16 DESIGN007 MAGAZINE I FEBRUARY 2019
• The propagation speed of microstrip is
always faster than stripline—typically, by
13-17%
• Dissipation factor is a parameter of a
dielectric material that quantifies its inher-
ent dissipation (loss) of electromagnetic
energy
• Dielectric constant and dissipation factor
contribute to the frequency dependent loss
and to degrade the bandwidth and speed
of the signal
• High-frequency dielectric materials gen-
erally have low Er and Df, enabling the
signals to propagate faster, have less loss,
and therefore, higher bandwidth
DESIGN007
References
1. Firger, J. "Tiny biosensor patches worn on skin show
big promise," CBS News, April 4, 2014.
Further Reading
• Browne, J. "Picking Materials for Multilayer PCBs,"
Microwaves & RF, February 17, 2011.
• Morrison, R.
Fast Circuit Boards: Energy Management,
Wiley, January 2018.
• Olney, B. "It's a Material World,"
Design007 Magazine,
August 2018.
• Olney, B. "Signal Flight Time Variance in Multilayer
PCBs,"
The PCB Design Magazine, December 2017.
• Olney, B. "Beyond Design: Microstrip Coplanar Wave-
guides,"
The PCB Design Magazine, March 2017.
• Olney, B. "Faster Than a Speeding Bullet,"
The PCB
Design Magazine,
February 2016.
• Olney, B. "Controlled Impedance Design,"
The PCB
Design Magazine,
May 2015.
• Olney, B. "Material Selection for SERDES Design,"
The PCB Design Magazine, September 2013.
Barry Olney is managing director
of In-Circuit Design Pty Ltd (iCD),
Australia, a PCB design service
bureau that specializes in board-
level simulation. The company
developed the iCD Design Integrity
software incorporating the iCD
Stackup, PDN, and CPW Planner. The software can be
downloaded from www.icd.com.au. To read past columns
or contact Olney, click here.
When we design a transmission line, as part
of a multilayer stackup, we are not just de-
fining the copper traces but also specifying
the dielectric to transfer the electromagnetic
energy. The traces and vias guide the energy
through the substrate. A field solver uses the
combined effects of trace width, clearance,
and thickness plus the dielectric constant and
material thickness to determine the imped
-
ance of the trace. However, the speed of prop-
agation is independent of trace geometry and
is totally determined by the dielectric material.
All dielectric materials that compose a sub-
strate have different properties, so one needs
to select the optimal materials for the required
purpose carefully.
Key Points:
• The impedance of transmission lines is a
function of substrate dielectric constant
• The CTE is a yardstick for expected PTH
barrel reliability
• Adding a coupled return path close to
the signal conductor in the presence of a
dielectric reduces the impedance
• Typically, for a digital design, a
characteristic impedance of 40–60 ohms
and differential impedance of 80–120
ohms are used
• A good transmission line is one that has
constant impedance along the entire
length of the line, so that there are no
mismatches resulting in reflections
• Coplanar impedance is determined by
the ratio of trace width to clearance,
so size reduction is possible without
limit—the only penalty being higher
losses
• A transmission line does not carry the
signal itself but guides electromagnetic
energy from one point to another through
the substrate
• With their relative timing requirements,
the signals essentially ride the EM carrier
wave
• The speed of a computer does not depend
intrinsically on the speed of electrons but
on the speed of energy transfer between
electronic components