On Tue, 23 Mar 2004 14:44:30 +0000 (UTC),
(Ken Smith) wrote:

In article ,
Tam/WB2TT wrote:
Roy,

We were talking about bypass type ceramics. See the 3/20 10:13AM posting.
BTW, I think with SM you are pretty much forced into using ceramics.

Cornell Dubilier / Waldom makes surace mount siler mico caps. You can
get them from Digikey for under $10 US.

The CDE "MC" series of cazapitors are "mica", not "silver mica". The
difference is that silver mica caps have to be sealed (dipped) or the
silver plating reacts with everything. I'm not sure what plating is
used for the "MC" series of surface mount mica. My guess(tm) is
aluminum.
http://www.cornell-dubilier.com/mica/mica.htm
http://www.cornell-dubilier.com/film/hmc.htm
http://www.cornell-dubilier.com/catalogs/MC.pdf

The big advantages of silver mica is stability, wide temp range, very
low dissipation, and tolerance to over voltage spikes. Many years
ago, I wasted a month working over an HF xmitter, trying to design out
the expensive silver mica and porcelain cazapitors and replace them
with cheaper ceramics. It was possible for the low power drivers but
a waste of time in areas that had high RF currents or required good
stability. A similar cost reduction exercise was also being done on
the automagic antenna tuner (by someone else) with similar results.
The project ended when someone suggested using high temp silver solder
to prevent the ceramic caps from reflowing their solder connections
and falling off the board.

I guess(tm) the reason that silver mica caps are difficult to find is
that there are few companies producing high power RF products as
compared to the huge number of low power RF products. It's not a big
market that probably can only support a few specialty component
vendors.


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