David, two suggestions that may help
with your problem.
(1) If you are using board brushing and
not pumice scrub then you could have thin slivers of copper breaking away from
the edge of the track and causing embedded shorts in the solder resist. This is
almost unavoidable with mechanical brushing so I would always recommend pumice
scrubbing.
(2) Plated shorts can be caused by the
laminate weave giving an uneven copper surface. This can lead to tunnelling of
the tin-lead or tin plate which results in very fine shorts corresponding to the
weave. On half ounce copper this effect is more pronounced because of the
reduced time in etch. Oddly enough, better results can often be achieved with
one ounce material because the thicker foil is less prone to print through and
the increased etch time often means these fine shorts disappear. Look for spikes
along the edges of tracks as a sign that this is happening; they may be
localised in one area of the panel.
Both
of these effects are very difficult to see without at least 10X magnification
and are almost impossible to find after solder resist application.
One method of
finding a short of this nature is to apply a current, say 1 amp, across the two
shorted nets. You will see a brief flash and the short will carbonise. This
makes it much easier to locate and remove.
You won't experience (1) with pumice
scrubbing and you can reduce the incidence of (2) to zero by getting better
surface topography on the laminate, slowing the resist lamination speed, and
increasing board entry temperature to the laminator to help the resist to flow
and conform to the copper surface.
Hope this helps,
Paul Gould
Technical Director
Teknacron Circuits Ltd,
UK
Chris
I am not
sure who I should address this to at IPC but I have you on my address list,
so your the lucky one.
I am
having a problem with an SMOBC board that has .008 spaces and traces running
the length of the board (about 18 inches). It seems that after etch I
am getting some small hair like slivers that can not be seen with the naked
eye and are only picked up at bare board test. But at test we might
find two shorts, cut them out and then different ones will pop up on
retest. We are being very careful not to over etch but the problem
persists. Do you or anyone else at IPC have any
suggestions?
We start
on half oz material
After
etch we scrub to clean the boards, visually inspect and then tacky
role during soldermask.
Thank you
for your assistance.
David
Ducey