7 Essential PCB Routing Methods for Electronic Engineers

Digital circuits often draw discontinuous current, which generates surge currents in high-speed devices. Excessively long power traces lead to high-frequency noise due to surge currents, and this noise couples into other signals. High-speed circuits inherently contain parasitic inductance, resistance, and capacitance, so the high-frequency noise will eventually couple into other circuits. Parasitic inductance reduces the maximum surge current handling capability of traces, causing voltage drops that may disable the circuit. Therefore, placing bypass capacitors in front of digital devices is critical. Larger capacitors are limited by energy transfer speed, so a combination of large and small capacitors is typically used to cover the full frequency range.

Signal vias create voids in power and ground planes. Improper via placement increases current density in certain areas of power or ground planes; these high-current-density areas are defined as hotspots. Via placement must avoid this condition to prevent plane segmentation and subsequent EMC issues. The optimal method to avoid hotspots is to place vias in a grid pattern, which ensures uniform current density, prevents plane isolation, shortens return paths, and eliminates EMC problems.

High-speed signal traces should avoid bends as much as possible. If bends are necessary, acute or right-angle bends are prohibited; only obtuse-angle bends are permitted.

Serpentine routing is commonly used to achieve length matching for high-speed signals, and serpentine traces are also a form of trace bending. Reasonable selection of trace width, spacing, and bending mode is required, and spacing shall comply with the 4W/1.5W rule.

Excessively close spacing between high-speed signal traces easily induces crosstalk. Due to layout constraints or board dimension limits, the spacing between high-speed traces may fall below the minimum requirement. In such cases, maximize the spacing between high-speed traces at the bottleneck sections. Whenever space permits, maximize the spacing between any two high-speed signal traces.

Long stub traces act as antennas; improper handling causes severe EMC issues. Stubs also introduce signal reflections and degrade signal integrity. Stubs are most commonly generated when adding pull-up or pull-down resistors to high-speed traces; daisy-chain routing is the standard solution to eliminate stubs. Empirical rule: a stub exceeding 1/10 wavelength functions as an antenna and becomes a defect.

Trace impedance is determined by trace width and the distance from the trace to its reference plane; wider traces yield lower impedance. The same principle applies to interface connectors and component pads. When a large connector pad connects to a narrow high-speed trace, impedance discontinuity occurs (low impedance at the large pad, high impedance at the narrow trace), which causes signal reflections. To resolve this, place a keep-out copper area under large connector or component pads, and add a reference plane for the pad on an adjacent layer to raise impedance and ensure continuity.

Vias are another source of impedance discontinuity. To minimize this effect, remove unused copper connected to vias on internal layers. This can be implemented via CAD tools during design or coordinated with PCB manufacturers to eliminate excess copper and maintain impedance continuity.

High-speed differential pairs must maintain uniform width and constant spacing to achieve the target differential impedance, so differential routing shall be as symmetrical as possible.

No vias or components are allowed between differential pairs; such placement induces EMC issues and impedance discontinuity.

Some high-speed differential pairs require series coupling capacitors, which shall be placed symmetrically. Capacitor package size shall be minimized; 0402 is recommended, 0603 is acceptable, and packages 0805 or larger (including parallel capacitors) are prohibited.

Vias cause significant impedance discontinuity, so minimize vias for high-speed differential pairs. If vias are necessary, place them symmetrically.

High-speed interfaces (such as parallel buses) require control of signal arrival time and skew error. For example, all data lines in a high-speed parallel bus must meet strict skew tolerance to ensure consistent setup and hold times, requiring length matching. High-speed differential pairs demand zero skew between the two traces to prevent communication failure. Serpentine routing is used to achieve length matching and meet timing requirements.

Serpentine compensation shall be placed at the source of length mismatch, not at the far end, to ensure synchronous transmission of the positive and negative differential signals for most of the path.

Trace bends are a common source of length mismatch. Length compensation for bends shall be placed within 15mm of the bend.

If two adjacent bends are spaced less than 15mm, their length mismatches cancel each other, and no additional compensation is required.

Segmented high-speed differential pairs (separated by vias, coupling capacitors, or connectors) shall be independently length-matched per segment. Most EDA tools only check total length in DRC, so segment-level matching requires manual attention.

Interfaces such as LVDS display drivers contain multiple differential pairs with strict inter-pair timing and skew requirements; all such pairs shall be length-matched on the same layer, since signal propagation speed varies between layers.

Some EDA tools include internal pad traces in length calculations, leading to actual length mismatch after compensation. Extra caution is required when using such tools.

Always use symmetrical fanout whenever possible to avoid serpentine compensation for length matching.

If space permits, add a small loop at the source of the shorter differential trace for compensation, rather than using serpentine routing.

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