"Jeff Liebermann" wrote in message
...
On Fri, 27 Aug 2010 01:27:14 +0100, "Arfa Daily"
wrote:

Anyone
else got any constructive comments on the subject of thermal interfacing
of
coolers to high power chips ?

In a past life, I used to design HF SSB marine radios. The typical
transmitter was Class AB 150 watts with about 30% efficiency. That's
two devices, dissipating about 125 watts each, over an area of about
70 sq-cm. Oh yes, no fan allowed.

This is quite a bit more dissipation than the average desktop, causing
some things to be more critical. In the process of getting it to
work, I learned a few things.

1. The less silicon grease used, the better. The idea behind silicon
grease is to fill in the gaps, scratches, and gouges in the power
transistor base and aluminum heat sink. Cross sectional
microphotographs show metal to metal contact on the peaks, but huge
gaps, filled with silicon grease, in between. Under ideal
circumstances, maximum metal to metal contact, with minimum gaps is
the target practice.

2. All heat sinks and transistor bases are NOT flat. I made a
dramatic improvement to the measured thermal resistance by polishing
flat the base of the xsistor and the face of the heat sink. That
meant removing the gold from the copper base, but that's what was
necessary. I used a Moire pattern to measure flatness. A mirror
finish was best, but difficult to achieve. To prevent corrosion, I
plated the exposed copper with electroless tin or silver. For the
aluminum heat sink, I just used abrasive polish and a glass polishing
plate to obtain a mirror finish and flat surface.

3. Compression pressure is important. None of the standard spring
clip CPU heat sink holders come even close to optimum. Compression
adjusts for the bends, and also provides some level of galling to
provide metal to metal contact. If done correctly, adding silicon
grease actually increases the thermal resistance. However, this is
difficult to do with a CPU that has components on the bottom side,
thus preventing compression. Applying pressure only on the top center
of the CPU, will cause the substrate to bend, and eventually break. I
have some ideas, but nothing that can be retrofitted to an existing
motherboard and CPU socket. This is close, but not optimum:
http://www.frostytech.com/articleview.cfm?articleID=2273
Note the comments on base finish and flatness.

So, if you want the best head sinking, polish flat the CPU top
(removing all the laser scribbled markings, polish the heat sink face,
use very very very very little silicon grease, and compress the
sandwich until it nearly breaks the CPU.

--
Jeff Liebermann


Thanks for the insights Jeff. All interesting stuff. These are dedicated
games machines, not based on a PC in any way. The power supply is specced to
deliver 12v at 23 amps, yes, that's twenty three amps ...

Almost all of this is potentially going into these two processors, so not
far off 300 watts between them. No mean task shifting the heat off them !

Arfa