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April 1998

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Subject:
From:
"Stewart, Dougal" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 28 Apr 1998 09:47:13 +0100
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Black oxide to make handling easier, piling up around nailheads, proving
cleanliness of surfaces !!!!!!!!!
Time to stop old wives tales in their tracks before more people who
don't fab boards start believing this c**p.

Multilayer boards require a key for the inner layer copper to adhere to
the B stage prepreg RESIN (glass is not sticky). In the early days this
key was provided by either brushing the cores or by using double treat
copper. Double treat copper has had a roughened surface grown onto the
copper by electroplating means, and due to that extra processing is more
expensive than the standard foil, which has a rough side (used for the
same purpose by the laminators) and a shiny side or 'drum' side.
Handling of double treat cores is a problem because finger grease is not
removed as no further surface prep is needed (otherwise you wouldn't use
double treat!). Brushing the shiny side gives a rougher surface and
creates a mechanical bond during multilayer lamination.

Brushing has a down side, - as cores got thinner, the material became
more difficult to handle through a brushing machine, and the copper on
those thin cores then stretched, which caused problems with material
shift at bonding. The next option was to use pumice, but this had
similar problems with thin core handling and machines self destructing.
A chemical solution was the obvious choice.

Oxidising the layers gives a surface roughness which encourages a
mechanical bond to the resin system, and a weak atomic bond between the
resin and the copper oxide. The process of black oxide also includes a
preclean stage which removes grease and resist residues. There is a
problem with black oxide, in as much as it will continue to grow in the
bath and if you have too much, the crystals break during the pressure
part of the bonding cycle and suddenly you have poor adhesion again!
Brown oxide was brought about specifically for bonding high Tg materials
such as polyimide, cyanate ester and BTepoxy which have a different
affinity to standard epoxy. It is basically a different crystal
structure which is what gives it a brown appearance.

Pink ring hit us all in the late 80's partly due to drilling smaller
holes through solid planes, but the solution was to change to reduced
black oxide ( reduced back to copper). The reduction process maintained
the crystal structure grown onto the copper but would not be attacked in
the same way as the oxide.

Other systems have also been used for creating bond adhesion including
Durabond which is an immersion tin process - this time trying to improve
adhesion by using chemical bonds, and today there are a number of
microetch systems which create the mechanical surface on the copper
surface by purely chemical means, but allow for a long hold time between
processing and bonding - reduced oxide reoxidises over time.

So, the purpose of oxide was not to prove surface cleanliness or to
improve handling, nor does it 'pile' up around nailheads causing
reliability issues. It is there to promote adhesion of copper foil to
resin by creating a mechanical structure.

Lastly, a word about peel strength. Multilayer boards delaminate in a
vertical direction, peel strengths are performed in a horizontal
direction. If peel strength really did correlate to delamination
tendency, then nobody would ever use black oxide. Peel strength was used
as a marketing tool because it allowed certain suppliers to show how
their system was better than someone elses. Unfortuneately, like many of
these tests, end users believe the marketeers and specify a figure,
without fully understanding the issues.

I hope all this stops any further myths evolving on the net!
Dougal
Dougal Stewart
Product Development Manager
Viasystems Selkirk Ltd
Selkirk, Scotland, TD7 5EJ
Tel: +44 1750 21601
Fax:+44 1750 22513
email   [log in to unmask]

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