AUGUST 2018 I DESIGN007 MAGAZINE 47
• High harmonics of the fundament can
suffer excessive losses in amplitude and
a degradation of edge sharpness which
results in distortion of the signal eye.
• When the frequency exceeds 1GHz, cop-
per roughness, conductor loss, skin effect
and skew, due to variations of glass weave
in the dielectric, begin to come into play.
• Standard FR-4 has a Tg of 135°-170°C,
whereas the high-speed materials are
generally over 200°C.
• Dielectric material selection is usually
driven by the frequency and rise time of
the digital signal with lower values of
loss most suitable for high frequency
applications.
• A small difference in dielectric constant
between materials can impact impedance,
line widths and clearances, and thus
losses significantly.
• Embedded capacitance technology pro-
vides distributive decoupling capacitance
and takes the place of conventional dis-
crete decoupling capacitors over 1GHz.
References
1. Barry Olney's Beyond Design columns:
Material Selection for Digital Design, Material
Selection for SERDES Design, Effects of Sur-
face Roughness on High-Speed PCBs.
2. Selecting PCB Materials for High-Speed
Digital Circuits, by John Coonrod, Rogers Cor-
poration.
3. Using Embedded Capacitance to Improve
Electrical Performance, by Joel Peiffer, 3M.
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 contact Olney, or read past
columns, click here.
to improve signal propagation speed through
the medium. However, for planar capacitance,
a high Dk creates a high value of capacitance,
between the planes, to effectively decouple the
PDN at high frequencies.
ECMs offer many benefits when used for
decoupling high-speed digital circuits, including:
• Lowers the impedance of the PDN
• Dampens plane resonance
• Reduces power plane noise and thus
coupling of plane noise to signals
• Reduces radiated emissions
• Replaces large numbers of discrete
decoupling capacitors
From an engineering perspective, noise mar-
gins are increased, which can translate into
improved performance and less time devoted
to troubleshooting and fixing issues further
down the track. In addition, the component
count reduction saves time in board layout and
assembly which reduces cost.
In conclusion, the designer needs to be able
to quickly evaluate the best, most cost-effec-
tive material for their application based on the
vast array of choices available. Sorting through
numerous datasheets is a very time-consuming
process. And an extensive table of numbers
does not paint a memorable picture. However,
a direct visual comparison, of dielectric mate-
rials, based not only on manufacturer's prod-
uct lines but more importantly on what one's
preferred fabricators stock is undoubtedly the
most efficient approach.
Key Points:
• Materials, used for the fabrication of the
multilayer PCB, absorb high frequencies
and reduce edge rates causing signal
integrity issues.
• Dielectric constant and dielectric loss are
a function of the dielectric material, their
distribution in the PCB stackup and the
applied frequency.
• The glass epoxy material (FR-4) has negli-
gible loss for digital applications below 1
GHz. But, at higher frequencies the loss is
of greater concern.
