Louh,
At a previous employer we did not accept this type of defect,
especially where conductors were impedance controlled and on
component pads near the knee of holes. I would say >90% of all
incidences are not related to the copper chemistries. The operators
and/or plating shop managers are very knowledgable about their plating
bathes (this case Electrolytic Plating). They insure that chemistries
are balance through set and random sampling for each shift (based
on volume). Although a sceince, the electrolytic plating chemistries
are pretty much defined.
Most burnt copper cases, as with my previous employer, where related
to the external PWB layouts.
When calculating the total current per side, an operator is supplying
current via rectifiers to exposed copper regardless of its
geographical location. So low density areas with thin conductors will
tend to overplate and eventually burn due to the supplied current,
whereas high density areas will tend to plate evenly. By re-routing
your pads or conductors closer to the body of copper will minimize if
not eliminate this condition. Also provide deviations to suppliers by
allowing theiving patterns to isolated conductors. Other than
cosmetics, the theiving does not affect fit, form and function.
Due to the topography of burnt copper it will leave an impression on
solder dewetting on pads or lands.
Although slow, this is ONE reason why Japanese PWB shops still use
Electroless Plating as the major copper depositor following a quick
electrolytic. There are other benefits for using Electroless Copper.
General, but I hope it helps.
John Gulley - QA
Inet Inc. - A Smarter Vision
Plano, TX
972-578-3928
______________________________ Reply Separator _________________________________
Subject: FAB, ASSY: "Copper Burning" during plating
Author: <[log in to unmask]> at Internet
Date: 10/21/96 11:54 AM
This morning, I was looking at some boards that had pads covered with
copper bumps, blisters, pimples, or whatever you want to call them. I've
been told this was a product of copper "burning", resulting when boards
are plated with current too high. Is this burnt copper unacceptable? I
didn't see any pictures in IPC-A-600 to suggest it wasn't, but don't like
the looks of the plating. Any comments? Thoughts from assemblers would
be welcome, too.
Lou Hart
Compunetics Quality Assurance
412-858-6117
.
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