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Re: [Amps] SSPA & Heatsink Compounds

Subject: Re: [Amps] SSPA & Heatsink Compounds
From: Steve <>
Date: Wed, 18 Dec 2013 17:39:46 +0000
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The flanges on the common four lead transistors with round caps are soft copper, and bend as you say. When the screws are tightened, metal is displaced inwards and the large central patch tends to lift away from the heatsink. There's an interesting article in Feb '84 Microwaves and RF by J Scholten from Philips - 'Modeling transistors when the heat's on'. He found the centre area about 7mm dia. contributed almost nothing to the heat transfer from chip to heatsink.

The flanges on the high power push-pull parts (such as MRF151G and the current crop of 1kW+ transistors) is a copper tungsten alloy and is much harder.


Steve, and all,

>One word of caution with Manfred's technique - be very careful if
>applying clamping pressure onto a transistor with a ceramic cap.

Yes, that's right! I had plastic parts in mind, like TO-220 and

>The cap is strong round the outside edge, but weak in the centre. If
>your clamp happens to push too hard in the wrong place you can end up
>breaking the cap and the broken pieces fall in and mash up the bond
>wires. The magic smoke is still inside but your transistor is dead.

True. With ceramic encapsulated transistors, it's a good idea to be
reasonably gentle when pressing down in the middle. Like finger
pressure only.

With those, I usually tighten the screws very slowly, and help
pressing down with my thumb. The flange on these deforms easily, so
one has to be really gentle with the screws too. Heating helps, to
soften the compound.

Also be aware that some compounds are MUCH softer than others! I
wouldn't use any of the stiff ones with this sort of transistor!
Instead the stiff ones work fine with plastic cases.

The bad thing is that compounds with better heat conductivity tend to
be stiffer too, because they have more densely packed granules, and
less fluid between them!


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