Ioan,

As I think I have mentioned before in this forum a study that Nihon Superior
did some time ago on hand reworking of wave soldered joint indicated that
there is no significant reliability issue with mixed alloys.   I can send a copy
of that study to anyone interested.

SAC is Sn-3.0Ag-0.5Cu and SN100C is Sn-0.7Cu with <0.Ni    Although SAC305 is
nominally 0.5Cu in a working solder bath it is likely that the Cu will have
crept up towards 0.7% so mixing the two is probably not going to have much
effect on the Cu content of the alloy.    So the possibilities are a SnAgCu alloy
with anything between zero and 3%Ag.   As the Ag increases the effectiveness
of the Ni in doing anything beneficial for the Sn-Cu system is diminished.
There would be a gradual transition from one alloy to the other.   Since the
evidence that is accumulating indicates that, overall and on average there is not
a great difference between the reliability of SN100C and SAC305 there should
not be any nasty shocks.   Metallurgically speaking there is no catastrophic
reaction between any of the constituents that would result in a combination
that is dangerously unreliable.

One of the industry consortia in which Nihon Superior is involved is
considering a project that would look at the properties of joints that result from
minipot reworking a wave soldered SAC joint with SN100C and also the bulk mixed
alloys and that work will find its way into the public domain.

Keith Sweatman
Nihon Superior Co., Ltd.

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