Thermo Pad or Thermo Grease?

[AMPREPAIR]

I have a nagging question. What is BEST for heat transfer; a thermo pad or thermo grease?

[soundguruman]

The answer is BOTH.
Even with a thermopad, there are air gaps that can be filled with zinc grease. AND it's better to do it that way.
Yes, you can use BOTH at the same time.
The thermopads tend to harden, split, and crack after many years, and I replace them with mica insulators, and grease.
Although the thermopads are more neat and clean, and require less effort, I think the grease transfers more heat, faster.

As far as the GREASE, SILVER grease is better, and transfers much more heat, much faster. It is superior in all ways!
BUT it's conductive, and you gotta be careful not to short out your components if you are using it.
I would rather use SILVER grease on many things that were formally thermopads or zinc grease.

[Enzo]

I think it important to keep in mind whether cooling is adequate. It is fine to say one thing is "better" than another, but if both ways provide the needed cooling, then "Better" doesn't change anything.


If a manufacturer can use silpads and save money on materials and labor and still have proper cooling, then that is what they will do.

It is like stepping off a cliff edge. You either fall into the abyss or you don;t. Doesn't matter if you step a little farther than you have to, you still fall to your demise.

[soundguruman]

I think it important to keep in mind whether cooling is adequate. It is fine to say one thing is "better" than another, but if both ways provide the needed cooling, then "Better" doesn't change anything.


If a manufacturer can use silpads and save money on materials and labor and still have proper cooling, then that is what they will do.

It is like stepping off a cliff edge. You either fall into the abyss or you don;t. Doesn't matter if you step a little farther than you have to, you still fall to your demise.

A little thermopad doom and gloom from Enzo.

[Enzo]

Doom and gloom? No, just advising against a lot of extra effort to no advantage. It is easy to look at everything and think how much better it would be if we only increased... whatever.

The radiator in my car does a fine job of maintaining the proper temperature in the engine. Installing a larger radiator would do no better. Same concept applies here.

[Jazz P Bass]

I have a nagging question. What is BEST for heat transfer; a thermo pad or thermo grease?

It really does depend on the application.
If the transistor collectors are common to the heatsink, then you only need the grease.
Maybe a better wording would be "what is best, thermo pad or mica?"

[R.G.]

Again, Enzo is right. If your application is so near the edge of the cliff that the micro-differences in thermopad and mica/grease or the differences in one type of grease or another (they're actually solids-filled pastes, not greases BTW) make a difference, then you are asking for a burnout if the heat sink surfaces get coated with a little dust and bar funk.

Not to mention that there is an optimum tightening torque/force on the screws and/or bolts that hold a transistor down. The right torque holds the thermal surface flat; too little leaves too much gook in the spaces between device and insulator, and too much deforms the heat sink or device so it's not flat. Getting the torque right is more important than the right grease or insulator. Do you use a torque-screwdriver or nutdriver to seat heatsink screws?

Neither do I.

If you're too close to the edge, you need another transistor pair to share the thermal load, not a microscopically better grease or insulator.

[Gtr_tech]

The pads work OK, but mica works a bit better. Plus, you can reuse the mica as long as its in good shape. Don't try to reuse the pads...its not a good idea as they compress and can be cut thru easily after that.

Arctic Silver is for computer dorks....not MI techs. 3.3vdc is one thing, 100vdc on a high power amplifier is completely different...and you damn well don't want anything conductive there.

[Steve Conner]

Take that, computer dorks!

Bergquist make several different kinds of Sil-Pad, some of which perform considerably better than the cheap generic grey ones. For my last solid-state amp build, I used a sheet of Sil-Pad A1500 and cut it to size myself. I also clamped the devices with a bar, instead of relying on the provided screw hole in the TO-3P package.

The high performance grades are naturally more expensive, but it may be worth it compared to, say, another pair of power transistors.

[Gtr_tech]

Good idea clamping the transistors with a bar. It holds the part of the x-istor mounting tab nearest the die firmly to the heatsink.

[R.G.]

There are some spring clamps made by ??? Aavid??? that use one screw hole to hold them down, and they press down on the insulated body of plastic power devices in about the center of the metal pad area. Very nice indeed.
See figures I and J in this link.

[soundguruman]

The pads work OK, but mica works a bit better. Plus, you can reuse the mica as long as its in good shape. Don't try to reuse the pads...its not a good idea as they compress and can be cut thru easily after that.

Arctic Silver is for computer dorks....not MI techs. 3.3vdc is one thing, 100vdc on a high power amplifier is completely different...and you damn well don't want anything conductive there.

You are right, mica and zinc grease works better. And it IS BETTER.
But has been said, that the heat dissipation is limited by the heat sink and fan, and after saturation occurs, everything is hot. And that means no matter what method is used, it's sometimes too HOT.
EXAMPLE: Hartke 5500, a woefully undersized heat sink, that cannot possibly dissipate the power of the amplifier, no matter what grease and insulators, no matter how fast the fan goes, it still runs way too hot. The heat destroys it's own components, it self destructs.

I have successfully used silver grease on many pro sound devices. It IS better than zinc grease. It dissipates heat much, much faster than zinc.
BUT it is conductive, and needs to be applied by a qualified installer.
It also improved processor temperatures by 30 degrees, just from a simple, quick application.
However, you can't rule it out for use on other devices, besides computers, there is an obvious advantage. I would RATHER use it when possible.
I am a "pro sound" tech, not an "MI" tech. And so that must be where the difference is.

[Steve Conner]

EXAMPLE: Hartke 5500, a woefully undersized heat sink

Woefully undersized heatsinks are quite common!

The argument goes that music signals don't consist of 100% power all the time, so you can get away with a smaller heatsink and power transformer. Once one manufacturer starts doing it, all of the others follow to keep their prices competitive.

If a designer does this, it's only common sense to fit a thermal cutout so that if the amp is driven at 100% sustained power, it'll shut down rather than blowing up. But they don't always get that right, or even bother to do it, since the cutout costs money too.

It follows that sustained sine wave bench testing of modern solid-state amps is not a good idea. One-third power is worse than full power.

[soundguruman]

If a modern amp cannot survive full power continuous testing
(crate amps fail this test)
Then I believe it belongs in the trash can.
I explained this to the techs at the Crate Factory many years ago: heat rises (surprise).
If your amp is not designed for continuous operation, time to buy another amp. Herein is the downfall of Crate.

[Steve Conner]

Well, most can't. Welcome to the 21st century, Mr. Van Winkle.