first place? If you face the possibility of shorting vias, then your design strategy needs to evaluated.

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