We have a problem which I suspect many of you have encountered and
satisfactorily solved.  Most of our PCB assemblies are through-hole, double
sided or multi-layered.

We have a relatively small but persistent percentage of boards that exit
the flow solder process with  solder bridges between adjacent
(clipped/clinched) leads on the circuit side of the board.  Visual
examination of the bridges typically reveals them  to be between leads
which are not bent toward each other.

This phenomenon has been accepted as part of our normal process along with
the subsequent touch up operation which removes the shorts.  I invite you
to comment on the following two hypotheses and to add any diagnoses and
successful corrective actions you may have taken.

--The tin-lead solder ration may sufficiently differ from the eutectic
point that the solder remains in the "semi-plastic" state too long when the
board exits the wave and cools before the bridging material can exit the
surface.

--The preheat temperature may be too low--especially with multi-layer
boards having heat sinking ground planes so that too rapid cooling is
induced and bridges form.

I suspect many of you have theories better informed by experience.  Please
let me hear from you.  I am on a team tasked with reducing our fairy
extensive after-wave touch-up operation.

PS:  I thank each of you who replied to my request for resolving a
conformal coating over LPI solder mask non-wetting and dewetting problem.
Each of your suggestions was enlightening and helpful.  Much testing has
determined that the problem is some still unidentified contaminant on the
silk-screened side only, which can be removed by a sufficiently "vigorous"
aqueous cleaning.

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