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September 2001

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From:
"Mcmaster, Michael" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 20 Sep 2001 18:45:33 -0700
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Here's a brief primer on foil.  I'm no expert.  I kept waiting hoping that
someone more qualified than me would provide this information.  I'll be the
first to admit that I plagiarized a lot this from literature from Gould and
Oak-Mitsui.  I looked for links to similar information on both of their web
sites and found none.  Perhaps someone from either or both of these
companies can provide links to better information.

Electrodeposited foil is produced by plating onto a cathode drum rotating in
an electrolytic solution.  Current density, contact time (drum speed) and
solution parameters determine the foil characteristics, surface roughness,
grain structure and other properties (tensile, elongation, etc.)  The two
different sides of the foil have very different appearance and
characteristics.  The "drum" side is typically very smooth.  The other side
of the foil is designated as "matte" though I've also heard it called the
"tooth" side since it tends to have dendrites from the electrodeposition
process.

The foil then goes through treating.  This is predominately chemical and
provides such features as mechanical and chemical bond enhancement, as well
as corrosion resistance.  One thing to remember is that both sides of the
foil aren't necessarily treated the same.

Foil thickness at this point is measured by weight, thus the designation 0.5
oz and 1.0 oz which stands for the weight of a sheet of a specified size.
And right now I can't find what that size is.  Once the foil is incorporated
into the board and etched into circuitry, it's no longer possible to measure
it by weight so thickness here is measured in more familiar units (mils and
microns).

Historically (and let me qualify that by saying I've been in this business
about 14 years), foil has been used by laminate manufacturers with most of
the treatment on the matte side which was placed against the prepreg when
making the cores.  The drum side was left facing out for the fabricators to
build the circuits on.  The only treatment on the drum side was a conversion
coating to keep the copper form oxidizing.  Fabricators would then use
chemical and mechanical surface preparation methods to improve photoresist
adhesion and optimize circuit yields.

The smooth copper surface on the drum side of the foil makes for a very poor
bond when laying up multilayer boards with prepreg.  As is so often the
case, those who get their first get all the good real estate, and that's
what happened here.  The laminate manufacturers used up that wonderfully
rough "tooth" side of the foil.  To address this, fabricators use an oxide
or other bond-enhancement treatment on the foil to promote adhesion between
foil and prepreg.

Then a few years ago, innovations started to occur.  I think double-treat
came first, which meant that many of the treatments that were used before
only on the matte side were now also applied to the drum side.  The benefits
and detractions of doing this are beyond the scope of this little primer.
Let's just say that double-treat has the benefit of not requiring a
subsequent oxide or bond enhancement step during board fabrication IF you
don't damage it in handling.

This was followed by someone having the idea of flipping the foil and having
the laminate manufacturers build cores with the matte or tooth side sticking
out. This is called reverse-treat foil.  Again, I'm going to avoid a
detailed list of benefits and detractions.  At it's simplest, putting the
tooth to the outside is better for building thin (and I'd call thin <
0.004") cores since you needn't worry about tooth-to-tooth issues.  It also
tends to result in higher yields mostly from two factors:  better resist
adhesion to the matte surface and fewer problems associated with trying to
etch the "teeth" embedded in the core dielectric.

Lastly, there's reverse-double-treat, which combines both aspects.  The foil
is treated for bond enhancement on both sides and cores are built with matte
side out. It offers the same benefits as reverse-treat, especially for thin
cores where the double-treat surface avoids the need to go through oxide.
This was important when 2 and 3 mil cores were first introduced.  Getting
them through an oxide line was nearly impossible without shredding them.  As
material transport improvements have been made, especially with conveyorized
oxide and bond enhancement processes, the need for double-treat on very thin
cores has abated.

> ----------
> From:         Earl Moon[SMTP:[log in to unmask]]
> Reply To:     TechNet E-Mail Forum.
> Sent:         Thursday, September 20, 2001 12:46 PM
> To:   [log in to unmask]
> Subject:      Re: [TN] External conductor starting thickness prior to
> plating?? ?
>
> Shirley,
>
> For some of us less informed, no joke, could you explain the ED foil
> process
> and how such things as drum rotation speed affects thickness. Also, if you
> can, please explaing various processes used to prepair outer layer foil
> surfaces for subsequent cleaning, imaging, plating, and etching processes.
> I'm especially interested in how thin is too thin under what processing
> conditions.
>
> Thank you much,
>
> Earl
>
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