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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