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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