Adam,
Everything you mentioned about etch back and desmearing is correct.
In our process we used new drill bits for multilayers to mainly limit
the amount of smearing. There could have been a requirement for this
in the MIL-55110P specification as well. Some MIL requirements would
require stuff like that. A new sharp bit helps big time on
multilayers since you need a good sharp cut through the inner layer
copper. These multilayer panels were so expensive that the bit cost
was a small fraction of the overall cost. It just wasn't worth the
risk drilling with used bits or resharps. The multilayer pre-preg
bonding materials have improved over the years so it's probably not
such a big deal any longer but we all know how hard it is to break
old habits.
When we went to the newer activated palladium catalyst in 1990 we no
longer needed desmearing for commercial multilayers. We only did a
slight etchback with straight technical grade (92.8%) sulfuric acid.
Desmearing was still performed on military boards though because of
the MIL specifications.
Tom
--- In
Homebrew_PCBs@yahoogroups.com, adam Seychell
<adam_seychell@y...> wrote:
>
>
> twb8899 wrote:
> >
> [deleted other interesting paragraphs]
>
> > When drilling multilayers we always used new bits and never
resharps.
> > Our bits were used first for multilayers, resharpened and then
used
> > on two layer panels for a couple of cycles and then junked. Many
> > shops now days are only doing multilayer work so they put out
tons of
> > bits that have drilled 2500 or less holes. These are the bits to
get.
> > For single sided and hobby boards you can just about drill until
they
> > break although the dull bits will make a larger burr.
> >
>
> Thanks Tom for sharing your knowledge with the group, although I
have couple of
> questions.
>
> What is different about the holes drilled in multilayer boards that
make it you
> cannot use re-sharpened bits ? Is it because the bore is left
smoother with a
> new drill bit and sometimes that makes a more reliable copper
connection
> (electroless plated) to the inner layers ?
>
> I read that they etch back the epoxy to make sure "drill smear"
does not cover
> the exposed copper foil of inneralyers inside the holes. The
process is called
> "desmear" and consists of three steps. An epoxy swell (a water
miscible organic
> solvent), epoxy etch (hot potassium permaganate/hydroxide),
followed by
> neutralize (sulfuric acid/ammonium persuflate). Is this what you
used to do ?
>
> I would imagine that after epoxy desmearing that it wont matter
what mess is
> left behind after drilling with old & blunt bits.
>
> Adam