Thermal pad disintegrating

[Mike Paff]

On Wed, 08 Mar 2017 20:55:46 -0800, isw <isw@witzend.com> wrote:

In article <o9of0e$bi6$1@dont-email.me>, N_Cook <diverse@tcp.co.uk
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"
<soundserviceleeds@outlook.com> 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/details/d12-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.