Hi TechNet!
I agree with Patty and would like to add that in the past I
have been able to 'fix' pink ring by increasing the
electroless copper plating thickness to over 125 microinches
in order to seal the wedge and keep out any contact with
acid copper plating solutions. I have also had pink ring
show up all over the place when my electroless copper
thickness (on a heavy dep electroless copper) dropped to 60
microinches.
Direct metallization systems have two disadvantages when
trying to plate within the wedge.
1. They are very thin and allow acid copper plating
solutions to penetrate the wedge exacerbating pink ring.
2. They are semi conductive coatings. Which means you
have to have an acid copper bath designed to be excellent at
throwing copper into recesses or you will have trouble
getting copper into these areas of very low current density
(like wedges). Electroless copper is much more conductive
and, consequently, more forgiving when trying to plate into
wedges. WITH EVERY DIRECT METALLIZATION SYSTEM, YOU NEED TO
KEEP YOUR ACID COPPER BATH EXCEPTIONALLY FINE TUNED! Saying
that, some are more forgiving than others.
With the direct metallization processes, the acid copper
plating mechanism is one of first knitting across the hole
surface from highest current density areas to lowest current
density areas, then building thickness. Your acid copper
system has to be adjusted to completing the first step as
rapidly as possible.
Regards,
Dave Sullivan
Rockwell Collins, Inc.
[log in to unmask]
______________________________ Reply Separator _________________________________
Subject: Re: Re[2]: Mechanism of Wedge Void Formation
Author: [log in to unmask] at ccmgw1
Date: 7/18/97 12:19 PM
Short synopsis of how wedge voids are born: During drilling, small amounts
of vibration of the bit against copper planes in an ML can do some damage to
the resin around the holes created. This damage or fracturing is then
invaded by the solvent/sensitizer solution of the permanganate desmear
process. The permanganate bath then preferentially etches this already
loosened resin, forming the "wedge" in severe cases. It can be big or very
small, depending on the amount of fracturing to that resin area. Acid
attack to the oxide can begin as early as the acidic neutralizer in the
permanganate process, but the microetch on the electroless or DM line is a
more likely candidate. Often the electroless copper will fully cover the
wedge area and "seal" it from further attack during acid copper plating.
Pink ring is known to be an indicator that there may be a wedge void.
Meaning that, if you have a wedge void, you most certainly have pink ring,
but you may have pink ring without the wedge void.
But remember, it all starts way back in the beginning, even before drill, if
the lamination process is not optimized (heat rise, pressure, temp., cool
down, etc.) or if the prepreg is aged and does not fully penetrate the oxide
structure during lamination. Optimizing a drilling operation helps to
minimize any damage done by a drill bit. See Glenn's paper - it can be done.
Small holes and closely spaced holes are usually the most affected.
I always preached that, unless you laminate and drill correctly (at least
with the best possible process), nothing you do later on will "fix" the
problem - it can only get worse. Then the oxide post-treatments came along
and either hid the pink ring so you didn't see this "process indicator", or
actually provided more/better adhesion of prepreg to the oxide. Note that
you won't see pink ring from the surface unless the oxide layer is facing
outward!
Patty
----------
From: Miscantor
To: TechNet
Subject: Re: Re[2]: Mechanism of Wedge Void Formation
Date: Thursday, July 17, 1997 6:49PM
Patty - good reading material - unfortunately haven't read till u mentioned.
would like to dispute Lenny's views - have seen too many cross sections to
know otherwise. Whenever pinkring occurrs - look at those cross sections
carefully - u will notice some sort of copper folding after plating - means
had to be minor space - most llikely filled with air - then solution -
whether plain shifting or whatever - the wall has either a gough in it - or
minute fracture in the oxide layer. Reduced oxide is nice - but is the oxide
layer reduced chemically or is it being chemically etched back - be careful
when ur chemical supplier says " we have reduced oxide".Can SERA determine
breakdown product is left in? When does the reducer have to be changed? And
when we speak about multi-layers - 4 - 6 - 8 - 10 - what happens on 16 layer
boards - 20 layers? Saw some occurances happen this year on some pretty
thick
boards - hydrolization of the reducer - ain't a pretty sight when a 20 layer
board seperates. Final conclusion as to how and why? Will know in a month or
so .... will try to remember to pass info on. Little pressed for time these
days .... once i get freed up will try to forward this info through the net.
Yes - reduced oxide does help out when using DM - to an extent - but
drilling
is still where it starts - from there - your guess is as good as mine as to
where the acid attack occurrs.
best regards
Richard Fudalewski
Atotech Canada Ltd
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