Thermal pad disintegrating

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This is a bit weird.

I have an audio power amplifier that uses what looks like strips of pink  
silicone sheet between the output transistors and the heatsink.
But these strips have degenerated and are kind of squidgy, a bit like when  
Speaker surrounds fall apart.
Not come across this before.

They have got so bad that an arc occurred through one strip, beneath one of  
the transistors, which has burnt a small hole in the heatsink, blowing the  
amp and the SMPS, which I repaired.


So what kind of material could this pad be made of?   Surely silicone  
wouldn't do this?



This is the amp, you wouldn't expect this kind of problem on something like  
this.

http://www.dbaudio.com/en/systems/details/d12-amplifier.html



Cheers,


Gareth.  


Re: Thermal pad disintegrating

I have not seen the pads break down. I have replaced mica with silpads to g
et away from heatsink grease.

You can likely find replacements here.

http://www.digikey.com/products/en/fans-thermal-management/thermal-pads-she
ets/218

BEWARE that some of the pads are electrically conductive.



Re: Thermal pad disintegrating
Gareth Magennis wrote:

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Hmmm, I've seen these pink ones in old power supplies and such, many years  
ago.  I haven't seen them in anything recent.  Some of the Bergquist pads  
have some kind of fabric in them that makes them quite robust.

They also have some Gap-Pad material that is designed to be very squishy.
It is not designed to provide electrical insulation, but to accomodate  
uneven parts that have to conduct heat to a heat sink.

Jon

Re: Thermal pad disintegrating


"Jon Elson"  wrote in message  

Gareth Magennis wrote:

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Hmmm, I've seen these pink ones in old power supplies and such, many years
ago.  I haven't seen them in anything recent.  Some of the Bergquist pads
have some kind of fabric in them that makes them quite robust.

They also have some Gap-Pad material that is designed to be very squishy.
It is not designed to provide electrical insulation, but to accomodate
uneven parts that have to conduct heat to a heat sink.

Jon



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The parts under the transistors seems to have been compressed by the heavy  
spring clips used, as it seems way thinner than normal, but firmly stuck to  
the heatsink.
Probably why one arced.

The parts not under transistors and exposed to the environment are squidgy  
and thicker, and not at all stuck on the heatsink, and just break into  
chunks when you move them.



Gareth.






Re: Thermal pad disintegrating
On Monday, March 6, 2017 at 4:56:49 PM UTC-5, Gareth Magennis wrote:
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I haven't had one fail in use, but have had a bunch fail when the device is
 replaced for other reasons.  Often they'll be so acclimated to their heats
ink they're virtually bonded, and they tear when removal is attempted. In a
 pinch I've used mica washers and a tiny schmear of 3M compound.  I also ha
ve tons of older plasma SMPS that used them, so I have donors.  They're als
o available from most supply houses.


Re: Thermal pad disintegrating
On 06/03/2017 21:56, Gareth Magennis wrote:
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Stay with mica slivers, been around for geological time, no plasticer to  
leach out. The only time I've come across failure of a mica insulator  
was when some swarf got under it at manufacture, that would do for all  
other insulators as well.
Trouble with mica is it is dirt cheap and not patentable.

Re: Thermal pad disintegrating
wrote:

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Also somewhat poorer thermal performance ...

Isaac

Re: Thermal pad disintegrating
On 08/03/2017 05:46, isw wrote:
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If you mean mica is poorer, don't go by product data sheets.
I did a realworld expt one time , comparing mica and sillypads on an  
amplifier. Running with fixed input and controls and load, and  
monitoring the temp of the heasink and IR thermo of devices. The mica'd  
version ran with noticably hotter heatsink and cooler devices.
IIRC the datasheets quote an (fraudulent?) impossible degree of  
squashing of pads, not obtainable with one screw thru a tab.

Re: Thermal pad disintegrating
 N_Cook wrote:

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** Thin mica (0.05mm) is still the best performing insulator material for semiconductors you can get at a sensible price AND will easily handle 200V or more. Just a smear of white, silicone grease on each side and you are there.

The reason makers ever use silicone pads is saving assembly time.  

If the power per device is high (ie over 20 watts) then forget silicone pads.  

Forget published specs too, pad makers lie.  



....  Phil  

Re: Thermal pad disintegrating

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Sometimes, pads are used to increase spacing and minimize  
device-to-ground capacitance.

Isaac

Re: Thermal pad disintegrating
isw wrote:


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** Never matters with audio amps, SMPS or the like.  

 With frequencies above a few MHz, alumina pads are a far better choice, allowing thickness of a few mm with good conductivity.



.... Phil



Re: Thermal pad disintegrating
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 semiconductors you can get at a sensible price AND will easily handle 200V
 or more. Just a smear of white, silicone grease on each side and you are t
here. "

Actually it should not be smeared. Just a dab will  do you, and when the fo
rce is applied whether by screw or other means it is better because there a
re no air pockets in it.  

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ads. "

Well my Phase Linear 400-2 seems to get along with them quite well. However
 they might be thicker or something, I never check. If I recall correctly t
hey say not to reuse them, like when you change the transistors to get new  
insulators, but I did it and it works fine.  

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They all lie. You are probably old enough to remember when we could trust b
ig companies. Banks even. Now, at least in the US you cannot trust any of t
hem. Even the water company. Recently those new water meters screwed up and
 overcharged the living shit out of a bunch of people. YOU HAVE TO PAY THAT
 OR LOSE YOUR HOUSE. Then later you can fight back for a refund. Of course  
that might be hard to do when you are homeless. That is how it is here dude
, and I got alot more. Seriously, banks on the phone told people to stop pa
ying their house payments to qualify for a refi and then swooped down and f
oreclosed.  

