Polygons L2 and L3 for dissipation ¿?

[AndJuan]

Hello,
I'm doing a printed circuit where employment 4 layers (L1, L2, L3, L4) in top layer (L1) there are 4 mosfets with encapsulated D2PAK covered by a big Polygono in L1 and L4 for dissipation with some Vias (1mm/0.5mm), these will support a 30A, like me,
I would like to know if it is good to use the L2 and L3 layers polygons to help dissipation. (View Photo)

THX.

[mairomaster]

It depends what is happening on layers L2 and L3. They are internal layers and cannot dissipate so much heat. However, if you have plenty of space in the area (and no ground planes, etc.), it doesn't hurt to create a big polygon under each MOSFET which will help a bit to spread the heat around the board. It most of the cases it won't be worth it though as I can imagine, since it introduces some limitation and normally you don't have that much space for those. Be careful not to short anything.

[robertferanec]

If the big Polygon is GND / Power, and L2 or L3 layer is a GND / Power plane, it will help to spread the heat. In case it is not GND, and there is space, we sometimes draw polygons on more layers to help the big currents, however, if you are expecting higher temperature you should provide an option for a heatsink and not relay on PCB heat dissipation. Many people are forgetting, that their boards may be placed in a box or in a hot place which will increase temperature of components and even if everything works oki on their table, it may be a problem in real applications. I have seen rapidly rising temperature of components when tested in environmental chamber.

If we are speaking about switching power supply circuit, I am careful about size of the polygon which is running at high frequency - I read that could make some problems with EMC.