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>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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