The old cliche' that a picture is worth a thousand words definitely applies
to this scenario.  Below is an outline of some of the causes I have seen in
the past and a couple of questions.

1) the design of the board (layer count, copper weights, size, thickness,
etc) will affect your profile.  Are you using any profiling devices such as
a thermal strip, mole, etc. to check your topside temp. This may help to
tune in your adjustments for diff. designs.

2) are the areas somewhat common?.  If so, check for the presence of a
Soldermask web.  lack of a web/dam may contribute to the bridging.

3)does the condition appear to be isolated to one vendor/one Soldermask
type?.  If this was the case, I would suspect solderballs as well.

4) what is the angle of your wave?  If it is not set properly, the "snap
off" of solder may be after it has started cooling and would show up in the
tighter spaced leads first.

    I hope that this will give you some starting points.  If you would like
to Email me, I may have some more suggestions and further details.  I would
suggest initially to check your angle and topside, temp.  It is the easiest
start.

JOHN


-----Original Message-----
From: [log in to unmask] <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Thursday, April 02, 1998 9:13 AM
Subject: [TN] Solder Bridges [Thanks for Conformal Coating Dewetting Advice]


>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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