Thermal Pad Recommendations

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Agegrouper

New Member
Greetings. I am refurbishing an old Adcom GFA-535II amplifier. I am replacing the power supply board with a Hoppe's Brain kit and am now working on the two main boards which each have several transistors that are screwed into heat sinks. I already disconnected to get the boards out and now want to replace the thermal pads that go between the transistors and the heat sinks but not sure what type or specs I should be looking for. I believe the transistors are TO-3 kit but not sure what that means. I've seen ceramic, silicone, and mica with paste discussed on the web. Any guidance would be greatly appreciated. Thanks.
 
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Interesting. So those look like they are silicone. Do you have to use thermal paste or some kind of grease with those or how do you otherwise attach? I also have a 555II which I want to do next but am starting with the 535ii first for practice since it is simpler.
 
Agegrouper said:
Interesting. So those look like they are silicone. Do you have to use thermal paste or some kind of grease with those or how do you otherwise attach? I also have a 555II which I want to do next but am starting with the 535ii first for practice since it is simpler.
Click to expand...

That is the benefit of the silicone sil-pads, no grease required.
 
With "Sil" pads you need to be careful that the case of the device does not cut through the pad when installing the device. The pad is pretty soft so go easy on tightening down the device or you will get a short. Do a continuity check after each installation to verify the install. I have used "Sil" pads but since have gone back to mica/paste. Over a buck each is too steep and I have not found a good reason to use the silicon pads vice mica.
 
I've stopped using grease and mi a pads. I use silicone pads now on everything. No mess! I use the thermal grease I have left to lube the plunger O-ring and tube on my heated solder sucker.
 
aldena007 said:
With "Sil" pads you need to be careful that the case of the device does not cut through the pad when installing the device. The pad is pretty soft so go easy on tightening down the device or you will get a short. Do a continuity check after each installation to verify the install. I have used "Sil" pads but since have gone back to mica/paste. Over a buck each is too steep and I have not found a good reason to use the silicon pads vice mica.
Click to expand...

Everything has its ups and downs, mica can crack. We used both at my old job.
I'm not 100% sure but I think sil-pads also offer better heat transfer.
 
Agegrouper said:
Interesting. So those look like they are silicone. Do you have to use thermal paste or some kind of grease with those or how do you otherwise attach? I also have a 555II which I want to do next but am starting with the 535ii first for practice since it is simpler.
Click to expand...
The soft gray rubber-like gray-colored silicone pads do not need any heatsink compound. The material conforms to all the irregularities of the surfaces that sandwich it to enable good heat transfer.
 
I agree that both heat pads have their ups and downs and both do the same thing, but I can't see enough difference to justify the higher cost.
 
aldena007 said:
The pad is pretty soft so go easy on tightening down the device or you will get a short
Click to expand...
read the installation guide for the devices to find the torque for the bolts. For theTO-3 transistors I read about it is under 10 INCH pounds. Not very tight at all. This amount should not cut a sil-pad or bend the device causing less contact area.
 
Most people overtighten the screws on silpads.
The standard is that they should not be compressed more than 20% of their thickness.
Without an inch-pound or equivalent metric torque wrench run the screw down to contact and give a light extra snugging, 1/8 of a turn is enough, 1/4 turn is probably too much.

The best results for the transistors that we are using will be found with 0.2mm thick pads. 0.3mm are available but I would only use that if the heatsink surface is not flat enough and you need some extra gap filler.

All good silpads will have a fine fiberglass mat imbedded in the pad for stability. The silicone is flexible in 3 dimensions, the fiberglass mat is not. When the pad is overtightened not only can the transistor cut into the pad but the movement of the silicone from the pressure tends to rupture the structure around the fiberglass mat, thus degrading the heat transfer through the pad.

All good silpads will have some amount of ceramic or other type of filler to increase the heat transfer. Boron is found in some of the more expensive pads.

A more expensive alternate is a kapton pad coated with a special heat conductive phase change wax, but that is a bit hard to find and used mainly in military and high cost applications.

Cheers,
James
 
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