Posted: Sat Mar 24, 2012 1:35 pm Post subject: Cooling question
Dear all,
Does anybody know which of these two alternatives is better from a heat conductivity perspective?
1. Placing for example a TO-252 on the top side of the PCB, with numerous vias to the bottom. The bottom copper plane is thermally connected to a cooling surface.
2. Placing the same component on the bottom side of the PCB. The plastic component material is thermalliy connected to a cooling surface.
In other words: do vias through the PCB conduct heat better than the plastic component case?
The best would be a direct center tab land pattern extensionon the same plane. Some datasheets mention a certain wattage in reference to X mm sq. Thru holes have a limited area depending on coating thickness and it also depends on the final mounting of the board for heat dissipation. If there are a lot of vias under the soldered center tab, it will probably be better than the plastic case connection.
If you have an array of D-PACs then the plastic case thermal connection is the most simple solution with a cooling plate on top. For single components I have seen metal cooling pieces glued to the top.
It depends a lot on the wattage. Copper is always better than Aluminium (US Aluminum).
You will have to take a look at the datasheet to see how much heat (wattage) to dissipate for your application and consider free air derating if no fan is involved.
One possibility would be to reserve (if only a few) an area on the bottom and use part of the separated plane area as center tab extension.
Hubert
If the room was available I would have it on the underside and a plated through mounting hole close to the tab so I could have a brass threaded standoff to go to the heat sinking area. In one board that I made by hand I had two holes close to the tab of a TO220 size Dpak then inserted some brass brazing wire into the holes and soldered both sides which transferred the heat to the ground plane on the underside, this worked well.
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