In message <[log in to unmask]>, Todd Rogers
<[log in to unmask]> writes
>Technetters,
>We have been experiencing some sporadic problems with soldermask being
>burned off the edge of the circuits at the hot air level process.  The
>soldermask on the knee of the trace seems to burn off and allows the solder
>to stick to the trace edge.  We are using a chemical clean process prior to
>soldermask, and are spraying the soldermask on with a Teledyne spray
>coater.  Has anyone else experienced these problems and what did you do to
>fix it?
The soldermask must be too thin on the trace edges, (around 0.1 mil to casue
this problem). The amount of solder mask applied will depend on trace height and
distribution. Isolated traces are the most awkward as they tend to be overplated
realtive to the rest of the board and the ink has more room to flow off the
trace. You can only resolve this by increasing the amount of ink applied or
double coating.

I assume you are drying horizontally and that this problem is on both sides of
traces and not worse in X or Y directions. Bear in mind that as the ink is
heated in the intial drying phase, it will become less viscous and flow more if
held vertically.

We use curtain coating and the only answer for us to achieve 0.5 mil minimum
trace edge coverage on some boards is to double pass. One advantage of spray
coating is that a more thixotropic ink can be used to reduce flow. Curtain coat
ink by its nature has to be very runny. 
-- 
Paul Gould
[log in to unmask]

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