Hi guys,
with all that talk about heat spreader to heatsink mounting here, I
can't help but wonder about how nonsensical it is to make such an
copper-to-aluminium sandwich! Why not make the entire heat sink from
copper? Then you don't need an extra copper heat spreader, and if you
make the heatsink from several pieces, you can solder them together.
While solder isn't as great a heat conductor as copper is, it's still
waaaaaaay better than thermal transfer grease! So much so, that a
soldered joint of two totally unfinished, rough copper surfaces is lots
better than a joint assembled with thermal grease, between finely lapped
surfaces!
You may jump at me and say that copper is too expensive or too heavy.
Yes, copper is almost three times as expensive as aluminium (per
weight), and almost three times heavier. But it also conducts heat two
times better than pure, soft aluminium, , and three times better than
the aluminium alloy usually employed for extruding heat sinks!
So, a correctly designed copper heat sink will weigh the same as an
aluminium heatsink of the same thermal capacity, and it will be a tad
more compact. The only downside is taht it will still cost almost three
times as much for the raw material. But is that significant? A heatsink
for a legal limit solid state amp might cost about 10 dollars in copper!
I think this is peanuts, compared to the cost of the transistors, and
considering the advantages it provides!
When I need large heatsinks for high power devices, I buy copper sheet
in two thicknesses: One might be 6 to 10mm and is used for the base
plate (acting as spreader), while the other is 0.5 to 1mm thick and is
used for the fins. I bend the fins into a tall L shape, with the
horizontal part of the L very small, and that small L base is the
contact surface for the fin. I thoroughly tin one side of the thick
copper plate, and the bottom of each L, then I place all the fins on the
plate, put a silicone rubber wall around the plate to avoid the solder
flowing out, and then I heat the whole affair so it solders together.
The product of this exercise doesn't look particularly beautiful, but
has excellent thermal performance, and costs a lot less than a
commercial aluminium heat sink plus copper heat spreader plus lapping
work plus thermal grease plus bolts!
Before you ask: OF COURSE, you need a blowtorch for this work. It's a
bit beyond the operational range of your 15 Watt SMD soldering iron! :-)
The second last photo on this web page shows one such heatsink:
http://ludens.cl/Electron/fmtx/fmtx.html
Manfred.
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Visit my hobby website!
http://ludens.cl
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