Having used DMAB for several years now, we can state that it does work
reliably. In our validation testing process, we performed extensive analysis
on the post DMAB surface and found that both the micro-topography was
dramatically reduced and the level of oxidation almost zero. This explains
why it works. Quite simply the chemical reaction is based on basic chemistry :
METAL oxide + Acid = METAL + water
CuO + 2H(+) = Cu + H2O
If DMAB eliminates, or at least dramatically reduces (no pun intended) the
oxygen from the left hand side of this equation, then there is no reaction
with the available acid that would result in pink ring. The natural chemical
reaction ceases to occur. On the other hand... if there is a high level of
oxidation as is found in typical BLACK oxide, then this natural chemical
reaction cannot be avoided.
----------
>There are some things that I can't understand about the black oxide, pink
>ring and the use of DMAB
>Pink ring is the result of the attack of BO by acids in the electroless
>copper line. DMAB prevents this attack.
>The black oxide treated with DMAB can resist a lot of time in a 10% HCl
>solution but only a mom in a sulphuric/hidrogen peroxide solution. Then, why
>we don't have pink-ring using DMAB?
----------
I suspect that by the description given, you are ETCHING copper using
Sulfuric/peroxide solution. The visible attack that you see is actual
removal of copper (and the slight layer of brown after DMAB processing).
This is not oxide attack but rather a chemical removal of copper including
the surface colouration.
----------
>
>The peel-off for the black oxide treated is lower than the non treated.
----------
Our analysis did exhibit slightly lower peel strengths after post-treatment
but remember... the adhesion you measured was on black oxide. Should the
oxide layer be removed by acid as per the above reaction... then you
eliminate the very layer that provides this 'good' adhesion. I would
recommend that you consider an alternative measurement for bond integrity
such as TMA260.
----------
>During the drilling , I think it would be easier to delaminate. Again ,why
>we don't have pink-ring using DMAB?
>
>I would like to understand how DMAB works, because sometimes it doesn't do.
----------
When DMAB fails to perform properly it is usually due to:
1) Poor chemical control of DMAB solution (concentration, pH etc.)
2) Surface contamination after treatment (fingerprints etc.)
3) Post oxide baking at too high a temperature (reoxidizes copper)
4) Poor rinse quality after treatment (watch you water purity)
5) Improper drying and/or storing conditions after processing.
----------
>Is there another systems to protect de black oxide?
----------
There are other post treatment reducers but all are based on the avoiding
the chemical reaction outlined above.
----------
>
>I would like also to find some papers, articles, etc. about this process.
----------
There have been several good papers on pink-ring dating back several years.
Probably the best source of these is your chemical supplier of DMAB. If not
please E-mail your address and I'll send you some reference material.
----------
>
>Thanks and excuse my bad english.
>
>
>Toni.
>
>
>
================================================================
Please find these previous E-mails regarding Pink ring :
================================================================
Pink Ring can better be termed "Pandora's Box". This is because
it's source(s) can be many. Isolating the source(s) can easily
confound a process engineer (trust me, I know). Rudy has outlined
where the dissolution of the oxide takes place. To name just a few,
a couple potential sources could easily be improper Rate of Rise in
lamination (rendering a non fully wetted oxide to laminate
interlaminar bond), drilling parameters, the new (fun) HTE copper
foils that are more malleable than the the lower (harder) grade
foils,...etc. The bottom line is that somewhere, somehow a small
weak point develops around a drilled hole. That weak point is where
the acid chemical attack to the oxide on the plane develops and can
be seen. They key point is that pink ring has been with us for years
and has plagued the PCB fab shops to try to isolate it's source.
Minor Pink Ring has been found to not impair the electrical or
mechanical performance of the PWA. (Cosmetically it's not appealing
but it meets Fit/Form/and Function) Major pink ring is an issue.
If the pink ring is significant enough to the point to where there
is a profound separation at that innerlaminar bond location, then
during subsequent wet processing, a plating "wedge void" could easily
develop. This is where a plating thickness can easily fall well below
.001" thk and could possibly even have some copper voiding (abscense
of copper). Generally, those locations have massive pink ring way up
the plane(s). Micro section evaluation will detect this condition or
PTH NDT (Non-destructive testing). With the advent of post oxide
conversion or reduction coupled with better copper plating chemistry
that produces better distrubution, leveling, and throwing power we
(fabricators) are able to produce far better product than say 10 years
ago.
Yes, reducing stack heights and shortening times in acid solution
will minimize the amount of oxide dissolution on the plane(s). And
this may be a fine fix for a shop that has the capacity and scheduling
to accommode for this, but for a larger shop that is QTA and
semi-production orientated, we must optimize our processes to produce
premium quality at a "fast pace". Yes, sometimes the cost (like the
plating chemistry) is expensive, but the competition is fearce.
(Sometimes compromises are not an option)
Shops now a days should not be plagued with pink ring. It should be
a thing of the past. I don't believe oxide conversion/reduction is
hiding the issue. If a fab shop runs routine cross sectional analysis
and cross checks PTH integrity by PTH NDT, then they would know and
capture any PTH anomaly that would be considered an issue as
previously outlined.
Groovy
(PS-Rudy, DMAB is not the only post oxide process out there you know)
______________________________ Reply Separator _________________________________
Subject: Re: pink ring
Date: 2/5/96 10:36 PM
-----------------------------------------------
Just to make sure that we all are beginning at the same point, Pink Ring is
caused by the dissolution of the oxide layer on the innerlayers. The oxide
is black, and when it dissolves, the"Pink Ring" of Copper under the oxide is
exposed.
As has already been written, Pink Ring has many causes, and even when the
process is seemingly perfectly in "spec", it is still possible to get Pink
Ring, which is the principal reason why most PCB fabricators use a DMAB
reduction step at the end of the oxide line. The DMAB renders the oxide
insoluble to any of the subsequent process chemicals, thus effectively
eliminating Pink Ring. This step eliminates the appearance of Pink Ring, but
it is thought to be a largely cosmetic effect.
Many studies have been done on Pink Ring causes, and after you get by the
obvious process problems, such as leaving the panels too long in the HCl
containing steps, there were two main "causes" of Pink Ring. Pink Ring could
be avoided in most cases by drilling shorter stack heights. The other place
that Pink Ring was observed to occur was in the Acid Copper Plating bath.
This is clearly due to having porous, or voids in the electroless Copper
deposit.
It has been pretty clearly demonstrated that moderate Pink Rink has virtually
no effect on the performance of the board in service, but yet for years, it
was a major cause of rejection of finished PCB's.
Rudy Sedlak
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D. Rooke
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