Homebrew PCBs

Fantastic explanation, thank you very much!

I can easily see why multi-layer boards can and do get so expensive -
there is a lot of work in them!

Thanks again!

Chris








--- In Homebrew_PCBs@yahoogroups.com, "twb8899" <twb8899@...> wrote:
>
> 
> 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
> >
>