This relates partially to the "about free samples" thread, but is different enough to warrant its own discussion...
I requested a sample of DuPont's Kapton MT polyimide film, and got word back that they can send me a paper-size (8.5x11) sheet as an engineering sample. (Data on this stuff can be found here.)
When I get the sample I'll cook up a little report on how well it works, how easy it is to use (incl. cutting to fit oddball packages like multiwatt15s) and how well it works.
In the meantime, if anyone has decent sources for tabbed-IC and very large transistor packages, or raw material sheets that can be trimmed to fit applications where pre-cut insulators are impossible to find, please post 'em here so the next visitor will have an idea where to find the stuff.
oO, who's strange enough to use Artic Silver thermal compound (which is the best small-contact-area compound you can get for use on processors, etc.) on transistors...
I requested a sample of DuPont's Kapton MT polyimide film, and got word back that they can send me a paper-size (8.5x11) sheet as an engineering sample. (Data on this stuff can be found here.)
When I get the sample I'll cook up a little report on how well it works, how easy it is to use (incl. cutting to fit oddball packages like multiwatt15s) and how well it works.
In the meantime, if anyone has decent sources for tabbed-IC and very large transistor packages, or raw material sheets that can be trimmed to fit applications where pre-cut insulators are impossible to find, please post 'em here so the next visitor will have an idea where to find the stuff.
oO, who's strange enough to use Artic Silver thermal compound (which is the best small-contact-area compound you can get for use on processors, etc.) on transistors...
sam9
When faced with the need for an od sized pad, I've tried a strip of kapton tape. The warehouse in the plant where I work uses it for packaging. I just drop by and ask for a few feet or a stub roll.
It looked like pretty insubstantial stuff. I've tried deliberatly to pass a current trough, but at voltage/current combinations one would find in a SS amp I've been unsuccessful. I also tried melting it with a solder iron - no luck.
This seems too easy, but based on what I've seen so far, it works very well.
PS: I still put a thin film of thermal grease on top of it.
When faced with the need for an od sized pad, I've tried a strip of kapton tape. The warehouse in the plant where I work uses it for packaging. I just drop by and ask for a few feet or a stub roll.
It looked like pretty insubstantial stuff. I've tried deliberatly to pass a current trough, but at voltage/current combinations one would find in a SS amp I've been unsuccessful. I also tried melting it with a solder iron - no luck.
This seems too easy, but based on what I've seen so far, it works very well.
PS: I still put a thin film of thermal grease on top of it.
![xa200diff[1].jpg](/community/data/attachments/11/11341-de8e8a5a0d7a8b6f9229b7f77b9d9816.jpg)
Why Kapton? It's not a particularly good thermal conductor and tears easily. Its only advantage is its tolerance for very high temperatures, which is not the issue here (the epoxy in the transistor package will go long before the Kapton). It seems that a loaded silicone should perform better at temperatures more typical of audio amps (<100-150 degrees C).
Now, if you're just going for the coolness factor, you can't beat PEEK.
Now, if you're just going for the coolness factor, you can't beat PEEK.
In an effort to avoid that messy grease, I found an Aavid Thermalloy product (Wakefield makes one too) that is much neater; its a waxy stick that comes in a push out applicator (kinda like a small rectangular stick deoderant!) that goes on real easy. It is supposed to be 20% more efficient than grease, no runout or "bleeding", expands once it hits 160 degree F (undergoes phase change), and cleans up easily. You can use it on any two surfaces that you would use the grease on, I think.
DigiKey carries it on the same page as other Thermalloy stuff. Look for the picture in upper left catalog page. Not sure if it has a number. Only drawback: costs about $17 per stick, good for 1000 sq. inches. I used it to interface some heat sink modules to a big thick aluminum plate, after getting rid of the grease that used to provide the thermal interface.
Larry Wright
Seattle area
DigiKey carries it on the same page as other Thermalloy stuff. Look for the picture in upper left catalog page. Not sure if it has a number. Only drawback: costs about $17 per stick, good for 1000 sq. inches. I used it to interface some heat sink modules to a big thick aluminum plate, after getting rid of the grease that used to provide the thermal interface.
Larry Wright
Seattle area
I find it only tears when one has first cut a nick cut in it. Trying to tear an undamaged strip could give one a coronary. As far as thermal conductivity goes, I Rod Elliot's article on heat sinking. Not too different from mica, better than some other stuff. And of course not of them are in the same league as beryllium oxide toxic though it may be.
Nelson Pass mentioned in a previous thread that.....
"I like Thermalloy's white thermal grease, but it's kinda messy"
Messy isnt the appropriate word to describe that stuff and I'm not sure there is a word that can! I once had a big tube of it squish out all over the inside of my tool box cause I forgot to place it back in its "safety" container . Man, one daub of that stuff and you can paint your whole house!!
