I am routing a PCB with MOSFETs in LFPAK (aka SOT-669, Power SO8) packages for the 1st time. They look like D2PAK. The manufacturer, NXP, advises to draw identical polygons on top and bottom sides, connected to the drain tab, and linked together by vias, to transfer the heat from top to bottom side (bottom side cooling). They suggest to put the vias right through the drain pad, and around it.
Vias
They mention vias with 0.8mm (32 mils) hole diameter, but they don't comment on how they came up to this value. Isn't it too big ? I am concerned about the solder paste filling all the vias and the MOSFET not being soldered well. I'll have them soldered in an oven.
My reference : https://assets.nexperia.com/documents/application-note/AN10874.pdf, page 16.
Polygon connect
Should I use thermal relieves to connect the MOSFETs to the polygons ? I've read I shouldn't use them for vias.
Best Answer
No, don't use thermal relief vias. The reason they are that large is so that they WILL fill up with solder, that will make a strong heat connection to the other side of the PCB. I have done several PCB's using this method, and it works well, the MOSFETS will solder just fine.
The PDF used 1 oz for its simulation, which usually is 1/2oz base and 1/2 oz plate. I would recommend using at least 2 oz copper, (1oz base, 1oz plate) if you have other smaller SMT parts on the board, or a heavier plating if just the MOSFETs. If you can get 3oz or 4oz, you will have a much better heatsink.
Look on page 17 of that PDF, more holes used to link the layers the cooler the MOSFETS ran because they had better thermal connection to the other layers of PCB.