Hi Thomas,

Over the past year or two something approaching 20 million boards, ranging
from single-sided to multilayer have been wave soldered under commercial
production conditions (for commercial use) with the stabilised variant of the
Sn-07%Cu eutectic that the company with which I am associated has developed.
In many cases wave soldering is carried out after one or more reflow stages.
For reasons of economy most of these boards have a resin preflux or an OSP
finish.   As expected there is some decline in solderability with each stage
of reflow but defect rates in wave soldering have in general been kept within
acceptable limits by fine tuning process parameters along the lines now well
established for lead-free wave soldering, i.e. close control of process
parameters like preheat, solder temperatures and conveyor speed, wave
dynamics that ensure good heat transfer to the board and components, slightly
longer contact times (~4 seconds) and, for plated through hole filling,
slightly slower conveyor speeds and/or the use of a first turbulent wave even
if there are no surface mount components on the underside of the board.    In
some cases consideration has been given to relaxation of the requirements for
topside fillet formation where this will not affect the integrity of the
circuit but in general acceptable topside fillets can be achieved.    A point
that has to be acknowledged, however, is that the fluxes used have generally
been the higher solids content (12-15%) resin-based  (but still no-clean)
type that have remained popular in Japan.   These results might be difficult
to achieve with the very low solids (3% or less) "no-residue" type fluxes
that have been popular outside Japan but the indications are that they are
achievable with the medium solids (~5%) resin-containing fluxes that are now
fairly widely used in Europe and America, provided they are of the "sustained
activity" type, i.e. with activators that can survive the higher preheats
(130 C) required for through-hole boards and slightly longer contact times.
Solder bath temperatures usually do not exceed 255 C.

Good luck with your lead-free wave soldering project.

Regards
Keith Sweatman
Nihon Superior Co Ltd

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