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Old 03-04-2010, 06:28 PM   #3067 (permalink)
CamLight
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Quote:
Originally Posted by ElectricZX2 View Post
Sounds like you've done your homework CamLight...I like those numbers. I do agree with you though to go as thin as possible while still being effective as an electrical insulator. In the Piston example the coating was much thicker than 1/2 a mil, and you're talking about primarily radiant heat transfer rather than conduction, which might account for a significant difference.
At 1/2 a mil are you really talking about Type II coating at that point? I thought the primary difference between Type II and Type III anodizing was the thickness...with a "hard coat" starting at around 2 mils.
I don't know the small details about the differences (as I've only used Type III myself) but both Type II and Type III are offered in an overlapping range of thicknesses. But, it's a different process for both of them. Type III is a colder (below 40F IIRC) process that uses a higher current setting than Type II (which is room temp). It's also a darker solution so it's harder to use dyes to achieve different colors.

I've seen hardcoat offered down to 0.1mil but have only looked at vendors who have experience doing heat sink related anodizing. It's critical to go thin for this. For hardness (and not electrical insulating) purposes, I'm sure that the low end for the coating is much thicker as the mechanical properties of the coating are then more important.

Another reason to use a vendor who has experience with heat sink coating is that you might need to remove a clear coat of Type II anodizing from the heat sink. Many are plated this way and you can't apply the hardcoat until it's removed and you have bare metal. But, it's critical that the vendor not overetch the surface when removing the standard (Type II) anodizing!!! This leaves a matte surface that, while feeling pretty smooth and is still perfectly flat, is now a LOUSY conductor of heat from the sink to the MOSFET (or whatever). The matte surface is comprised of a much larger number of peaks and deep valleys that significanly reduce the surface area of metal actually touching when the MOSFET and sink are pressed together. Thermal compound fills in these low spots, but it's nothing like the results you get when two pieces of metal with mirror surfaces are used.

I got zapped this way on a batch of heat sinks two years ago. The thermal performance of my fan/sink combination dropped 20% just because of the over-etching of the surface.
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