Thermal pads are supposed to be single-use, and not applied and re-applied. Thus, mica is the only option for tinkering.
You do not smear the grease, you put a small blob in the middle of each side of the mica sheet, then press it against the heatsink - once - so that it sticks (and never move it after this), then put the transistor on and tighten the screw. This will cause optimal grease flow and the best surfaces possible.
Smearing causes air pockets, adds dust and dirt and other suboptimal surfaces.
You beat me to it. I agree. And I have been the power expert for decades, not necessarily only in audio either.
You get a feel for how much to put on. Where to put it ?
On a TO-220 you put it right by the hole. You just barely finger tighten it and then press down with your thumb or whatever and let it ooze out the sides. It should only ooze a little bit but it is important to apply that pressure. If too much comes out, use less next time. But better it be out there than in between the heat sink and the part that needs it.
On a TO-3 it goes almost between the pins but on the bigger side, close to the pins as possible. Yes it squeezes through the holes, so what ? Actualy that means put a ittle bit more on to compensate. Again, you get a feel for it.
Incidentally I got a couple of things to say. Alot of times I have run out of things and had to make due. Those gray rubber insulators can be reused, but I wouldn't try it too many times.
Another thing is to INSPECT the mica insulator for internal delineation. That means the layers separating inside. If that happens the thermal compound is almost useless. Now the tricky part is that in many cases all those layers together make for like 2,000 volts insulation. There are lower rated ones that are thinner, you almost never need 2,000 volts in audio. That was for horizontal (line for those across the pond) or high voltage output transistors in TVs.
If you do find one with the layers separating you can separate it and only use part of it. But make sure you can use at least half of it...
And if you ever accidentally bend it and it makes like a crack in it, you are done.You need a new one because that separates ALL the layers and there is no way to make it right.
That is one of the reasons I do not recommend re "gooping" transistors when they do not need to be replaced. That cement the stuff turns into does not depend on moisture to conduct heat. Leave it alone. Seriously, you are not doing any good at all and run the risk of cracking the insulator.
I'll be hitting four decades in the business soon, so if anyone wants to disagree with me, state your case. Do not try to tell me what Thermalloy or anyone else said, because they sold nice TO-3 insulators with the compound already on them. A useless concept in my professional opinion. What's more they sold them in paper packaging which could bend and cause splittig, and you would not be able to see it without taking the compound off of it. Defeats the WHOLE purpose.
I am qualified to criticise them.
