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, 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 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.
Place one carrot seed in each via and cover it with planting soil