Mica versus Silicone pad insulators

yes

you have too much time on your hands :)

only joking honest

Ron

Your fingertip response change is very steep in the area 32-33 C.

-- John

Silicone pads are terrible thermally.

My finger is pretty well calibrated from 50 to 60C. I can stand 50C forever, 60C for about one second, and interpolate between.

John

My favorite coffee temperature :)

D from BC

I didn't quite follow your entire post, but I would have used a IR thermometer pointed on the devices, then the heatsink, then the room. I never thought silicone was better than mica, except in the case of a poor fit, or warped pieces. I have also tried to sand down surfaces that were not flush, and even remove the oxide on aluminum, to optimize transfer.

grge

I prefer silicon pads in high power amps both RF and audio as do quite a few manufacturers.

The previous post, that said it was the transistor temperature that would be variable, was correct; remember, the whole heat output is dumped into the heatsink, and that isn't affected by the thermal resistance of your mica. Heatsink-to-air temperature difference is whatever it takes to drain that heat.

It is possible to fit a second heatsink onto the TOP of the transistor, if you really need better thermal transfer (using both bottom and top surfaces of the metal plate).

It's interesting that the tongue is the most temperature-sensitive part of the human body, and the most resistant to temperature damage.

John

Most people have known for decades that mica and good goop beats those stupid silicone things hands down.

Cheers! Rich

...

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

That's only because it's the wettest part that ever sees the outside. ;-)

I heard some standup comic talking about hot food - "It's too hot to pick up with my hands, so I'm going to put it into my mouth?" ;-)

Ever blistered your palate with hot pizza? ;-)

Cheers! Rich

On Tue, 11 Sep 2007 17:30:33 +0000, GregS wrote: ...

Unless you're under oil or something, you _can't_ remove the oxide on aluminum, because it so aggressively sucks up oxygen.

But, fortunately, aluminum oxide (especially in the thicknesses that it spontaneously forms in air) is practically as good a heat conductor as pure aluminum. :-)

And it's a much better electrical insulator. ;-D I've heard that if you get the surface flat enough, and type III anodize it, that you don't even need the mica.

Cheers! Rich

Yup; the silicone is too thick. And the vendors measure theta with insane clamping forces applied, then lie by another 2:1 for good measure.

John

The interface between the device and the heatsink is probably the most critical piece in the puzzle, and is the piece that a builder/repairer has most control over. The device temperature is what you're trying to control. You have no control over the internal thermal impedances of the device... they are determined by the manufacturing process and materials. Given a heatsink has adequate capacity for dissipating the heat (heatsink to air thermal impedance), then that only leaves the device to heatsink interface to manipulate. In comparison between greased mica and silpad insulators, the mica/grease combination has the better thermal characteristics. Mica can be problematic though, mainly because there seems to be no quality control on the manufacturing process. The thickness of a mica slab can vary from 0.010 to 0.050 or more. Mica can withstand about 1000 volts per mil, so it really doesn't need to be so thick. You can usually split a mica slab several times and still be safe in using it. The thermal impedance goes down somewhat with thinner slabs, too.

Overall, mica/grease can exhibit almost half the thermal impedance of a silpad. Given the same device power dissipation, lower thermal impedances mean lower device temperature.

That being the case, why is it so easy to burn your tongue on a hot drink or hot food ?

Arfa

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I think I will rely on the evidence of my own observations and not performance tables produced by the manufacturer's with an obvious vested interest. I've no reason to believe the original silpads had aged, been affected by WD40 or anything. I will assume they are , all manufacturers, all generically bad until a similar personally conducted experiment, in a real situation, proves to me to be otherwise.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Exactly what is the item of equipment in question?

Ron(UK)

Surely a degree of faith in what manufacturers say, has to be assumed, otherwise we would all go around our whole lives with our heads up our collective arses ! Do you for instance do a complete analysis of every piece of processed food you eat to see if the manufacturer is lying about how much salt it contains or how much calorific value it has ? Do you check your bottles of beer to see if the ABV that the manufacturer quotes is true ? Do you run a complete set of tests on a resistor type before stocking it as a spare to see if what the manufacturer says about its tolerance stability is correct ? No? Neither do I.

I can understand scepticism about 'waved in the air' figures that appear to have little or no foundation in reality, but figures for these washers that quote specifics on the applied pressure to get the results that are published, don't seem to fall into that category to me. I can also understand manufacturers of some products getting into 'mine's bigger than yours' specmanship contests, but silicone heatsink pads ? Do you really believe that these are such a big part of the manufacturer's total output of products, that he needs to engage in the sort of behaviour that involves lying, or at least distorting, the 'true' facts about the product, just to sell a few more than the other guy ?

As it happens, I cannot remember seeing pads distorted in the way that you describe, so maybe they were from a bad batch or something. Maybe, the amp manufacturer torqued them up too much, and the heat did the rest. Maybe the amp was used on midsummer's day in Ibiza at full volume with a blanket over it, and a fault ...

I agree that many amps have output devices that appear to run hotter than we might like, and that the thermal transfer across the insulators seems less than ideal, but if that's the way the manufacturer made it and is happy with, and the item is not on my bench with failed output transistors, then I tend to just note the fact with interest, and work on using the "If it ain't broke, don't fix it" principle.

And to Graham, I HATE amps that have live heatsinks. Just a few days ago, I had a big AV amp on the bench that nearly threw me off my chair when I laid the edge of my hand across the two heatsinks ready to take some measurements with the meter probe in the other hand. This thing had rails of about +/-

Arfa

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Unless big G has a defendable patent on mica then why do I not see like-for-like , in real situations, comparison tables of BigCorp GeeWizz SiilyPad versus mica in the performance figures.

"Live" heatsink amps would be ok if the damn things were labelled with big fluorescent stickes stating such. But of course such amps should only be serviced by their own service agents is what they would say.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

What other part(s) of your body would you be likely to immerse in hot food or drink?

Best regards, Spehro Pefhany