Visiting a PCB manufacturer provides important insights into the manufacturing process that can help designers. There are many variables and tolerances involved in PCB fabrication that designers need to understand to avoid issues. Seeing the process first-hand helps designers understand how their board will be built and avoid pushing the limits of what can be manufactured. It also helps them use the least expensive technology needed and follow design rules that account for manufacturing tolerances and constraints.

1. Why should you visit a PCB Manufacturer?
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2. Why should you visit a PCB Manufacturer?
•If you design PCBs, you should learn first-hand
how they are built
•A tour of your supplier will advance your
skillset and thereby your professional value
•Many manufacturing processes are involved
•Many variables, each with its own tolerance
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3. Why should you visit a PCB Manufacturer?
•If the rules constraining layout do not accurately
account for excursions within tolerance for all
variables, tolerance stackup can defeat a design
•Good design practice means using the least
expensive technology to achieve performance
objectives
•Whenever possible, designs should not push the
limits of manufacturing processes
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4. Why should you visit a PCB Manufacturer?
•For example, 2-mil-wide traces can be
manufactured with 2-mil spaces, but only under
limited circumstances
•Only on inner layers when plating is not
required
•Absolutely not in buildup layers with buried
vias
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5. Why should you visit a PCB Manufacturer?
•2-mil traces present no manufacturing
difficulty but 2-mil spaces can be held to
no less than plus or minus 25 percent
variation
•That tolerance rules out impedance
control in a differential pair of those
dimensions
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6. Why should you visit a PCB Manufacturer?
•Though finished trace thickness has negligible
bearing on impedance, understanding how
copperweight influences manufacturing is
essential, which a plant tour can clarify
•Copper weight governs how readily fine traces
can be etched within tolerance but there are
other implications
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7. Why should you visit a PCB Manufacturer?
•Ten percent above or below nominal thickness holds true for
dielectrics down to 0.031-in. cores but a 6-mil core, for example,
has a 1-mil tolerance.
•There is no way to stretch the aspect ratio of laser-drilled
microvias beyond the range of 1:1 to .75:1
•If a HDI design is based on a 6-mil dielectric, but in error uses 4-
mil vias with 8-mil pads and the via locations and trace spacing
are too tight to enlarge the pads and therefore the vias, the only
way the board can be built is by decreasing the dielectric height
to achieve the necessary via aspect ratio.
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8. Why should you visit a PCB Manufacturer?
•If impedance must be held to 50 ohms for traces on the top or
100 ohms for differential traces, and the dielectric thickness
must be reduced by half to achieve the necessary via aspect
ratio and thereby salvage the layout, trace and spacing
width, too, must be scaled by half.
•Dielectric, trace, and spacing tolerances at such dimensions
would make impedance impossible to control within useful
limits
•Manufacturers can adjust processes only so far to compensate
for imbalanced designs
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9. Why should you visit a PCB Manufacturer?
•The general (but not absolute) rule to follow for inner layers,
except HDI layers, is use half-ounce copper
•However, an inner layer with vias that will be stacked should
start with quarter-ounce copper
•The vias must be filled after plating, then planarized and wrap
plated
• If half-ounce copper were specified, the trace thickness would
exceed 1 mil after those operations, which may be too tall for a
single thickness of dielectric in prepreg to conform and fill
around features
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10. Why should you visit a PCB Manufacturer?
When transmission-line effects must be considered, there is an
optimum combination of rules that must be in place to guide
layouts: A certain dielectric drives a certain minimum via
size, which dictates a certain pad size, which determines trace
width and therefore spacing
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11. VISIT SIERRA CIRCUITS
CONTACT SIERRA CIRCUITS TO SET UP A SITE VISIT
» Amit Bahl,
Director of Sales & Marketing, Sierra Circuits, Inc.
1108, West Evelyn Avenue
Sunnyvale, CA 94086
Phone: 408-735-7137
Fax: 408-735-1408
Toll free: 800-763-7503
Sunnyvale Office Timing:
8 AM to 5 PM Pacific Time, Mon-Fri
TO KNOW MORE ABOUT OUR PCB MANUFACTURING & ASSEMBLY CAPABILITIES VISIT
www.protoexpress.com
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