Mike Bailey wrote:
We have long had a practice regarding the processing of thick panels with
high aspect ratio holes. We perform standard desmear/electroless deposition
followed by a 10 min electrolytic strike plate. We then re-process the
panels through electroless copper. This has served us well in terms of
protection from "bubble" voiding, but has become burdensome on our pattern
cu throughput. In all honesty, we have not seen bubble voids, but are
probably a bit paranoid and hesitant to break the habit. The logic of this
process has been that if you must re-process the panels, why subject 99.99%
of the holes to additional electroless. It seemed better to lock them in
with a strike plate first.
Anyway, I was in a meeting the other day and it was suggested that we simply
re-process the panels a 2nd time thorough electroless copper and eliminate
the strike plate altogether. Being the composed individual that I am, I bit
my tongue, but was thinking. WHAT PART OF INTERCONNECT INTEGRITY DON'T YOU
UNDERSTAND?????
While I know that we have all done this double pass at one time or another,
and it probably appeared to work satisfactorily, you must admit that the
main objective at the time was just to get the panels through and avoid
scrapping them at that point.
I would appreciate a reality check and some feedback on this issue.
Thanks in advance
Mike Bailey
Director of Engineering
McCurdy Circuits Inc.
4900 E. Hunter Ave.
Anaheim, CA 92870
Phone: 714 507-4900 Ext 253
FAX: 714 507-4911
e-mail: [log in to unmask] <mailto:[log in to unmask]>
Ed Cosper wrote:
Mike,
I for one am in favor of a double electroless step on high aspect ratio
holes. Only because I have used it many times in the past as a last resort
and found marked improvement. We have shadow now so this doesn't apply to
our current processing.
However, you seem to indicate that your copper strike is producing
acceptable yields but someone wants to eliminate it..because of time
consumed in the copper tanks I suspect. The question that comes to mind is
if you have voids due to bubbles ( air entrapment) in the electroless
process, I don't see how a copper strike would solve it. The void would
already be there. If electroless was missing, what is the strike plating to?
Perhaps your electroless process is better that you think. I wonder what
would happen to your yields if you simply removed the strike and see how
your original electroless deposit would stand up.
Just my thoughts,
Ed Cosper
Patricia Goldman wrote:
Mike,
Can I/we assume that you have put into practice all the other "tricks" for
eliminating entrapped air and gas?
Such as, the Bump (all process tank stages on electroless line) or
ultrasonics, angling panels, and the best possible cleaner/conditioner for
wetting of the holes?
Double electroless is a kind of band-aid... You remove some part of the
first layer in the microetch, you know. It always seems to me that
"band-aids" or re-works at one part of a process tend to produce more
"band-aid opportunities/requirements" downstream.
Patty
Patricia J. Goldman
PPG Industries, Inc.
412-492-5516
[log in to unmask]
In the many years that i have literally " worked " with circuit houses,
setting up processes, the only time I can recall using a strike (before
medium depth) was either with a low dep deposit - 10 - 30 millionths -
electrolizing flex - teflon.
In the flex and teflonprocessing - (there is a chemical that can be used
prior to cleaner conditioner)- usually a ring void would be visible if the
preplate additive was not used, hence - a strike was introduced. the strike
would 99.5% of the time bridge the void - produced by original electroless
deposit.
In low dep applications - a strike was introduced - so as 2 assure sufficient
copper when running the panels through persulfate - prior to plating.
In the last 10 years - majority of pc houses have and are utilizing a double
pass thru the electroless copper - especially on high aspect ratio panels,
was and is working quite well. Why the second pass Mike? how does it work.
Well...same effect as striking the panel. You are basically bridging whatever
voids u have in the thru hole.
Other techniques, as Patricia has mentioned, work as well. Other than a
higher concentration of palladium is required in the process. Why? The actual
mechanical force of the " bump ", as it was referred to, will have a tendency
to break down the tin/palladium colloid, thus a slightly higher percentage of
palladium is required due to breakdown. At the same time, one must kep in
mind, that 2 high a concentration of palladium will cause resin poisoning,
leading to such defects as "hole wall pull away" and other phenoms.
Patricia mentioned wetting of the holes. If you are referring to actually
watering down the holes - with water - DI or City - as I have seen done in
several parts of the continent...I personally cannot believe that this has
any effect on the thru-holes themselves. Y? well....majority of the chemistry
on-line has a dyne count lower than water, at the same time, there are
certain additives that are added to the cleaner which wet 100% better than
water. If by the time the panels have reached the electroless bath, and the
holes are not wetted, then the processing thru the preplate line should be
seriously investigated.
Another phenom I have seen, is where the silenium ( large traces found in the
holes) used in bonding the copper laminate to the resin, had been smeared
thru the holes, caused by wrong entry/exit/sharpening/hits - even after
desmear traces were found. The only way we were able to process those panels
was to use a high-pressure deburring machine - running at about 325-375psi -
then processed thru the electroless pre-plate line.
Mike - if u don't have confidence in a double pass electroless - then I would
suggest - keep striking. The strike is actually doing the same thing as the
double pass - with a little thicker bridging of the copper. But just maybe,
you'd want to see what a double pass could do for you. Especially if they
are high aspect ratio multi's.
The opinions mentioned above are mine and mine alone.....but they have been
conceived thru actual hands-on work and many hours of sectioning.
Best Regards,
Rick Fudalewski
Florida CirTech
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