Hi All,
Well, we sure got a mish-mush of opinions ouit there.
I have written on the subject, most recently in the August 2001 issue of
Global SMT & Packaging magazine and the February 1997 of Circuits Assembly
magazine.
Surface scientists do NOT regard a surface as having no depth; quite the
contrary. A surface from the surface science point of view is at least a
number of atomic layers (not a mono-layer) thick; in fact, the crystal
arientation and atomic lattice structure at the surface will determine
surface characteristics, such as work function, surface tension, etc.
Solder wetting [an unfortunate choice of a word if there ever was one] does
not just include the 'wetting' of the surface in the saponifier sense, but
also the disolution of base metal atoms into a solder component, typically
tin. For copper that means the formation of Cu3Sn (near Cu) and Cu6Sn5. IMCs
are brittle, do not unlike the solder creep, and have melting temperatures
higher than their constituents. With the exception of IMCs of Au/Sn (AuSn,
AuSn3, AuSn4) and Ag/Sn (Ag3Sn), IMCs are much stronger than the solder. The
Au/Sn and Ag/Sn IMCs are weak and brittle; the former causes what is referred
to as ‘gold embrittlement’. The problem with too mch IMC formation is
primarily that the Sn is consumed from the solder volume immediately adjacent
to the Cu leaving a Pb-rich region--this is where cracks form when they occur
near the interface, not in the IMC layers.
The reason why soldering to Alloy 42 or nickel requires more heat [higher
temperatures/ longer times] follows from the different dissolution rates of
the various metals. To illustrate the point, the relative dissolution rates
in Sn, pegging Sn in itself at 100 mm/s at 230°C, are: 100 Sn: 2.5 Au: 0.85
Ag: 0.085 Cu: 0.0021 Ni [Ref.: Klein Wassink, Soldering in Electronics].
I sure have seen a lot of solder joint faillures because IMCs never formed
[no wetting], but not a one because of too much IMC. So I do not agree that
IMCs should be as thin as possible; I certainly have no problem replacing
HASL with OSP; ENIG is a different story--I see too many people having
problems. Of course, if the HASL process is not well controlled, there are
likely to be problems--but that goes for any other uncontrolled process. I
certainly can put OSP onto a non-solderable Cu surface; and boy can i screw
up ENIG.
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 386-437-8747, Fax: 386-437-8737
E-mail: [log in to unmask], Website: www.engelmaier.com
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