Relative Permittivity; Loss Tangent; Skin Effect And Resistive Losses; Choosing The Substrate Material - Xilinx Virtex-4 RocketIO User Manual

Chapter 10: PCB Materials and Traces

Relative Permittivity

Relative permittivity (E
conductor. The higher the relative permittivity, the slower a signal travels on a trace and
the lower the impedance of a given trace geometry. A lower E

Loss Tangent

Loss tangent is a measure of how much electromagnetic energy is lost to the dielectric as it
propagates down a transmission line. A lower loss tangent allows more energy to reach its
destination with less signal attenuation.
As frequency increases, the magnitude of energy loss increases as well, causing the highest
frequency harmonics in the signal edge to suffer the most attenuation. This loss is a linear
effect. For example, the loss at 5 GHz is twice the loss at 2.5 GHz. Conversely, at a given
data rate, the dielectric loss can be halved by using a material with half the loss tangent.
Lower loss materials generally have lower dielectric constants. A lower dielectric constant
also helps to lower the dielectric loss somewhat.

Skin Effect and Resistive Losses

The skin effect is the tendency for current to flow preferentially near the outer surface of a
conductor. This is mainly due to the larger magnetic fields in higher frequency signals
pushing current flow in the perpendicular direction towards the perimeter (skin) of the
conductor. At gigahertz data rates, most of the current is constrained to within a few
micrometers of the outer surface of the conductor. Therefore, the effective cross-sectional
area of the conductors decreases and the conductor resistance increases with frequency.
This effect changes with the square root of the frequency. For example, the conductor
resistance is twice that at 4 Ghz than it is at 1 GHz.
As an example, an 8 mil wide trace at 1 MHz has a resistance on the order of 0.06Ω/inch,
while the same trace at 3.125 Gb/s has a resistance of just over 0.5Ω/inch. Given a 10 inch
trace and 1.6V voltage swing, a voltage drop of 80 mV occurs from resistive losses of the
fundamental frequency, not including the losses in the harmonics and dielectric loss.

Choosing the Substrate Material

The goal in material selection is to optimize both performance and cost for a particular
application.
FR4, the most common substrate material, provides good performance with careful system
design. The Virtex-4 RocketIO Multi-Gigabit Transceiver Characterization Report
a BER performance of 10
longer trace lengths or higher signaling rates, a more expensive substrate material with
lower dielectric loss should be considered.
Substrates, such as Nelco, have lower dielectric loss and exhibit significantly less
attenuation in the gigahertz range, thus increasing the maximum bandwidth of PCBs. At
3.125 Gb/s, the advantages of Nelco over FR4 are added voltage swing margin and longer
trace lengths. Above 6 Gb/s, Nelco is considered necessary unless high-speed traces are
kept very short.
The choice of substrate material depends on the total length of the high-speed trace and
also the signaling rate. For more information on the transmission lengths possible at
various speeds and with various materials, refer to the Virtex-4 RocketIO Multi-Gigabit
Transceiver Characterization Report.
240
) is a measure of the effect of the dielectric on the capacitance of a
r
–12
for 44-inch FR4 traces up at speeds of 3.125 Gb/s. However, for
[Ref 1]
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is almost always preferred.
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Virtex-4 RocketIO MGT User Guide
UG076 (v4.1) November 2, 2008
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