Hi Todd,
I've run into the SnPb not properly soldering to pure Sn in a typical
leaded reflow profile when the Sn thickness is exceptionally high.
Apparently, the high Sn content of the component was changing the alloy
of the eutectic SnPb solder joint to a high Sn content SnPb solder, with
a corresponding higher melting point. This caused the solder to
prematurely solidify before a proper meniscus could form. The suppliers
response was that we should use a lead-free reflow profile (with our
leaded paste) to properly melt the 'now' high Sn solder. Our response
was that they can either bring their plating thickness under control or
sell their parts to someone else!
I would compare your suppliers plating thickness to your solder joint
volume. If their plating thickness is trivial compared to the solder
volume (it should be!), then I don't see why a SnBi plating would need
to become liquid to solder to it.
Quite frankly, I wouldn't allow a SnBi plating in a SnPb soldering
process anyway because of the concern of SnPbBi eutectic and heat
refining localizing that alloy...
Regards,
Ryan
-----Original Message-----
From: Leadfree [mailto:[log in to unmask]] On Behalf Of MacFadden, Todd
Sent: Monday, April 17, 2006 4:16 PM
To: [log in to unmask]
Subject: [LF] Backward compatibility of SnBi Finishes
Hi folks,
Three different suppliers of components with SnBi termination finishes
have warned us that their parts are not backward compatible with our
SnPb process because our reflow profile is apparently not hot enough. We
specify a peak temp range of 213-225 for 10-30 seconds within actual
peak and a total time above liquidous of 30-90 seconds. One supplier,
for example, recommends 230 C for 10 seconds.
This seems unnecessarily high. We've used components with pure Sn
(melting point ~230 C) in our SnPb process for years with no problems;
intuitively, wouldn't an alloy with a lower melting point (e.g., Sn-4Bi
m.p. ~225 C) promote better wetting and IM growth than pure Sn? Is there
some other concern with reflow of SnPb paste on SnBi finishes I am
missing?
(By the way, the concern in all cases is with reflow heat, not with the
possible formation of the super-low SnPbBi eutectic; the Bi content of
the parts in question is <4%, far too low for that eutectic to form
anyway).
Thanks in advance for any input.
Todd
--------------------------------------
Todd MacFadden
Component Engineer
Bose Corporation
The Mountain, MS 31
Framingham, MA 01701-9168
508.766.6259
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