On Thu, 12 Apr 2018 13:31:11 -0700, Jeff Liebermann
wrote:

On Thu, 12 Apr 2018 11:59:48 -0700, wrote:

I use some very fine particle size diamond lapping compounds in my
shop.

What's the grain diameter? If it's larger than the depth of the
cracks and crevasses in the aluminum extruded heat sink, then you're
taking a step backwards. The bulk of the heat is passed by metal to
metal contact.

Smallest grain size is 1 micron. Or .0000039
Inches. My best surface plate is flat within 30 millionths of an
inch. My best set of gauge blocks, the ceramic ones, are +1 to + 3
millionths in size except for the 4 inch block which is +4 millionths.
Eric

Maybe I should consider using some for heat sink compound next
time I want to overclock the crap out of something. In fact, it could
first be used for lapping the parts and then just left in place.

As I vaguely recall, the typical flatness specs for extruded heat
sinks is something like 0.007 in/in. So, if the heat sink is 2 inches
wide, it can be warped 0.014" and still be considered usable. Ugh. It
will take an awfully large amount of heat sink goo to fill a 0.014"
gap.

The lack of flatness also causes problems when one tries to using such
a heat sink as a lapping plate. If the CPU can is softer than the
aluminum, it will convert the formerly flat can into a warped version
of the not very flat heat sink. I did stuff like this in the distant
past and found a really flat lapping plat to be a necessity.

If you want to see how bad it can get, take a black felt tip pen and
"paint" the top of the CPU or the heatsink black. Find a flat surface
(glass is good). Drop a sheet of fine emery cloth (2000 grit) on the
flat plate. Move the heat sink very slightly over the sand paper.
Inspect the heatsink. Where the heat sink was raised, the sandpaper
will remove the ink. Where there was a depression, the paint will
remain in place. Extra credit for repeating the test with a hot or
cold heat sink. Thermal expansion in an extrusion isn't uniform and
you will see bending in the heatsink.

There's also a problem with diamond dust and grease. While the grease
is great of keeping the diamond dust in place and making it easy to
apply, thermal cycling tends to pump the grease away from hot spots
leaving rather voids. You can sometimes see this when disassembling a
CPU and heatsink combination that has been running for a few years. If
you take it apart immediately after assembly, the thermal goo is
uniformly distributed (as long as the heat sink is reasonably flat).
However a few hundred thermal cycles later, it will have voids.