Kirk Van Dreel wrote:
>We due not believe the plated solder thickness is the root cause of the
>problem, rather a measure of the severity of the problem.  We had done a
>regression on the thickness to the severity of the problem and found an R^^2
>of .33.  Secondly, the cross sections of the traces show very little solder
>remaining on the trace, approx. .3 um.  The remaining solder, which was
>present on the trace, is consumed in the glob.  Finally, we have seen
>larger bussing lines dewet, and in measuring the composition of the
>dewetted area we find from a eutectic mix a 70% Sn composition in the
>dewetted areas and a 50% Sn composition in the pooled area.  Due you have
>any additional avenues, theories, or testing which can be done?
>
Kirk,
This is difficult to comment on without knowing more about your process
and equipment. However, I will try and indicate some areas which might
contribute to your problem.

Is it possible that you have a residual current on your tin-lead plating
rectifier at the end of the plating cycle. If so, you will get a lead
rich surface as lead plates preferentially at low current density.
Current should go to zero at the end of the cycle even if the panels
cannot be removed straight away.

If you plate a complete panel with copper and t/l and then fuse it, do
you get a uniform shiny finish with no dewetting or pasty appearance?
You can fuse a test piece immediately after plating without etch and
solder conditioning to check the t/l and flux.

If you are getting good results in the centre of the panel and poor
appearance around the edges, or the opposite, this means that your t/l
additives need to be adjusted. Usually, it is the larger areas which are
more problematic, whereas fine tracks always look good even if the t/l
is out of balance. It can be a tin-lead solution problem when the reflow
is not perfect, but dewetting is more often down to etch balance, solder
conditioning, or the flux. I have seen serious dewetting due to all
these at one time or other.

Do you check the tin-lead with a hull cell, and look for throw around
the back of the plate? If your throw is not good it will affect the
balance of the deposit in different areas depending on the agitation. Is
there any link between holes in the circuit and improved deposit due to
agitation through holes. I am assuming you have wave and not knife
cathode movement. Perhaps you have too much agitation?

There are so many variables that it is difficult to help. I think you
will have to devise some test pieces to check different aspects of your
process. Start with the tin-lead solution, and work forward from there.

Good luck,
-- 
Paul Gould
Teknacron Circuits Ltd
[log in to unmask]
Isle of Wight,UK

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