I'd like to connect a D2pak device to a heatsink underneath a PCB. How can I optimize heat transfer, and how do I calculate the thermal resistance of the board?

I know lead is a poor heat conductor compared to copper...... but should I insert 10 smaller vias vs. 1 large via? These would be plated through holes obviously, and probably filled with solder to help conduct heat.

Ultimately I just need to know if I have 10 watts in, and a 5 degree C/W heat sink on the other side, how hot is the whole dang thing going to get?

 

http://rfdesign.com/microwave_millim...n_temperature/

Excellent article.

Summary: lead heat conduction is negligible compared to copper (order of magnitude). You can find the thermal resistance of 1 via by knowing the PCB thickness, the via drill diameter, and the copper plating thickness.

Using a 0.02" drill and 1oz copper plating (finished via = 0.018") I get 111 deg C/W per via. If I use 25 of them I get 4.4 deg C/W... which isn't bad.

 

I recommend using as many as you can so that you can assume a sufficiently low thermal impedance for the calculations. Then of course, when real hardware arrives, measure it.

 

It is not standard practice to put via under Dpaks in fact the best solution is to increase the copper pads area!!

As for 5 degree c/w it is just simple maths 10 watts on 5 c/w = 50c increase above ambient temp so if ya at 20 degee celcuis(room temp) you can expect 70 degree celius on the heat sink Ouch ... too hot rule of thumb if you can not put your thumb on it and hold it there ya got to much heat (up the heat sink)

 

except for the fact that you have 0.060" of fiberglass (an excellent insulator) between the Dpak and heat sink..... so in reality you're looking at melting Dpak.



The 4.4+5= 9.4 C/W is too large, so it would need a better heat sink anyways unless i reduce power dissipation.

 

um why arent you putting it on the copper side like a normal person !!!

 

I am. 10 watts is too much to dissipate through just a copper plane or those cheap clip on heat sinks.

 

ok here is a real cheap easy way ... around the main pad/tab off the D pak make a super large pad then use large amout of excess solder( ie buid up the free area around the DPAK(ie make a huge pool of solder as a heat sink) but realistical 10 watts is a bit much for a poor little old DPAK cant you get the component in a TO -220 ?

 

Err.. simple solution.. get a TO220 (or what ever pin config you need in thru hole mounting package) version of the device, and use a proper heatsink!!

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thicker copper (2-4oz) is recommend for such applications.

Mike

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Agreed on the TO-220 + heatsink.

The OP is finding it difficult because well, maybe it is not so practical to dissipate this much power onto a PCB. TO-220 sounds like a much better design if the OP has the height to do it.

There are also larger packages to consider like a TO-247, TO-3, TO-3P etc..

 

I have to use surface mount because of restrictions on the enclosure. (can't drill and tap holes, no room for clip on heat sinks and the like)

This was the best (and only) method I could think of. It will work, but as was mentioned I might have to opt for the copper plating to get some extra copper thickness for the vias.

 

if you've got room on the backside of the board for a heat sink, how about using a to220, laid horizontally, and have a squarish hole routed in the board for the heat sink to mate with the transistor.

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