Factors while routing signal traces on PCB

Poor routing causes crosstalk, reflections, EMI, long delays, difficult manufacture and design-rule errors. The main factors are:

Signal-propagation and electrical factors

1. Trace length: keep traces as short and direct as possible, particularly clocks and high-speed lines; match lengths of buses and differential pairs to avoid skew.
2. Characteristic impedance: keep trace width and distance from the reference plane constant (e.g. 50 ohm) and terminate lines to prevent reflections.
3. Crosstalk: use adequate spacing (3W rule), avoid long parallel runs, route adjacent layers orthogonally, and use guard traces.
4. Return path: keep a continuous ground plane under the signal; do not cross plane splits; use stitching vias when the layer changes.
5. Bends and branches: use 45-degree bends or arcs, not 90-degree corners (which change the impedance); avoid acute angles (they form acid traps) and avoid stubs and T-branches (use a daisy chain instead).
6. Vias: minimise their number, as they add inductance, capacitance and discontinuity.
7. Current capacity and voltage: choose a width suited to the current and temperature rise (power traces wider), and keep clearance and creepage for high voltage.
8. Separate signals: keep analog, digital and power separate, and keep sensitive signals away from noisy ones.

Manufacturing factors

• Follow the fabricator's design rules: minimum trace width/spacing, hole size, annular ring, and clearance from the board edge.
• Use teardrops at pads, avoid thin necks, and avoid acid traps (acute angles).
• Use uniform copper distribution to prevent warping; consider solder-mask and silkscreen aspects.

Design-error and testing factors

• Run design-rule check (DRC) and electrical-rule check (ERC); place test points, and label components; consider thermal relief for pads connected to planes.
• Leave room for rework and component accessibility.

Following these controls keeps the design reliable, manufacturable and low-noise.