Homebrew PCBs

I owned a commercial multilayer shop for many years. I'll describe the
basics of laminating multi layer boards. All board shops used similar
methods with modifications to meet their particular needs.

On small jobs and for quick turn prototypes we used no tooling pins
and allowed the panel to float in the lamination press. For a .062"
thick four layer board we would use what is known as "cap and core"
construction. The inner layers were processed on the top and bottom of
.015" FR-4 double sided material. After this inner-layer "core" was
etched the "tooling" holes were drilled out using an optical scope
drill. At this point the panel was cleaned and processed through an
oxide chemical bath which caused a thin oxide layer to grow on the
copper surfaces. We used a brown oxide process but other processes
available are black and red oxide. Small pieces of teflon tape were
applied to the drilled tooling holes which covered them and prevented
epoxy flowing in during the lamination cycle.

Two "cap" panels were prepared to form the top and bottom layers.
These panels were .015" thick and single sided. These cap layers had
no tooling holes and would simply be laminated to the previously
prepared inner-layer panels.

The bottom "cap" layer was laid copper side down on a table and three
pieces of 1080 prepreg material were laid on top of the bottom layer.
Next step was to lay down the double sided oxide coated inner-layer  
  panel. Three more pieces of 1080 prepreg material were laid on top
of the inner-layer and finally the top "cap" layer was placed on the
stack. 

1080 type prepreg would have a final pressed thickness of .0025" for
each piece used. Every thickness had to be added into the construction
including the copper foil thickness. So a four layer .062" cap and
core had three .015" layers, six .0025" prepreg layers and the copper
foil added to that. These panels were always a bit less than .062 but
when the final electro plating was complete it was very close to a
.062 thickness.

Now the stack was ready for lamination. Top and bottom "caul" plates
made from .25" thick 4140 steel and "separator" sheets made from .062"
thick stainless steel were used to laminate the multilayer panels. A
separator plate was laid on the bottom caul plate and this separator
was covered with a sheet of DuPont "tedlar" film. Tedlar is a high
temperature release film that prevents the laminated board from gluing
itself to the lamination plates. 

The board to be laminated was laid on the tedlar and another sheet of
tedlar was applied then another stainless steel separator sheet and
finally the top caul plate. Several boards could be stacked up with
separator plates and tedlar between each board. Caul plates were used
only on the top and bottom of the entire lay up.

Our multilayer press for small jobs was a manually operated 50 ton PHI
hydraulic press with 18" x 18" heated platens. This press had one
opening. We also had a 100 ton automated press with three openings for
larger jobs. Both presses had water cooling

Now the lamination lay up was inserted into the lamination press that
was pre-heated to 350 degrees F. Pressure was applied until it reached
approximately 275 PSI. We used 250 to 300 PSI depending on material
specifications.

The press operator would keep an eye on the process especially during
the first few minutes. In the beginning moments as the heat would
reach the prepreg, the epoxy would start to flow out of the board
edges which would allow the pressure to drop. The operator had to pump
up the press a bit to compensate for this pressure loss. As the epoxy
started to gel and then harden the pressure would increase and the
operator would bleed off a little pressure. On our larger press all of
this would take place automatically but the small press required an
operator to control it. The entire lamination cycle including cooling
would take about 90 minutes. 

When the cycle was complete the press was opened and the lay up was
taken apart. Each board had to have the epoxy "flash" trimmed away
from the edges and then a modified drill press was used to "spot face"
the tooling hole locations. This cut into the board just far enough to
expose the tooling holes on the inner layer. Since these holes were
covered with teflon tape before lamination there was no epoxy in them
and they could now be used to locate the board on the CNC drilling
machine. 

The drilling machine was set up using "soft tooling" which is a
phenolic plate attached the machine base that has been drilled with
the tooling hole pattern. Dowel pins were inserted into this plate and
the boards were pinned to it as well. Now the boards were registered
to the exact XY tooling location and the normal drilling process could
be started.

After drilling a multilayer panel it was processed through a
"de-smear" chemical bath that removed any epoxy smear from drilling
that could possibly cover the hole connections to the inner layers.
After the de-smear cycle all further processing was the same as a
normal two sided board.

On larger jobs and boards with more than four layers we used caul
plates that had tooling pins inserted. This allowed total registration
from job start up to final drilling. We skipped this pinning stage on
quick turn four layer jobs.

That's a basic description of mutilayer lamination. Hope this
information helps.

Tom


--- In Homebrew_PCBs@yahoogroups.com, "lcdpublishing"
<lcdpublishing@...> wrote:
>
> Hi guys,
> 
> Last night I was thinking about 3 and 4 layer PCBS and can't seem to 
> figure out how they do them.
> 
> Single sided - easy, we all do that.
> Double sided - easy, we do that as well with some alignment issues.
> 
> But once you get beyond that, I realize a different manufacturing 
> process has to be done to get the layers.  How do they do it?
> 
> Chris
>