Thomas,

Clearly you have a good understanding of the issues and the Ni-modified
Sn-0.7Cu eutectic alloy that is widely known under the name "SN100C".

The story about this alloy has perhaps got a little confused in the retelling
so it may not be  inappropriate for me to provide a little clarification.

Temperature
Something that has emerged from recent studies is that the equilibrium
melting point of an alloy is only one factor in determining its behaviour as a
function of temperature during soldering processes.   In wave soldering and HASL in
particular fluidity (a property different from viscosity) is an important
characteristic since, for example, it affects the ability of the solder to
spread, flow and fill plated-through holes at temperatures close to the freezing
point.   That is a property that is affected by whether the alloy passes through
a pasty range and it seems that, for example, a Sn-Ag-Cu alloy of nominally
eutectic composition which should not have a pasty range does in fact pass
through one in the non-equilibrium conditions that obtain in practical soldering
processes.    Certainly in its unmodified form the Sn-0.7Cu alloy which again is
nominally of eutectic composition does not freeze as a eutectic, which is why
it proved to be such a difficult alloy to use as a wave solder.    The
controlled addition of Ni has the effect of suppressing the pasty range (during
which there is precipitation of primary tin dendrites) and promoting freezing as a
true eutectic.     In any case the result of the addition is that the
Ni-modified Sn-0.7Cu alloy can be used in wave soldering with a solder bath
temperature lower than might be inferred from the melting point.    Of the ~900 wave
soldering machines running the alloy most are operating in the temperature range
250-260C.    Our observation is that most Sn-Ag-Cu alloys are being run in a
similar temperature range but we are open to correction on that point.   Of
course for thick multilayer boards where you are trying to get a topside fillet
on heavy connector pins temperatures over 260C are used and as far as I know t
hat is the case for Sn-Ag-Cu alloys also.   And to work with these sort of
solder temperature more preheat is usually needed than has been commonly used
with Sn-Pb.

Dross
It is acknowledged that it is difficult to compare dross rates since that is
a characteristic affected my many factors in addition to the composition of
the solder.    However, the chemistry of the Sn-Ag-Cu and Sn-Cu-Ni alloys is
different and the experience of customers running both on similar machines is
that character of the dross of the latter alloy is different and the weight of
dross that has to be removed significantly lower.  Of course a condition is that
the machine is properly adjusted and maintained and that the composition of
the alloy is kept within specification.   Again we would be interested to hear
reports of any different experience.

Bridging
There are a number of factors which make most lead-free alloys more inclined
to bridging.    The specific claim about the Ni-modified Sn-0.7Cu alloy is
that the incidence of bridging is much less than experienced with the unmodified
Sn-0.7Cu alloy under similar process conditions and we believe that this is
related to the improved fluidity mentioned earlier.   In practice, with some
attention to the design and layout of the board rates of bridging with that alloy
have been found to be comparable with those of alternative lead-free solders
and close to that typically obtained with Sn-Pb.

Copper leaching and erosion
As you acknowledge it has been confirmed independently that the rate at which
the Ni-modified Sn-0.7Cu alloy dissolves copper is lower than that of
Sn-Ag-Cu alloys and, as you say, this seems to be related to the effect of the Ni on
the Cu6Sn5 intermetallic that forms on the interface between the molten solder
and the copper.    The thickness of the intermetallic is initially a little
greater but thereafter it remains smooth and stable even in elevated
temperature exposure.     This lower rate of erosion has implications for the management
of the composition of the solder in a wave solder pot as well as for damage
to tracks and pads during soldering and rework.

Machine Corrosion
What I think we are talking about here is erosion of stainless steel.   All
high tin alloys are more aggressive in the dissolution of other metals than is
Sn-Pb but the issue is their aggressiveness towards the oxide film that is
what makes "stainless steel" stainless.   When that protective film is penetrated
and the underlying steel wetted all high tin alloys will start eroding the
steel faster than Sn-Pb.   However, the Ni-modified Sn-0.7Cu alloy is
significantly less aggressive towards the oxide film on stainless steel than the
commercial Sn-Ag-Cu alloys so that it can be run in plain uncoated pots.  This can
provide a solution for companies that need to run lead-free in an existing
machine and there are hundreds of examples of pots that have been running for
years with the Sn-Cu-Ni without erosion problems.    However, the oxide film can
be damaged in ways other than chemical attack by the solder alloy (e.g.
scratching, local overheating) so that where budget is available it is certainly
worthwhile investing in a coated pot.

Regards
Keith Sweatman
Nihon Superior Co., Ltd



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