56 DESIGN007 MAGAZINE I JUNE 2024
tion or short-circuiting adjacent paths. With
most PCB manufacturing processes, registra-
tion is done using a heavy-duty steel caul-plate
with pins that keep the various layers in place.
Each copper layer of the multilayer PCB is
then stacked, bottom to top, on the caul-plate.
Between each layer is an insulator that either
the core that the copper was processed with
(usually processed in pairs like layers 2 and
3) or layers of "prepreg," consisting of a com-
posite of pre-impregnated glass fibers and a
partially cured epoxy. e prepreg, which is
solid during the layup
process, partially lique-
fies in the press when heat
is added. is epoxy layer
presents most of the challenges
with designing for manufactur-
ability.
Material Considerations
and Challenges
e interaction of materials
like copper and epoxy is a criti-
cal step in the lamination process.
Copper is generally smooth, which
is ideal for electrical conductivity, but not
ideal for getting epoxy to stick. To ensure a
robust mechanical and chemical bond, a chem-
ical process is necessary to roughen the copper
to help the epoxy adhere better.
One challenge that PCB design soware
doesn't help with is pressure differentials. If
more than 50–60% of the layers have corre-
sponding copper pads, for instance, the alter-
nating stack of copper and prepreg can create
a high-pressure area on the PCB. is oen
causes the prepreg's epoxy to be forced away
from this area. Likewise, removal of copper
from layers creates low-pressure areas that also
tend to attract the flowing epoxy.
is kind of epoxy build-up and uneven dis-
tribution can cause a defect called delamina-
tion, where the laminate ends up leaving gaps
between layers. is can lead to catastrophic
failures in the PCB, such as shorts or even fires.
One strategy to mitigate this is to avoid creat-
ing high pressure areas, i.e., stacking copper
pads directly above one another through all
layers.
Design, Collaboration, and CAD Tools
Collaboration between design engineers
and manufacturers is essential to designing
PCBs for manufacturing. Early discussions
about stackup and material properties can pre-
vent costly reworks. While modern CAD tools
allow and even facilitate intricate designs,
they do not necessarily simplify the
manufacturing process. Under-
standing the practical implica-
tions of design choices is there-
fore important.
PCB design for manufac-
turing is a delicate balance of
technological insight, mate-
rial science, and precision
engineering. By under-
s ta n d i ng a n d i m p l e -
menting advanced regis-
tration techniques, manag-
ing material properties effec-
tively, and early collaboration between
designers and manufacturers, PCBs can
be designed to be manufactured properly the
first time around.
For a deeper dive into the lamination pro-
cess, listen to the lamination episode of On
the Line with..., where we also discuss in more
detail the challenges of heat application and
the complexities of sequential lamination for
high-density PCB designs.
DESIGN007
Matt Stevenson is vice presi-
dent and general manager of
ASC Sunstone Circuits. To
read
past columns, click here.
Download Matt's book, The
Printed Circuit Designer's
Guide to… Designing for Reality and listen
to
the podcast here.
Collaboration
between design
engineers and
manufacturers
is essential to
designing PCBs for
manufacturing.
