Digital Signal Processing Reference
In-Depth Information
16.7
Steps to Take When Circuit Board Traces Are Too Lossy
At high frequency the total interconnect losses are improved by reducing dielectric
losses and conductor losses (losses caused by the resistance of the traces).
Reduce the trace length, since loss increases exponentially with length.
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Use a 2D field solver to determine if the major contributor to the total loss is
due to conductor loss or dielectric loss. Do not use a high-cost low dielectric
loss laminate system if the loss is predominately due to the conductor.
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If conductor losses dominate, consider using low surface roughness copper.
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If dielectric losses dominate, consider using a low loss tangent laminate sys-
tem (but unless very high-performance laminate systems are used, this only
slightly reduces dielectric loss and so may not be acceptable in cost-sensitive
applications. However, this may be the only viable solution when signaling
at high speeds).
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Increase the trace width, but note that thicker traces will not improve high-
frequency losses.
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Use high-impedance traces. This is especially effective in lowering dielectric
and conductor losses in wide microstrips, but conductor losses in striplines
will also improve.
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Consider routing critical signals as microstrip. This can be especially effective
when signaling at high-data rates since the dielectric loss usually dominates at
high-frequency, and dielectric losses are smaller for microstrip than stripline.
However, because manufacturing tolerances are generally looser on the outer
layers, impedance control may not be as good and reflections may increase.
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If the traces are microstrip, consider removing the solder mask and treating
the exposed traces with a surface finish instead. This creates an exposed
microstrip rather than an embedded microstrip and reduces dielectric loss.
However, this can cause assembly problems. Be sure to check with manufac-
turing to verify that this option is acceptable.
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If only a few critical signals are experiencing unacceptable loss, consider
using a mixed laminate system where a high-performance laminate is used
on one layer in the stackup, while the remaining layers are traditional FR4
material. By segregating the critical stripline routes to the high-performance
layer, the loss is reduced for those signals and the remaining lower-cost layers
are used to route the majority of the signals.
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Verify that the signals are referenced to the proper power plane.
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16.8
Options to Reduce Circuit Board Thickness
Using more routing layers than necessary is the most typical cause of thicker than
desired circuit boards. To reduce board thickness:
