A belated comment on this issue is that it is rather meaningless to talk
about the realignment characteristics of lead-free solder pastes unless the
characteristics of the flux medium are also considered.

Because of their intrinsically higher surface tension lead-free solders
should have the potential for stronger realignment than tin-lead solder but to get
that effect you first need a flux medium that can achieve good interparticle
coalescence and good wetting of the substrates.   I think that it is generally
acknowledged that not all solder paste formulators have caught up with the
special requirements of lead-free solder surface chemistry so there is still
quite a lot of variation in the reported performance.   And if the flux medium
used with tin-lead is not so good its realignment capabilities might well turn
out worse than those of a lead-free alloy with a well matched flux medium.

I am sure all solder paste formulators are working hard on this issue so that
solutions will emerge in the not too distant future.

Keith Sweatman
Nihon Superior Co., Ltd

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