In the US you have to be very careful about making automatic payment becaus
e they will use a big fat ten inch to do your colonoscopy. It might be diff
erent where you are, but here you cannot even trust the phone or cable comp
any. Or the gas company. They are a bunch of crooks and are only stopped bb
y the chance of getting caught, or worse, screwing someone who can get medi
a coverage about it.  

Due to this, I prefer to just write checks. Plus, if you want to pay by pho
ne they charge you ! And then, instead of going into the bank and dealing w
ith a teller who is PAID they charge you to go to the ATM. They have been i
n place for a damn long time and should be paid off by now, but they keep o
n charging, as if they are still amortising the cost. They are a bunch of t
hieves. You cannot trust anyone, at least here. You might have better laws  
there but here it is open season on us. And it is looking like Trump is not
 going to do shit about it. He has been somewhat disappointing.  

The one good thing about the state of affairs in the US is that I am old en
ough not to give a shit anymore.

Re: Thermal pad disintegrating
snipped-for-privacy@gmail.com wrote:
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** Smear = thin layer applied to each side.  

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** It's not bloody Brylcreem  !!!

  
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 ** Yawnnnnn.....

 Use that amp in a PA system and it will blow up.  

 I have a PL400 mk2 here, with all the pink pads replaced with mica.  

 The TO3 devices run about 25 degrees cooler because of the change.  


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** Not all.

The big lies are from makers of sil pads and TO220 high current mosfets.
  
Oh, and loudspeaker makers with fake power handling specs.  




....  Phil  





Re: Thermal pad disintegrating
On Thursday, March 9, 2017 at 1:16:50 AM UTC-5, snipped-for-privacy@gmail.com wrote:
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or semiconductors you can get at a sensible price AND will easily handle 20
0V or more. Just a smear of white, silicone grease on each side and you are
 there. "
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force is applied whether by screw or other means it is better because there
 are no air pockets in it.  
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In small transistors, I put a tiny dab right in the center of the transisto
r.  On larger devices, I tend to spread it to cover at least half the heats
ink.  On hybrid modules I tend to give it a complete thin peanut butter coa
ting.

I was taught that compound should be applied by a dot and let it spread out
 by the compression of the mounting technique to prevent an air pocket.  Wh
at I've found out is that very few mounting methods compress the device suf
ficiently to spread and force most of the compound out.

You've done your share of STK replacements - how many did you remove to dis
cover the compound was in the center but never spread towards the outer edg
es?

I use Dow Corning 340 which is a very dense and heavy compound, and this st
uff takes a lot of force to compress it all the way across the back of a la
rge heatsink.

Re: Thermal pad disintegrating
wrote:

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If your claim is that the mica version "ran with noticably hotter  
heatsink and cooler devices", then something went wrong with your  
measuring technique. It's a sort of requirement that heat flows from  
warmer to cooler, and so the devices necessarily must be warmer than the  
heatsink, no matter what (passive) material you use in the interface.

A better-performing thermal interface would be one which *minimized* the  
difference in temperature between the device and the heatsink.

Also from your description, it sounds like you were measuring the  
performance of a specific mounting method, rather than the thermal  
resistivity of one of its components.

Isaac

Re: Thermal pad disintegrating
isw wrote:



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** You must be illiterate.  

 The OP did not say the devices were cooler than the heatsink.  



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** Which is consistent with the OP's finding.  


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** Hogwash.

  Try learning to read.  


....  Phil  

Re: Thermal pad disintegrating
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If I am understanding this correctly, this is the reason that amps now have
 floating transformer secondaries and have eliminated the insulators.  

Seems like it was not so big an issue in the old days, but now they are fan
 cooled and they DO depend on the temperature difference to keep from fryin
g out.  

And mounting methods ? Jesus H Christ, taking apart this shit now is ridicu
lous. You got one bad pair of output, the whole damn board has to come out  
and then all of the outputs have to be dismounted. This is one of the reaso
ns I quit. I am to the point where I do not want to work on anything not ol
d enough to drink. I mean it, and these MFs with their glue, I want to go i
n that factory with weapons of mass destruction and remove all glue. I do r
ealise why they do it, but damn. The way they do it they might as well just
 glue the lid on and call it a toaster.  

And even supposedly better amps are built in a pan. There is no bottom acce
ss. In many you can't even get the board out without the heatsink on which  
makes for a very awkward situation, and running it without the heatsink is  
a big nono.  

Shit like this is why I am pretty much retired and just selling off my equi
pment. Perhaps I am inferior on that respect, but I also have physical prob
lems that make it very difficult. But manufacturers do not think much of se
rvice, all they want to do is sell and have the product in the customer's h
and, working. After that they pretty much do not give a shit.

Re: Thermal pad disintegrating

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I think you are misunderstanding N_Cook's statement.

I read it to be that he ran two setups, one with a mica insulator,
and one with a sillypad.
The device using the mica insulator was cooler than the device
using the sillypad, and the mica insulator device's heat sink
was warmer than the sillypad device's heat sink.
This implies that the mica insulator is more efficient at moving
heat from a device to the heatsink than the sillypad.

Re: Thermal pad disintegrating
On 06/03/2017 21:56, Gareth Magennis wrote:
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How old, to get an idea of service life for this silicone junk?

Re: Thermal pad disintegrating



On 06/03/2017 21:56, Gareth Magennis wrote:
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How old, to get an idea of service life for this silicone junk?


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These amps are currently extensively used in the Pro Audio (Hire, Touring,  
Venue) world, and are probably still in production.
I suspect this one might me a rogue event.


Gareth.  


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