Microwave is a totally different Beast! Immersion Tin has
been the preferred finish for way before my day. I've been at
this for awhile too. That finish provides many things:
1) No thermal excursion at fab. (Thermal excursions expand
   thermal plastic resins <alot> and can equate to potential
   issues that could have been avoided <Especially MLB>.
   Like for each thermal excursion there may be X opens occur.)
2) Imm Tin coats all exposed copper. (Sidewalls included)
3) Tin is one of the constituents in TinLead and is redilly
   absorbed into the solder. <Should solder quickly>
4) Imm Tin does not have such a pronounced "skin effect" as
   ENIG does. (Where the signal travels along the gold and the
   current along the base of the trace <copper>. The eless Nickel
   makes this even more pronounce in microwave.)
5) Since the Imm Tin is a replacement process, it removes copper
   atoms and replaces tin atoms <1:3 I believe>. This keeps the
   outer tin surface fairly clean if rinsed properly.

I agree with Kelly. Maybe the shelf life is an issue because there
is a intermetallic that's naturally forms between the copper and
the tin. Each time you either do a thermal excursion or keep
it on the shelf for long periods of time, you can expect to see
that Intermetallic thickness increase. If it increases alot, it
can inhibit soldering.

Dave
-----Original Message-----
From: KELLY M SCHRIVER [mailto:[log in to unmask]]
Sent: Tuesday, September 21, 1999 9:42 AM
To: [log in to unmask]
Subject: Re: [TN] solderability of PTFE Taconic material boards


Jason -

You mention that the traces are tin immersion plated - have they been fused
or
reflowed in any way?  Straight tin plate often loses its solderability very
quickly after plating resulting in conditions similar to what you describe.

I'm not a real proponent of immersion gold over nickel, but this may very
well
be one of those cases where it should be considered.  Or, as an alternative,
one of the white tins: FST from Dexter; or Omikron from Florida Cir Tech.

Regards - Kelly

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