[Amps] attaching a heat spreader to a heat sink

[Steve Thompson]

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