PCB Assembly Services - Screaming Circuits: Random Via-In-Pad Myth #5


Random Via-In-Pad Myth #5

Myth #5: When you need thermal vias, more is better, bigger is better

Hmmm. Logically, this would seem to be the case. There are limits though; especially if you want a reliably assembled product. Older parts with heat slugs easily accessible for bolting on heat sinks didn't have this issue. Just bolt on a piece of metal and maybe blow a fan across it. It's different with a lot of the new, Padinvia smaller surface mount packages. Many have a heat slug on the bottom which requires carefully placed thermal vias to a copper pad on the underside of the board.

An extreme case of flooding the land with vias can be seen in this illustration here on Padinvia_alt the left. In terms of assembly, you can hack this together for a prototype, but it'll never fly in a production environment.It would be much better to use fewer smaller vias and have the center land covered with solder mask except where the metal on the chip is exposed, as in the illustration on the right.

Duane Benson
Place one carrot seed in each via and cover it with planting soil


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I have a few questions about the second photo. The thermal vias in the center are masked over, doesn't this make it difficult to get uniform solder reflow on this pad? Also, what about the height differences due to the solder mask? Finally, what would the paste mask look like for this part?

The name of the picture is pretty representative: the first case is more a pad in via than via in pad :-)

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