At low DC or audio frequencies, copper traces act as simple wires. But once digital signals transition in sub-nanosecond edges (USB 2.0, SPI at 40MHz+, DDR, Ethernet), every trace behaves as a transmission line. Here are the 5 most frequent high-speed routing mistakes observed in prototype reviews.
1. Crossing Split Reference Planes
High-frequency return currents do not take the path of least resistance; they take the path of least inductance directly beneath the signal trace on the reference ground plane. Routing a high-speed trace across a plane split forces return currents to loop widely around the void, creating massive loop antennas that fail EMC compliance.
2. Length Mismatch in Differential Pairs
Differential signaling (USB D+/D-, CAN, Ethernet) relies on the receiver rejecting common-mode noise. If one trace is physically longer than its sibling, the differential phase balance is destroyed, transforming differential energy into radiated common-mode noise.
3. Acute 90-Degree Angles vs 45-Degree Chamfers
While modern etching techniques have reduced acid-trap concerns, 90-degree corners create a local impedance discontinuity due to the trace widening by roughly 41%. Stick to smooth arcs or 45-degree mitered bends for consistent characteristic impedance.
Every via introduced on a controlled-impedance trace introduces parasitic capacitance (~0.5 pF) and inductance (~1.2 nH). Minimize layer transitions on critical clock, data, and strobe pairs.
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