Mark
"I like Thermalloy's white thermal grease, but it's kinda messy"
Messy isnt the appropriate word to describe that stuff and I'm not sure there is a word that can! I once had a big tube of it squish out all over the inside of my tool box cause I forgot to place it back in its "safety" container . Man, one daub of that stuff and you can paint your whole house!!
Mark
Bio Degradable Insulation.....
Philips in the past used a Mylar/Kapton like insulator under the line output transistor in televisions.
In time these fail by darkening and becoming brittle under the middle of the TO-3 device used, and breaking down to the heatsink (peak voltages are over 1000V).
Standard practice is to replace with a mica insulator.
Eric.
Philips in the past used a Mylar/Kapton like insulator under the line output transistor in televisions.
In time these fail by darkening and becoming brittle under the middle of the TO-3 device used, and breaking down to the heatsink (peak voltages are over 1000V).
Standard practice is to replace with a mica insulator.
Eric.
Messy Grease
I too hate the white thermal compound, it's ability to spread over the widest possiblwe area, to include my clothes etc. is irritating to put it mildly. It works very well though, with a mica insulator.
A word of warning about some of the alternatives, more than one commercial early adopter of the silicone-based pads experienced significant failure rates from such products in their power amps.
I only use the Kapton-based pads (I use Bergquist ones) and these have proven to be utterly reliable, and have good thermal performance for the higher grades.
I have also experienced failure, after time, of the earlier silicone based pads, the material creeps with time, causing eventual breakdown.
Andy.
I too hate the white thermal compound, it's ability to spread over the widest possiblwe area, to include my clothes etc. is irritating to put it mildly. It works very well though, with a mica insulator.
A word of warning about some of the alternatives, more than one commercial early adopter of the silicone-based pads experienced significant failure rates from such products in their power amps.
I only use the Kapton-based pads (I use Bergquist ones) and these have proven to be utterly reliable, and have good thermal performance for the higher grades.
I have also experienced failure, after time, of the earlier silicone based pads, the material creeps with time, causing eventual breakdown.
Andy.
Kapton
Hi All,
I have seen some very alarming reports on Discovery Channel about Kapton used as electrical wire insulation in commercial airplanes of Boeing and Airbus.
According to the report when the isolation gets hot as a result of a shortcircuit it spontaneously gets into fire and EXPLODES causing the fire to spread.
For this reason this compound is no longer used in military planes.
For commercial aviation the situation is simply denied by the manufacturers.
In the view of above I would never think about using Kapton as a transistor insulator in my home equipment.
Hi All,
I have seen some very alarming reports on Discovery Channel about Kapton used as electrical wire insulation in commercial airplanes of Boeing and Airbus.
According to the report when the isolation gets hot as a result of a shortcircuit it spontaneously gets into fire and EXPLODES causing the fire to spread.
For this reason this compound is no longer used in military planes.
For commercial aviation the situation is simply denied by the manufacturers.
In the view of above I would never think about using Kapton as a transistor insulator in my home equipment.
1. Kapton in power amps should not be a problem. Aircraft failures are typically due to wear and water causing sparcing.
2. The best material for electrical isolation and thermal resistance is Beryllium Oxide which is illegal in many places due to the toxicity of dust if ground. Availability is limited -- but I have seen TO-3 form factor.
3. The second best material is probably the relatively new fired Aluminum Oxide washers. Various form factors are available.
4. If you don't want a mess, consider using SilPad 2000 or equivalent. I bought some of these from Bergqvist. You will not get the performance benefit of reduced capacitance offered by the fired ceramics (2 and 3 above), but you will be much more unlikely to mount the devices wrong and loose performance that way.
5. Thick thermal grease is not recommended -- it is too hard to work with
6. Arctic silver etc. are not recommended as they contain conductive material.
There are further posts on this in the archive if you are interested.
Petter
2. The best material for electrical isolation and thermal resistance is Beryllium Oxide which is illegal in many places due to the toxicity of dust if ground. Availability is limited -- but I have seen TO-3 form factor.
3. The second best material is probably the relatively new fired Aluminum Oxide washers. Various form factors are available.
4. If you don't want a mess, consider using SilPad 2000 or equivalent. I bought some of these from Bergqvist. You will not get the performance benefit of reduced capacitance offered by the fired ceramics (2 and 3 above), but you will be much more unlikely to mount the devices wrong and loose performance that way.
5. Thick thermal grease is not recommended -- it is too hard to work with
6. Arctic silver etc. are not recommended as they contain conductive material.
There are further posts on this in the archive if you are interested.
Petter
Bergquist
The Bergquist web site is:
www.bergquistcompany.com
not as much fun as relating personal experiences/opinions but it does include specs and application notes on all manner heatsink pads including a number of products not even listed in digikey.
The Bergquist web site is:
www.bergquistcompany.com
not as much fun as relating personal experiences/opinions but it does include specs and application notes on all manner heatsink pads including a number of products not even listed in digikey.
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