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/de...amplifier.html



Cheers,


Gareth.

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

You can likely find replacements here.

http://www.digikey.com/products/en/f...ads-sheets/218

BEWARE that some of the pads are electrically conductive.

G²

Gareth Magennis wrote:

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.
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

On Monday, March 6, 2017 at 4:56:49 PM UTC-5, Gareth Magennis wrote:
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/de...amplifier.html



Cheers,


Gareth.

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 heatsink 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 have tons of older plasma SMPS that used them, so I have donors. They're also available from most supply houses.

On 06/03/2017 21:56, Gareth Magennis wrote:
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/de...amplifier.html



Cheers,


Gareth.

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.

On 06/03/2017 21:56, Gareth Magennis wrote:
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/de...amplifier.html



Cheers,


Gareth.

How old, to get an idea of service life for this silicone junk?

In article , N_Cook
wrote:

On 06/03/2017 21:56, Gareth Magennis wrote:
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/de...amplifier.html



Cheers,


Gareth.

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.

Also somewhat poorer thermal performance ...

Isaac

On 08/03/2017 05:46, isw wrote:
In article , N_Cook
wrote:

On 06/03/2017 21:56, Gareth Magennis wrote:
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/de...amplifier.html



Cheers,


Gareth.

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.

Also somewhat poorer thermal performance ...

Isaac


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.

N_Cook wrote:



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.

Also somewhat poorer thermal performance ...

Isaac


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.



** 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

In article , N_Cook
wrote:

On 08/03/2017 05:46, isw wrote:
In article , N_Cook
wrote:

On 06/03/2017 21:56, Gareth Magennis wrote:
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/de...amplifier.html



Cheers,


Gareth.

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.

Also somewhat poorer thermal performance ...

Isaac


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.

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

In article ,
Phil Allison wrote:

N_Cook wrote:



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.

Also somewhat poorer thermal performance ...

Isaac


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.



** 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.

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

Isaac

isw wrote:





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.


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.



** You must be illiterate.

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



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


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


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.

** Hogwash.

Try learning to read.


..... Phil

isw wrote:



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


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


** 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

"** 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. "

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

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

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 they say not to reuse them, like when you change the transistors to get new insulators, but I did it and it works fine.

"Forget published specs too, pad makers lie. "

They all lie. You are probably old enough to remember when we could trust big companies. Banks even. Now, at least in the US you cannot trust any of them. Even the water company. Recently those new water meters screwed up and overcharged the living **** 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 paying their house payments to qualify for a refi and then swooped down and foreclosed.

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

Due to this, I prefer to just write checks. Plus, if you want to pay by phone they charge you ! And then, instead of going into the bank and dealing with a teller who is PAID they charge you to go to the ATM. They have been in place for a damn long time and should be paid off by now, but they keep on charging, as if they are still amortising the cost. They are a bunch of thieves. 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 **** about it. He has been somewhat disappointing.

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

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

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 frying out.

And mounting methods ? Jesus H Christ, taking apart this **** now is ridiculous. 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 reasons I quit. I am to the point where I do not want to work on anything not old enough to drink. I mean it, and these MFs with their glue, I want to go in that factory with weapons of mass destruction and remove all glue. I do realise 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 access. 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.

**** like this is why I am pretty much retired and just selling off my equipment. Perhaps I am inferior on that respect, but I also have physical problems that make it very difficult. But manufacturers do not think much of service, all they want to do is sell and have the product in the customer's hand, working. After that they pretty much do not give a ****.

wrote:


"** 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. "

Actually it should not be smeared.


** Smear = thin layer applied to each side.

Just a dab will do you,


** It's not bloody Brylcreem !!!


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

Well my Phase Linear 400-2 seems to get along with them quite well.


** 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.



"Forget published specs too, pad makers lie. "

They all lie.


** 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

On Thursday, March 9, 2017 at 1:16:50 AM UTC-5, wrote:
"** 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. "

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


In small transistors, I put a tiny dab right in the center of the transistor. On larger devices, I tend to spread it to cover at least half the heatsink. On hybrid modules I tend to give it a complete thin peanut butter coating.

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. What I've found out is that very few mounting methods compress the device sufficiently to spread and force most of the compound out.

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

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

On Wed, 08 Mar 2017 20:55:46 -0800, isw wrote:

In article , N_Cook
wrote:


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.

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

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.

On Mon, 6 Mar 2017 21:56:45 -0000, "Gareth Magennis"
wrote:

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/de...amplifier.html



Cheers,


Gareth.


Back in the 70s they used Mica insulators, and coated them with silicone
grease to transfer heat. Unless the installer cracked them during
installation, they probably last forever.

I suppose they now came up with some cheap crap substance to save a
buck. If I ran across this, I'd change all of them with the old mica
types before more stuff burns up.

http://www.bergquistcompany.com/

I have been using as a matter of preference materials from Bergquist for roughly 40 years, albeit in small quantities. Typical output-per-device is at/around 30+ watts. None have failed over dozens of amps at 4 - 20 pads each.. Some caveats:

a) There are a LOT of counterfeits out there. Make sure what you use is from a reliable source.
b) DO NOT reuse such pads. They will last 40+ years if left alone. But if re-used, they could have micro-tears or punctures from the removal and re-tightening.
c) DO NOT use them if the heat-sink or substrate is not smooth and flat.They are not grease that can be used in excess to fill such gaps.
d) DO NOT use them with grease/heat-sink compound or any similar material. They can be incompatible.

They are pretty simple, pretty basic, inexpensive items with a single significant virtue - they are not sloppy. So, compound on leads that interferes with soldering becomes a thing of the past (although that is easily controlled with care at application). And there is no measuring issue.

I have also used these:

http://www.talonix.com/shop/item.aspx?itemid=3639 - only because they were OEM. They can be reused. Expensive. For a number of years, AR used them on their US-origin amps and receivers.

With all that in mind, I keep a small stash of Bergquist materials, mica & compound materials and some mica sheeting (also cheap and easy to find at a Jewelry Findings, stove or lamp supply) for unusual situations and/or applications. Such as adjustable mica caps on vintage radios. NOTE: Mica does not like to be drilled - better to punch it. And to make a small diameter mica punch, get a piece of brass tubing with the correct ID, and file it sharp. Punch onto something fairly hard but with some give - I have a piece of maple flooring that I use. You will get perhaps 4 holes and need to resharpen, but you will not flake the mica as you would with a drill. Very sharp tin-snips or professional fabric shears do a nice job of cutting the stuff.

Horses for Courses.

Peter Wieck
Melrose Park, PA