Earl,
Rogers website has a lot of info on materials for RF applications... Some of
the things we learned about RF were not necessarily intuitive. We made
transitions from Wave guide to PCB and from RF connectors, SMA's, N-type,
SMB's etc... Platings and process variations were problems. We typically
used aluminum housings for RF products. Many screw locations to help with
grounding, each discrete circuit was located in a shielded environment with
a small pass thru to allow the RF signal to go on to the next circuit...
originally we mounted all our PTFE boards on aluminum carrier plates and
screwed them to the base. We found that the "one board" approach was the
most cost effective and created a cover from machined aluminum to create the
isolation needed.
Thermal grease is still used in the industry, but nobody likes it. The
silicone 'migrates' all over the inside of the assembly and can dry out in
time and become non-functional. I suggest using the 'Sil-Pad' approach for
thermal transfer where you can... but I doubt you will be able to avoid the
silicone grease entirely.
I saw some cool products for special thermal applications, a sort of foam
loaded with ceramic particles was interesting, its thermal conductivity was
much higher that I expected and it conformed to non-flat surfaces well. I
have seen EMI reducing SIL-PADS used with a foil sandwiched between layers
of silicon rubber that had a soldering tab that could be connected to ground
for instance.
Thermal expansion needs to be accounted for in RF assemblies, I have seen
connecters break their connection to the board or lift the pad just due to
the CTE differences in materials.
Bill Brooks
PCB Design Engineer , C.I.D.
DATRON WORLD COMMUNICATIONS, INC
3030 Enterprise Court
Vista, CA 92083
Tel: (760)597-1500 Ext 3772 Fax: (760)597-1510
mailto:[log in to unmask]
IPC Designers Council, San Diego Chapter
http://www.ipc.org/SanDiego/
http://home.fda.net/bbrooks/pca/pca.htm
-----Original Message-----
From: Earl Moon [mailto:[log in to unmask]]
Sent: Wednesday, December 19, 2001 9:12 AM
To: [log in to unmask]; Bill Brooks
Subject: Re: Assembly of RF boards
Sure like what you, and others, say about gold, OSP's, and silver. Wish you
could convince 90% of the other engineers designing as if some very old
"traditions" had merit.
Also, thermal is a concern. Some of the amps get very hot. Some engineers
really fight using any type thermal transfer improvement as grease or
whatever.
Earl Moon
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