DECEMBER 2024 I DESIGN007 MAGAZINE 51
final-cure the panel, we will put the silkscreen
(legend) layer on top of it so there is a good
bonding adhesion in place. en, we bake the
panel again to remove the remaining solvents.
Once it comes out of the oven and cools, the
solder mask becomes hard and durable, expos-
ing all the solderable pads.
Solder mask has always been a relatively
straightforward element of the manufacturing
process, but as designs become tighter, high-
speed requirements more frequent, and cop-
per on the layers thicker, solder mask becomes
a more critical part of the manufacturing pro-
cess. In tight pitch areas of the board, designers
need to be sure they have enough solder mask
web or dam between fine pitch pads.
Most manufacturers can resolve and main-
tain a 2 to 3-mil wide web between pads. Keep-
ing as much of the spacing between pads to
accommodate a smaller dam or web between
features is important. If pad spacing leaves
something like a 1-mil web, most manufactur-
ers will struggle to produce boards that meet
design requirements. Narrow pad spacing can
create solder bridging during assembly and
shorting defects.
Why Are PCBs Green?
PCB boards can come in more than one
color, but these days, they are oen green.
Green became the standard mostly because
of the contrast of color it provides with other
components of the board, such as copper
traces. It offers the most visibly inspectable
color to contrast the PCB's features.
Over time, manufacturing best practices
increasingly aligned with using green solder
mask during production, and now it is the pre-
dominant color for PCBs. Other colors were
and are used for several reasons. Some manu-
facturers use different-colored boards to dif-
ferentiate prototypes from production-ready
boards, and many designers choose different
colors for aesthetic reasons. For high perfor-
mance, green is the best choice because it is
the most well-understood across the industry.
Solder Mask Design Considerations
As with any phase of production, designers
should be aware of what can go wrong during
solder mask and how to prevent issues before
they occur. ickness can impact the manu-
facturing process, and so can the type of sol-
der mask. When we get down to an imped-
ance tolerance of plus or minus 5%, every little
thing can have an impact, including the thick-
ness and pigmentation of the solder mask.
Solder pad shapes, sizes, and locations should
allow sufficient room for solder mask webs
and dams—considering the swell of the solder
mask. Knowing your tolerances and making
sure you have design consistency is key to a
smooth solder mask process.
We are nearing the end of the manufactur-
ing process and are ready for the surface finish.
Keep an eye out for my next article on that sub-
ject. If you can't wait, listen to the Designing
for Reality: Surface Finish episode of On the
Line with… and to hear our in-depth conversa-
tion about solder mask and legend, please lis-
ten to episode 10. DESIGN007
Matt Stevenson is vice president
and general manager of ASC
Sunstone Circuits. To read past
columns, click here. Down-
load The Printed Circuit Design-
er's Guide to… Designing for
Reality by Matt Stevenson and
listen to the podcast here.
"
As with any phase of
production, designers
should be aware of what
can go wrong during solder
mask and how to prevent
issues before they occur.
"
