JULY 2021 I DESIGN007 MAGAZINE 55
• Broadside-coupled striplines should ide-
ally be used on a core
– Try to avoid having prepreg between
them. is is required to control the
Z-axis alignment between the two
signal layers
• Specify the design trace-to-trace spacing
for correct modeling
• Give the impedance lines a different
aperture (usually 0.0001" difference)
than non-impedance traces
– Makes it easier to adjust line widths
if needed
• Specify reference planes
Minimizing Impedance Costs
Using the following guidelines will help to
minimize impedance costs:
• Specify impedance only on layers that
really require this
• Route all controlled impedance traces
onto the same layer
• Specify a ±10% tolerance when possible
• Designate one target impedance value to
be tested per layer
• Couple power/ground on adjacent layers
when possible
– Allow for modifying the construction
to meet overall tolerance
Conformance Tests
100% Testing
All impedance coupons are 100% tested
when specified by the customer. Aer com-
parisons against specified values, the measure-
ments are electronically stored.
Serialization
Serialization is a traceability process for con-
trolled impedance jobs. Impedance is typically
measured with a TDR (time domain reflectom-
eter) by adding test coupons to the panel. Before
the impedance has been tested, the coupon and
PCB have a serial number marked on them.
Understanding the cost drivers in PCB fab-
rication and early engagement between the
designer and the fabricator are crucial ele-
ments that lead to cost-effective design suc-
cess. Following your fabricator's DFM guide-
lines is the first place to start.
DESIGN007
Anaya Vardya is president and
CEO of American Standard
Circuits; co-author of The Printed
Circuit Designer's Guide to…
Fundamentals of RF/Microwave
PCBs
and Flex and Rigid-Flex
Fundamentals; and author of Thermal Management:
A Fabricator's Perspective.
Visit I-007eBooks.
com to download these and other free, educa-
tional titles. He also co-authored "Fundamentals of
Printed Circuit Board Technologies."
Figure 2: Controlled impedance test systems such as this Polar Instruments example can perform
a series of impedance tests automatically. (Source: Polar Instruments)
