tempcos and silicone glue question

[Paul R. Higgins]

Thanks to everyone who posted info on this topic.  It sounds to me like the 
consensus is that tempcos should be in some kind of physical contact with the 
expo transistor(s), and that what you glue (or goop) them together with is far 
less important.  That being the case, I think I'll try the automotive RTV 
sealant, because I really hate that silicone heatsink compound.  In my shop, 
that stuff ONLY gets used on heatsinks.  Yuck!

The idea of thermally conductive epoxy is also a neat idea, but with my luck, 
I'd blow up either the expo or tempco at some point, and I'd never be able to 
get them apart for repair without destroying the whole damn circuit board.  So 
RTV it is.

Terry B (KA4HJH) suggested that the acetic acid given off by RTV as it cures 
might be problematic if the sealant made direct contact with PCBs and 
components.  I know that some kinds of RTV silicone sealant emit ammonia fumes 
rather than acetic acid, although I'm not sure if this is a much better 
situation.  On the other hand, acetic acid is pretty weak stuff in its ability 
to corrode metal; it's not exactly in the same league with HCl or H2SO4.  From 
what I understand, the acetic acid is in the RTV mixture to prevent 
vulcanization in the tube; once you dispense some of the goop, the acetic acid 
evaporates and the rubber vulcanizes.  So I would presume that the acid wouldn't
be around long enough to do much damage.  And for adhering a tempco to the top 
of a plastic DIP, this is a moot point anyway.

-PRH

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Paul Higgins
email: higg0008 at tc.umn.edu
University College, University of Minnesota
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