Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!hplabs!hpfcdc!hpfcdj!myers
From: myers@hpfcdj.HP.COM (Bob Myers)
Newsgroups: sci.electronics
Subject: Re: Which is a better conductor: gold or silver?
Message-ID: <17660002@hpfcdj.HP.COM>
Date: 31 Jul 89 19:58:29 GMT
References: <14172@swan.ulowell.edu>
Organization: Hewlett Packard -- Fort Collins, CO
Lines: 29

>Does the military still consider that ceramic/cerdip packages also have better
>thermal characteristics, in general. Even with copper lead frames, a problem
>with plastic packages is that the plastic is (relatively) a heat insulator,
>so that little heat gets out the top of the package. Most heat must flow 
>through the leads. Thus, fan cooling is less effective with plastic packages.

I don't know about the military, but my experience in testing the beasts is
that a ceramic package is certainly better as far as thermal characteristics
go; ceramic has also been considered "more hermetic" than plastic, although
more recent (last five years or so) plastioc packages have been very good.
The best package, IMHO, is still the ceramic "sidebraze" (leads brazed on to
pads on the side of the ceramic package proper) with a metal lid.  
Unfortunately, it's also the most expensive.

One other little item to keep in mind is that a CERDIP package (ceramic
"sandwich" with a frit seal and a leadframe as found in plastic packages)
is NOT as good as the older ceramic sidebraze style.  Thermally, it's OK,
but I've found the frit seal to be fragile - and once it's cracked, it's
worse than plastic for letting moisture and assorted gunk inside.  Automatic
insertion equipment can do a real number on these packages if you don't
watch it carefully.  (Most AI machines squeeze the leads - which are normally
"sprung out" a bit, as with a plastic package - together slightly when
positioning the part for insertion.  This can stress and break the seal on
a cerdip part.)


Bob Myers            | "Writing is not necessarily something to be ashamed of -
myers%hpfcla@hplabs. |  but do it in private, and wash your hands afterwards."
hp.com               |                   - Lazarus Long/Robert A. Heinlein