Jason: Standard immersion Sn has an extremely short shelflife and is
difficult to solder even on it's best day. That's why it went away 25 years
ago, although I'm sure some people are still using it.

 The most bullet proof coating for high frequency(giga)PTFE boards is bright
acid electroplated Sn(BAT). Typically 0.2-0.4 mils. This will give you a
6-12 month shelflife due to it's fine grain structure, doesn't expose the
board to high thermal exposures such as fusing during fab(MLB delamination)
and will have enough Sn to provide decent solderability. We have used this
approach for many years on high frequency signal generators/calibrators.
With that said, I am not a fan of bright Sn. It additives are hard to
control, it can have problems with organics and is prone to causing shorts
from Sn slivers. You may also have a hard time finding a board fabricator
that can supply you with a good solderable bright acid Sn electroplate. Most
are using matte/semimatte Sn for etch resist and if they are using BAT it
may not be in proper balance for optimal solderability and shelflife.
If you can find a fabricator that can provide you with a properly controlled
and solderable BAT I would go with that. If not you may want to consider one
of the new generation immersion Sn's, however I have no idea how these
coatings may influence signal propagation and effects for high frequency
design. Good Luck.


Regards

Michael Barmuta

Staff Engineer

Fluke Corp.

Everett Wa.

425-356-6076

-----Original Message-----
From: jason gregory [mailto:[log in to unmask]]
Sent: Tuesday, September 21, 1999 5:39 AM
To: [log in to unmask]
Subject: [TN] solderability of PTFE Taconic material boards


Hello all you technetters,

Question for all the "high-freq" guys out there.
We have been building PTFE boards quite some tme and a long-running problem
has been the solderability of these boards. They are .018" thick and are
used for high frequency applications. The spec is for tin immersion plating.
No matter what reflow profiles I use, there are non-wetting signs all over
the board. The solder just won't flow properly on the pads. It looks as
though the solder flows, then stops. I've eliminated surface contamination
by washing the boards prior to build and still the same problem. Are there
any other platings available that will promote flow and won't affect high
freq reliability. Please help. Thanks.

Jason Gregory
Process Tech
Tadiran Microwave Networks
(281)263-6671
[log in to unmask]

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