Phil,

I do not want to turn this forum into a vehicle for a sales promotion so will
leave it to others to comment on the relative merits of SAC305 and SN100C.

However, on the point about hand soldering with SAC305 on a board that has
been wave soldered with SN100C there do not seem to be compatibility issues.
In fact a lot of people wave soldering with SN100C are doing hand soldering on
the same board with lower melting point SAC alloys.

I can send you directly a study that Nihon Superior did on mixed solder
joints but the main point is that, apart from the trace level nickel addition, two
of the three elements are common to both alloys.  If you solder with SAC305 on
a board that was waved with SN100C you get an alloy with a little more tin
and copper and, depending on the ratio of solders in the mix, a little less
silver than there was in the original SAC305.    The recently released IPC SPVC
study on differences between SAC alloys with different levels of silver and
copper indicates that there is no significant difference in the performance of the
alloy over the likely range.  And the nickel has no detrimental effect on SAC
alloys.  All that happens is that as the proportion of SAC305 in the mix
increases the joint looks less like the smooth bright finish you get with SN100C
and more like the grainy joint with the possibility of shrinkage cavities that
you get with SAC305.

Of course you can do hand soldering with SN100C flux cored wire.  Although
the melting point is higher, if you have a soldering iron with fast response and
good temperature control you can use the same tip temperature with SN100C as
you would with SAC305 (or 63/37 for that matter).

Keith Sweatman
Nihon Superior Co., Ltd

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