18 The PCB Design Magazine • August 2014
designer should calculate the highest frequency
of interest, taking the bandwidth into account,
then choose a dielectric material with the low-
est Er (Dk) and Df with a Tg about 180
o
C. And
remember, choosing the materials that are
stocked by your fab shop will result in up to 5%
better accuracy.
Points to Remember
• Selecting the best possible material will
not hurt—but may blow out the costs.
• Signals propagating in the dielectric mate-
rial of a PCB slow down.
• Signal traces on a PCB simply guide the sig-
nal wave, as the electromagnetic energy propa-
gates in the surrounding dielectric material.
•
The velocity of propagation in FR-4 is
about half the speed of light or 6 inches per ns.
• Dielectric constant and dielectric loss are
not a function of the geometry of the transmis-
sion line—they are a function of the dielectric
material in which the signal propagates, their
distribution in the PCB stackup and the applied
frequency.
• A low Er is desirable for high-frequency
design.
• It is best to use the value of dielectric con-
stant applicable at the highest frequency of in-
terest. However, the maximum bandwidth also
needs to consider the 3
rd
or 5
th
harmonic of the
fundamental.
• A high T
g
guards against barrel cracking
and pad fracture during reflow.
• A fiberglass-free material can be used to
eliminate differential skew but is costly.
• In order to select the correct dielectric
materials and variables for your substrate, you
need to consider dielectric constant and loss,
glass transition temperature, trace thickness,
width and separation and of course cost.
• Choosing the materials that are stocked by
your fab shop will result in up to 5% better ac-
curacy.
• The ICD Stackup Planner features 8,800
materials up to 40GHz.
PCBDESIGN
References
1. Barry Olney's Beyond Design columns:
Material Selection for SERDES Design, Trans-
mission Line—From Barbed Wire to High-speed
Interconnect, Matched Length Does Not Always
Equal Matched Delay, Mythbusting—There are
no One-way Trips!
2. www.frankswebspace.org.uk
3. Henry Ott: Electromagnetic Compatibil-
ity Engineering
4. Howard Johnson: High-Speed Signal
Propagation
5. Y. Shlepnev, Simberian Electromagnetic
Solutions: Simbeor Application Notes
6. The ICD Stackup Planner and PDN Plan-
ner extensions: www.altium.com
MATERIAL SELECTIoN FoR DIGITAL DESIGN continues
Figure 5: virtual materials substituted with iTeQ iT-180A material.
Barry Olney is managing
director of in-Circuit Design
Pty ltd (iCD), Australia. This
PCB design service bureau
specializes in board-level
simulation, and has developed
the iCD stackup Planner and
iCD PDn Planner software. To read past
columns, or to contact Olney, click here.
beyond design
