Heatsink approval needed for lm3886 - Page 4

AndrewT · 2nd August 2007, 02:43 PM

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.

Juergen Knoop · 2nd August 2007, 04:02 PM

professional Kapton isolator

gtforme00 · 2nd August 2007, 06:37 PM

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!

Regards,
David

AndrewT · 3rd August 2007, 07:58 AM

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.

KSTR · 3rd August 2007, 04:37 PM

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 50°C). 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

Juergen Knoop · 3rd August 2007, 05:02 PM

my picture shows a SOT-32 Kapton isolator with preapplied thermal grease.
Why should a manufacturer sell such stuff?
regards

KSTR · 3rd August 2007, 05:09 PM

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"...

2nd August 2007, 02:43 PM	#31
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	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. __________________ regards Andrew T.

3rd August 2007, 07:58 AM	#34
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	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. __________________ regards Andrew T.

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
LM3886 Heatsink size?	henkel	Chip Amps	28	11th April 2011 02:14 AM
LM3886 Heatsink	ormo	Chip Amps	27	27th July 2009 04:15 AM
Required heatsink for 3 LM3886 in parallel?	CJ900RR	Chip Amps	7	2nd January 2009 05:19 PM
LM3886 & Others Chip Amp Heatsink Clamp Solution	MartyM	Chip Amps	7	18th October 2007 04:36 PM
LM3886 pcb approval wanted	wxn	Chip Amps	21	7th April 2005 04:56 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

2nd August 2007, 06:37 PM	#33
gtforme00 diyAudio Member Join Date: Oct 2004	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! Regards, David

3rd August 2007, 04:37 PM	#35
KSTR diyAudio Member Join Date: Jul 2007 Location: Central Berlin, Germany	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 50°C). 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

3rd August 2007, 05:02 PM	#36
Juergen Knoop diyAudio Member Join Date: Jan 2007 Blog Entries: 3	my picture shows a SOT-32 Kapton isolator with preapplied thermal grease. Why should a manufacturer sell such stuff? regards

3rd August 2007, 05:09 PM	#37
KSTR diyAudio Member Join Date: Jul 2007 Location: Central Berlin, Germany	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"...

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)