Kirk VanDreel
05/20/97 07:54 AM
Paul,
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?
Respectfully,
Kirk Van Dreel
Hutchinson Technology
[log in to unmask]
Hutchinson, MN
>Kirk VanDreel
>05/19/97 11:25 AM
>Does any one have the capability to produce 50 um lines and spaces with 2
>um fused solder over the traces? Experiments have shown a dewetting of
the >solder into balls on the traces, however, the wetting angle is not
bad. Is >there a physical constraint we are running into which is causing
this >globbing of the solder?
>
Hello Kirk,
I have seen this problem on fine traces caused by excess tin-lead rather
than dewetting. When you etch, the undercut reduces the copper trace
width under the tin-lead. A 50um trace reduced to 40um will effectively
give 20% more tin-lead thickness when fusing, and above a certain
thickness it will bunch up due to surface tension and produce a solder
ball. Having said that, I would not expect this problem with 2um tin-
lead.
Can you do the following:-
Minimise undercut by using thin copper foil
Optimise etching parameters- (<8.5 pH in ammoniacal etch - 8.2 optimum)
Plate the tin-lead as thin as possible - (3-4um should be okay).
One final thought,if your trace edges are not covered with solder after
fusing, you are leaving more tin-lead on the surface. Check the solder
conditioning and fusing flux.
Good Luck,
--
Paul Gould
Teknacron Circuits Ltd
[log in to unmask]
Isle of Wight,UK
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