Thermal paste selection for MT200 Transistors

[Laurie]

G'day Ladies and Gents...Just a quick question, I need to re mount some MT200 Transistors and Mica pads for a Class A amplifier i am working on, I haven't done this before and I am just wondering what grease i should be using...I understand in needs to be a non conductive type...

I am sure its an easy question, unfortunately when you start looking for a product its all CPU stuff, and I am thinking its not the right stuff to be using...

[wraper]

Cheap zinc oxide/silicon based thermal paste (white color stuff) is adequate for this. Fancy CPU thermal pastes can be hit and miss if you want reliable insulation.
Like this one http://www.tme.eu/en/details/wps-1gram/heat-conductive-compounds/#

[Laurie]

Thanks for that mate.

[edpalmer42]

I don't understand your comment about 'non conductive type'.

It doesn't matter whether the grease is electically conductive because the mica sheet handles the insulation.  Just don't use a trowel to apply the grease.  Thermally, all forms of grease are insulators.  It's just that air is a much better insulator so you want the grease to push the air out of the gaps.

Were you thinking of something else?

Ed

[stephunk]

EDpalmer42: A thermal paste with a silver additive should be used when cooling CPU´s but for discrete power electronic shall be used an electrically nonconductive white Zinc thermal paste. Yes, there is mica which insulates transistor from the heatsink, but i´ll bet my shoes the paste will get into space where metal screw meets heatsink-and if the paste is electrically conductive, than the collector is electrically connected with heatsink. Or even better case, that the paste gets between the pins 

[edpalmer42]

EDpalmer42: A thermal paste with a silver additive should be used when cooling CPU´s but for discrete power electronic shall be used an electrically nonconductive white Zinc thermal paste. Yes, there is mica which insulates transistor from the heatsink, but i´ll bet my shoes the paste will get into space where metal screw meets heatsink-and if the paste is electrically conductive, than the collector is electrically connected with heatsink. Or even better case, that the paste gets between the pins 

I see now.  Depending on the type of case, the screw could cause a problem.  I was just working with some transistors in TO-3 plastic packages that were totally insulated.  I guess I had that idea stuck in my brain.

However, if the paste gets between the pins, you used way too much paste!

Ed

[SeanB]

World of difference between a TO3P ( or TO247 actually) package with an all plastic body and a steel bare metal TO3 package transistor.  There you really want an insulating white zinc oxide filled heatsink paste, and even then you need to have a thin layer to keep the insulation good enough to handle 500V, and for higher voltage you get aluminia insulators which are around 1-2mm thick which allows you to do 2kV of insulation.

The funny thing is that for a CPU in most cases the white compound will work perfectly, unless you are going for an overclocking record and need to have that 2C lower drop across the CPU and heatsink base at 200W of power dissipation.