[TenTec] Sil pads versus mica washers (mounting transistors to heatsinks)

[Dr. Gerald N. Johnson]

On Fri, 2008-06-06 at 07:06 +0800, Marinus Loewensteijn wrote:
> I am waiting for the new drivers and new finals for my transceiver to arrive (should have been here already).
> 
> One thing has just cropped up and perhaps some of the more up-to-date knowledgeable ones among you can give me advice.
> 
> When I was active 20 years ago the only thing that we had for insulation were mica washers. The finals are no problem - they go straight against the heatsink and do not need any insulation. The drivers, MRF 476, 3 watt dissipation, need to be mounted with insulation. Mica washers were used but when I went to get some the ones available were quite stiff and thick, approx 1/64" (0.75 mm) from a rough "guesstimate". I was told that no-one uses them these days anyway that sil pads were the ones to use instead. What's the life expectancy of this silicone robber insulation? What about reliability?
> 

The mica washers can probably be split to make them thinner. They
require thermal grease on both faces. Thinly applied.

The silicone pads don't crack like mica, don't have conductive
inclusions like low grade mica and don't need thermal grease. Not having
to apply thermal grease (and its often applied way too heavy) speeds up
production and prevents the over application of the thermal grease. Too
much thermal grease makes the transistor run hotter.

> Anyone able to shed more light on this? 
> 
> Many thanks in advance, Marinus, ZL2ML.
> 
73, Jerry, K0CQ