[Amps] attaching a heat spreader to a heat sink

[Robert Bonner]

I totally agree, the heat spreaders need to be a pancake and copper usually
is unless somebody bent it.  The heat sink should be flat as this one in
question is obviously warped.

The copper edges can be filed.  Copper is too hard to plane in a wood
planer, somebody will die.  Plus the copper surface is already smooth and
ready for the heatsink except for the edges.

The mating surfaces at the transistors need to be MATING perfectly.  You
cannot rely on the thermal compound to make the connection or cranking down
on the mounting screws to pull the pieces together.

Aluminum expands at a different rate than copper too.  So if you ask for the
screws to mount and pull the metals together the screws will snap the heads
off.

You might be able to get by with wackerized machine work on an amp built
with a pile of MRF150's.  You jump to an amp with MRF154/157's where 1
square inch is transferring 600 watts and you will be replacing $450 each
transistors.

You cannot have a heatsink flat enough, and the spreader thick enough. If
you could build the whole heatsink out of copper you'd be the real stud.
You couldn't stand the smell of the hot raw copper though.  YUK.

So while it sounds like OVERKILL making your heatsink and spreader flat.
Its not.  Now whether I'd run the stuff through a wood planer, not a chance.

It just might be time for a new heatsink. 

BOB DD

-----Original Message-----
From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
Behalf Of Steve Thompson
Sent: Thursday, May 24, 2007 4:49 AM
To: amps at contesting.com
Subject: Re: [Amps] attaching a heat spreader to a heat sink



kg7hf at comcast.net wrote:

> I would think that might be overkill, because when you bolt the spreader
to the heat sink, the two pieces are going to conform to each other, in
fact, they are going to expand and contract as the heat is applied and
dissipated anyway.  
I like the idea of trying as planer - although I shudder to think what 
my wife would do to me if I sent metal through her big thicknesser :-)

Given the thickness of metal, I doubt there will be much movement in the 
surfaces conforming. The issue is metal to metal contact at a much 
smaller scale - undulations and high spots in the scale of 1" and 
smaller, and also at a microscopic level.

The glass plate work shows how far from flat an extruded heatsink 
surface is. If you take something that really is flat then centre punch 
a hole, drill, tap and deburr it, go back to the glass plate and see 
what a mound surrounds the new hole. Your heat spreader or transistor is 
sitting on a small raised area and you've lost most of the thermal 
contact area.

Air is a very poor heat conductor, and thermal grease is only marginally 
better. Neither is a substitute for close fitting metal-metal contact.

Steve
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