tempcos + silicone glue question

KA4HJH ka4hjh at gte.net
Mon Nov 22 06:29:40 CET 1999


>The VCO's in the Roland Jupiter-8 use the CA3046 with the tempco
>cemented to the chip with silicone rubber cement. It looks and feels
>like white RTV (most silicone cements are RTV). Do not know if it is a
>specially heat conductive formulation or not. Some epoxies are made to
>have better than usual thermal conductivity. This stuff would be very
>difficult to remove!

RTV stands for "room temperature vulcanizing", which makes it a type 
of rubber. They make a LOT of different varieties of the stuff with 
all kinds of properties. Make sure you get the right kind for the 
job. Also, RTV gives of acetic acid fumes as it cures, and I've 
always wondered if that might be a danger to circuit boards and 
components. You can speed up the curing process by heating it.

Silicone IS a better-than-you'd-expect thermal conductor, although 
obviously not like metals. Silicone-based grease can be found between 
transistors and heatsinks everywhere you look. There are also thin 
silicone rubber insulators which, unlike mica, can be used "dry" and 
still have decent thermal conductivity for lower-power applications. 
Making your own thermally-conductive insulators from raw RTV might be 
a rather tricky, however. Sounds like a good way to fry something.

As for attaching tempcos, it's probably both conductiing heat AND 
providing some insulation from ambient changes (if you really cover 
everything). Certainly better than nothing.

These days microprocessors are still covered with a THIN layer of 
silicone grease (I clock-chipped my 180MHz 604e daughtercard recently 
and it was greased) or attached with a thermally-conductive epoxy. 
The latter is a bit more difficult to remove! (RTV just peels off). 
I've already told the story of what happened when Power Computing got 
a bad batch of epoxy--it took them over six months to figure why 
their computers kept flaking out (that was the official excuse, 
anyway).

---

A note to regular list-lurkers: I hope I don't sound like a 
know-it-all but the thoughts are really flowing tonight. Bear with 
me. I don't actually know everything. As soon as I get on "Jeopardy" 
my mind will go blank.  8)

Terry Bowman, KA4HJH
"The Mac Doctor"

ICQ: 45652354



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