What is the best aluminum for heat sink?

A friend is making a spacer to go between a tab on a device and a heat sink. The total thickness will be about 0.25 inch (6 mm?). Of course thermal grease
will be used at both matings.
I've been asked what kind of aluminum.
What (in descending order of preference) are the aluminum types that would be good raw material for this?
Thanks.
--
Al, the usual


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Suspect wrote:

Aluminum extrusions - may be not much less than that of "poured aluminum", which has heat conductivity of 161 w/m-k.
Die-cast zinc-aluminum alloy - 127 for "ZA-27", 115 for "ZA-8". Die-cast zinc "zamak" - 113 w/m-k Die-cast die-castable aluminum alloy - 96.2 w/m-k Annealed brass - 61 w/m-k
http://www.cs.unc.edu/~pxfl/papers/efficiency.pdf
Extruded copper - 300 w/m-k Extruded aluminum - 200 w/m-k
http://electronicdesign.com/Articles/Index.cfm?AD=1&ArticleID 26&pg=5
6061 aluminum alloy - 166 w/m-k, if I correctly translated from
http://www.espimetals.com/tech/aluminum6061-t651.pdf
6061 aluminum alloy - 180 w/m-k
(There are heat-treated versions of 6061 including T4 and T6, which have mildly increased electrical resistivity, and I suspect also slightly decreased thermal conductivity.)
http://www.efunda.com/materials/alloys/aluminum/show_aluminum.cfm ? IDͺ_6061&prop=all&Page_Titleͺ%206061
If this spacer has to conduct several 10's of watts or more and every degree matters, I say make it out of copper or silver.
Otherwise, extruded aluminum, poured aluminum and 6061 are close enough to each other for "best aluminum", and I expect ordinary aluminum plate and bar stock (to be machined into shape) to be similar.
- Don Klipstein ( snipped-for-privacy@misty.com)
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Don Klipstein wrote: [...]

Indeed. The most important factor would be that the spacer be as flat as possible on both sides, for best heat transfer.
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On Mon, 04 May 2009 18:52:30 -0700, Usual Suspect wrote:

All of the above.
You'll lose more delta-T at the mating surfaces than in the aluminum, most likely. Matweb (www.matweb.com) should have thermal conductivities.
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Tim Wescott wrote:

The mating surface should be as flat as possible. Many cheap extrusions fail this test. Using formed sheet material is very effective.
See Motorola's ( now On Semi's ) AN1040 IIRC for good thermal advice.
Graham
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I have seen devices with poor flatness. I have polished some things. Getting flat is hard.
greg
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GregS wrote:

When I was in college I was able to achieve significant flatness fairly consistently, given enough beer and other things.
I don't like being that flat any more, though.
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So long as the beer isn't flat that's fine :-)

We flatten worn oilstones, used for sharpening wood chisels and so on, by rubbing them with dry sand on a melamine board. Throw some sand on the board and rub the oilstone on top until all the hollows are gone - noisy and takes a little time but it does the trick.
Something similar ought to work for heatsinks
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Stuart wrote:

If you have access to a milling machine, it's easy to mill a surface flat. Aluminum machines really nicely.
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Then you have to worry about stress making that flat surface look round. Too much torque can do that.
greg
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On Tue, 05 May 2009 10:34:26 -0700, James Sweet wrote:

The point of the whole exercise was to do it by hand, with a file.
These days, you just upload the program, throw a piece of metal into the machine, and the finished part comes out the other end. ;-)
Cheers! Rich
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Rich Grise wrote:

Exercise? I thought he wanted to make a heatsink?
For a one-off of a simple operation like milling a surface flat, it's a lot easier to just stick it in a manual mill. If you want 100 of them, or you want a complex part, then CNC is the way to go.
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James Sweet wrote:

Some, perhaps even most, aluminum alloys machine nicely. Pure aluminum, and just about anything designed to be easy to bend up, is 'gummy', and hard to get a good finish on.
But 6061 is a treat, and many extrusions seem to be nicely machinable.
--

Tim Wescott
Wescott Design Services
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On Tue, 05 May 2009 10:34:26 -0700, James Sweet

Surface quality, as well as flatness, are the two factors here.
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Yes, very careful lapping on a piece of glass with finer and finer grinding paste should produce a suitable finish.
Of course you could try scraping. A friend of mine used to hand scrape surface plates.
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wrote:

Hand-scraped TO-220 packages. The mind boggles.     
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Stuart wrote:

Is scraping really better than lapping? If anyone here can speak definitively about the difference, I'd be very interested to hear about it.
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wrote:

http://en.wikipedia.org/wiki/Hand_scraper
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Spehro Pefhany wrote:

Very informative. Thank you.
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Bob Larter wrote:

Scraping has the advantage that it looks nice and retains lubricant on moving mating surfaces.
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Best Regards:
Baron.
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