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Re: [Amps] 'heat spreader' sizing ??

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Subject: Re: [Amps] 'heat spreader' sizing ??
From: "pfizenmayer" <>
Date: Fri, 4 Feb 2011 19:00:29 -0700
List-post: <">>
Like Roger , it has been years since I did this sort of work and even then 
there was a group that did most of our FEM for stuff like this and 
stress/strain analysis of composite structures.

But one thing I have not seen mentioned is that the size of the heatspreader 
for best use of it depends on the thermal conductivity of the surface to 
which it is mounted.

As the thermal conductivity of the "heatsink"  decreases , the heat will 
spread more in the heatspreader of higher thermal conductivity so how large 
it is depends on the "heatsink". If the spreader and the heatsink had the 
same thermal conductivity ,roughly a heatspreader only needs to be large 
enough that the dimensions are enough to accomodate about a 45 degree angle 
from to to bottom surface- ie: a 1/4 inch thick copper heatspreader only 
need to be 1/4 inch larger in all dimensions . If the heatsink is not that 
good then the heat spreads at a smaller angle and its size needs to be 
larger .

If you mount the devices with screws - be sure to use something like Bellvue 
washers to allow the differential thermal expansions between the copper 
heatspreader and the package to be relieved. Otherwise you stand a chance of 
cracking the ceramic if that is what is used in the package

Your thought of soldering also needs to take the differential thermal 
expansion to be taken into account - solders vary widely in their ability to 
take repetitive thermal cycles .

I would think the device manufacturer would supply you with the materials 
used in the package and their thermal expansion properties as well as 
recommended mounting techniques. It is certainly to their benefit to do so.

73 de Hank K7HP

>>> The project is to mount a 1KW, 2 transistor, board on a heat sink. The
>>> transistor centers are about 2 inches. The question is: What should the
>>> dimensions of the heat spreader be? I have seen dimensions of 1/4 x 2 x
>>> 4 up to 1/2 x 4 x 7; that represents quite a range. The questions are:
>>> 1. What thickness is required. Is 1/4 sufficient?
>>> 2. What is the minimum effective distance from the edge of the devices?
>>> 1/2 inch? 1 inch?


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