Question regarding thermal paste

Isotonic

Isotonic

A life soaked in Music and Sound
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I am recapping a Sansui 710 (717). I have heard from various restoration threads here and there that applying new thermal paste to output transistors is a good preventative measure in preserving them.

So I went to Fry's and asked for the thermal paste. I ended up with a "high performance" paste called Cooler Master. One of it's features is that it is "dielectric", which I assume, means that it is non-conductive. It also says that it is "heavily filled with heat conductive metal oxides".

I did a little reading on correct application of thermal grease and got to work.
As I pulled off the output transistors from the heat sink I noticed that the old, mostly dry grease was in the screw holes and as I pulled the pins, I noticed that there was some on them too.

After applying a thin layer of new grease to each side of the plastic (mica?) surface I re-inserted the transistors through the heat sink and into a part on the other side that has the leads connecting the transistor to the rest of the amp.

It occurred to me that some of the new grease had mixed with the old and that the new grease might cause an electrical pathway between the pins, or impeded contact at the end of the pins.

I don't undertand how these things work. I don't want to have a problem when I add electricity to the amp.

Am I doing something wrong?
 
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Dielectric properties indicate the degree at which a material's insulating properties will of break down and begin to conduct. Given enough voltage a substance that is a dielectric will start to conduct. Some examples are:

Helium 0.15
Air 0.4 - 3.0
Glass 9.8 - 13.8
Silicone oil 10 – 15

Most insulating heat compounds contains silicone which has a high dielectric constant.

I have never used the compound you noted, but read that it is designed for CPU chips sets. These run on < 12v and is low current. It has a dielectric constant of 4.4. This is just a little better then air. We have all seen how electricity can arc over an air gap.

Before I past this on transistors that have +60v/-60v or more on the rails, I would investigate further. Maybe some of the AK members can speak from experience. I use only silicone based paste with ceramic powder to enhance insulation.

If you have the right past, then you don’t need to worry about conducting current between transistor leads. Also, remember that you don’t need much of this. Just a small amount will provide the needed coverage and heat transfer. If the stuff is squeezing out all around the transistor when tightened down, you have used too much. keep it off the pins, use lacquer thinner and cotton swabs to clean the old off the mica waffers and the back of the transistors prior to applying the new.
 
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GC --Z9 compound
http://www.gcelectronics.com/order/Catalog_PDF/2007 Catalog 120407 54.pdf


9 - Thermal Compounds
One of the most common mistakes made by hobby electronics enthusiasts (and quite a few professionals too), is to assume that if a little thermal compound is good, a lot must be better. Absolutely not so! The amount of thermal compound should be exactly that amount which ensures that an air-free join is made between the mating surfaces. If too much is applied it will cause an increase in thermal resistance, since it is not really that good at conducting heat. Generally speaking, any electrical insulator is also a thermal insulator, so the thinner the final composite insulation - including thermal "grease" - the better.

Having said that, one must ensure that the electrical insulation is sufficient for the applied voltage or disaster will surely follow - usually in a spectacular fashion - especially if high voltages or currents are available.
Click to expand...
Quoted from ESP Articles page..
 
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I'm pretty sure that the paste on the pins was from the origenal paste as I was very careful to put a thin layer on the wafer and on the transistor, but it was late and the stuff is pretty messy to work with.

I chose this paste over the traditional kind because I thought that it may improve the ability of the heat sink to conduct heat, and therefore extend the life of the original transistors.

I think I will dismantle the whole thing, clean it and start over again. I'm not sure if I will use the high performance paste. I'll have to research that further.

Thanks for you help.

Has anyone here used high performance paste on output transistors?
 
This subject came up recently and Echowars said it was best to keep the silver type pastes for what they were designed for, CPUs, and to use the silicone based for electronics. I believe he know of which he speaks. Avionic's suggested product would be right up your alley.
 
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