Re: heatsinking a thermal padded IC ?

On Sep 25, 12:03 am, Adam S <not.valid@nosuchaddress> wrote:
John Larkin wrote:
On Mon, 24 Sep 2007 18:21:51 +1000, Adam S <not.valid@nosuchaddress>
wrote:

Normally any QFN packages and the like that need to dissipate heat will
have a thermal pad designed to transfer heat from die to a copper ground
plane via thermal vias. Is it possible to improve the thermal
conductivity by adding a aluminum heatsink directly on the PCB surface
opposite the QFN package ?

Yes, we do that. The important thing then becomes to establish a good
heat flow path from the thermal pad to the sink. That is typically a
copper pad on the top side that solders to the thermal pad on the ic;
a number of thermal vias; and a biggish pad on the bottomside that you
bond the heatsink to somehow. The vias may then be a major source of
thermal resistance, since they will probably be few and small.

An inner-layer heat spreader, a copper pour, can help, usually the
ground plane itself. Make that layer 2 if possible and nail it to the
heatsink pour with additional, largish vias outside the footprint of
the ic.

http://s2.supload.com/free/AmpBoard.JPG/view/
http://s2.supload.com/free/AmpBottom.jpg/view/

John

Thanks for the photos. thats some densely packed PCB !
I am using 2 layer board so unfortunately I cannot take advantage of the
extra copper you mentioned above. What have you used to bond the
aluminum heatsink to the solder pad, I'm assuming the pad is not covered
in soldermask ?

I was thinking epoxy. I would assume that the epoxy film thickness after
pressing it down firmly would reduce to less than a few 10s of microns.

Will a grid of large 1mm diameter vias improve thermal conductivity
between the IC's thermal pad and the copper pad on the opposite side of
the PCB ?

Are you familiar with the on-chip diode method to measure die
temperaure? If not, I'll write it up. Obviously, this is to be used to
figure out the effectiveness of whatever scheme(s) you come up with.
This is probably a situation where practical measurements are better
then theory.

Some heat leaves the chip via bond wires, so extra copper on ground or
supply also helps to cool the chip.

.

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