56 The PCB Design Magazine • March 2014
beyond design
• A trace routed on the inner stripline layer
exhibits 4–10 dB less noise then a trace routed
on the outer, microstrip layer.
• Microstrip layers are those that are fabri-
cated on the outside of the substrate, top and
bottom. The trace has a dielectric material on
either side—FR-4 below and air above.
• Microstrip traces are usually faster than
stripline traces.
• Stripline traces are totally embedded in di-
electric material between planes.
• The propagation delay of a serpentine
trace is less than the delay through an equiva-
lent length straight trace. The signal is sped up
because a portion of the signal will propagate
perpendicular to the serpentine.
• If two traces of equal length are reference
to different planes then the return paths may be
considerable different and add round-trip delay.
• Although two signals are exactly the
same length, they may exhibit different delays.
PCBDESIGN
References
1. Barry Olney, Beyond Design: Impedance
Matching: Terminations, Skewed Again, A New
Slant on Matched-Length Routing, Differential
Pair Routing, Embedded Signal Routing
2. Howard Johnson: High-Speed Digital De-
sign
3. The ICD Stackup Planner and PDN Plan-
ner: www.icd.com.au
Barry olney is managing di-
rector of in-Circuit Design Pty
ltd (iCD), Australia. This PCB
design service bureau special-
izes 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.
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Matched Length Matched Delay continues
