Thermo Pad or Thermo Grease?

[Enzo]

Mr Guru doesn;t seem to realize that full power testing to an audio amp is not sine waves. SIne waves are great for watching clipping, but they are a very unrealistic signal for amp power testing. Try using a music signal - the thing they were designed to do.

[R.G.]

You are right, mica and zinc grease works better. And it IS BETTER.

The problem with the word "better" is that it has no meaning whatsoever unless you can describe some metric for measuring what is better. Otherwise, it can only mean "I think I like it better."

I'm thinking you must mean "transfers the same heat at a lower temperature drop than [whatever]". If so, that's measurable. And in fact, it has been measured - a lot. The folks that make rubber pads know that they have to beat the combination of performance, cost, and reliability to get designed in.

However, the measurement has to include some factors of cost of the materials, ease of use, and reliability. After all, the highest performance material for a thermally conductive insulator is a layer of vapor-deposited diamond. Diamond conducts heat many times better than copper, and is a very good insulator. But there may be some price and usability issues with diamond, right?

It's worth reading Mounting Considerations for Power Semiconductors. The bottom line is that there are a large number of variables, and that the difference between filled silicon rubber pads and greased mica is miniscule, and in favor of the (in this case) Themafilm rubber insulators. See page 5, figures 5 and 6. Figure 5 shows the thermal resistance of the silicon rubber pads to be about 1.2 - 1.5 C/W on a to-3 without grease. As a side-line, mica without grease is up at 4, so getting the grease right with mica means the difference between 4C/W and 1.5C/W; getting the right amount of grease is critical. Figure 6 shows the thermal resistance of greased mica at 1.5C/W.

I think you may be going on experience with much older thermal pad insulators; they did used to be not as good as mica. They're better now.

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.

Not necessarily. Thermal resistance is funny. It's obvious when thought about the right way, but that's not always the obvious way.

Thermal interfaces are limited by the thermal resistance of the material *and the area it conducts through*. The whole point of thermal paths and heat sinks is to spread heat out. 1W in a 10W power resistor won't get it warm. 1W in a 1/4W resistor will cook it. 1W in a miniature lamp will get so hot it glows yellow-white. The idea is to get the heat out of the silicon junctions and into the surrounding air at the lowest temperature build up. This is why NASA and military space hardware actually has used diamond heat spreaders. Most commercial power devices solder the semconductor dies to copper heat spreaders, and then solder these to the (steel or aluminum) mounting plates of the devices. Whatever thermal resistance a surface has, increasing its area decreases the temperature drop across it.

If you want a cooler power device-to-heatsink path, spread the heat. One way is with something like a copper sheet under the device, and a kapton film and grease to the heat sink. The copper spreads out the heat from the device better, and the wider area of the kapton film allows a lower temperature drop than using just the area of the semiconductor. More heat sink fin area is doing the same thing at the sink/air interface.

Notice that there is more to be gained by spreading the area out than there is by making the insulator/grease better. The typical TO-3 transistor has a resistance from chip to case of 1C/W. If you just double up semiconductors, this effectively drops to 0.5C/W, as the same number of watts go through twice the area. If mica/grease is 1.5C/W and you use a copper heat spreader, it's easy to get 4 times the area of one TO-3, and you can cut that 1.5 C/W by four to under 0.4C/W.

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.

And if the semiconductors have a limit of 150C on the junctions, and an air temp of 40C, they have a temperature drop they can stand of 110C. The semiconductor has 1C/W drop. The insulator is 0.5 to 1.5 C/W. The sink itself can be 0.5 C/W (if it's huge!!) up to 10-15C/W. So changing the 0.5 to 1.5C/V of the insulator is not the biggest effect. Even if the sink itself is too high, you can halve *both* the junction to case and case to sink part by doubling the semiconductors. So you can partly compensate for a minimal sink by doubling semiconductors, and/or by using heat spreaders.

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.

You'll want to worry about the word "migration".