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Old 2nd August 2007, 02:43 PM   #31
AndrewT is online now AndrewT  Scotland
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The relatively "hard" surface of Kapton tape will not deform into the air gaps of the metal surface. The deformable isolators are designed to do exactly that and should be used dry.
Kapton should be gap filled.
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Old 2nd August 2007, 04:02 PM   #32
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professional Kapton isolator
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File Type: jpg kapton.jpg (57.3 KB, 146 views)
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Old 2nd August 2007, 06:37 PM   #33
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AndrewT,

What do you think of using the adhesive style Kapton tape? I have always used this out of convenience, but if there is a better method, I would love to know. Would it be better to use non adhesive Kapton with a gap filler on both sides? I would suspect the answer is yes, but it sounds like you have experience in this field, and I would like to use the best practical method!

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David
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Old 3rd August 2007, 07:58 AM   #34
AndrewT is online now AndrewT  Scotland
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Hi,
I have no experience with Kapton.
The only thickness I found in the UK was far too thick (0.006" if I recall correctly).
The adhesive is often 0.002" thick (and worse). I don't know how thermally conductive the adhesive is that is used. It would be nice if the suppliers/manufacturers would supply thermal data for both parts of the isolator and what the combined performance is with actual "To" sizes. But the few that specify don't tell us.

What is the big secret? Why all the competing measurement units? Consistency and comparability would help.

I have a feeling that 0.001" Kapton with a normal thermal compound on both sides will perform as well as the best out there.
But that solution will not appeal to the assemblers. It is as unclean and slow as mica.
The next best would be 0.001" Kapton tape + 0.001" to 0.0015" thermal adhesive (total thickness ~0.05mm). I suspect this may be outperformed by the best of the deformable but a lot cheaper.
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Old 3rd August 2007, 04:37 PM   #35
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A general rule for when to use thermal grease is: Hard surfaces -- Yes, soft surfaces -- No. But many apparently hard surfaces (like many of the polyimide aka "kapton" foils) have a polyphase coating that gets soft and fills the gaps when reaching a certain temperature and gets hard again (but keeping its shape) when coming below that phase change temp (on the order of 50C). With such polyphase coating, use of thermal grease is not recommended. Of course one needs it with mica and Al2O3 washers or with direct mounting as those are permanet hard surfaces. Compressible thermal pads like those made from silicone may only be used with components fixed to the sink at a single point (single bolt mounting). Components with multiple fixing points can easily be damaged when using compressible pads -- bendings stresses can cause the substrate to crack.

Some details on thermal accessories, catalog excerpts in a PDF:
http://www.fischerelektronik.de/fisc...cher/E.1.E.pdf

Similar stuff from another manufacturer:
http://www.heatmanagement.com/kunze/...e/Products.pdf

Regards, Klaus
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Old 3rd August 2007, 05:02 PM   #36
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my picture shows a SOT-32 Kapton isolator with preapplied thermal grease.
Why should a manufacturer sell such stuff?
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Old 3rd August 2007, 05:09 PM   #37
KSTR is offline KSTR  Germany
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Juergen, read a little more careful, please. I did NOT write something like "all kapton isolators have a polyphase coating and therefore don't use grease with these"...
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