PCB routing is the core link in printed circuit board design, connecting discrete components into a functional electronic system through precise wiring. For beginners and even experienced designers, mastering the professional terminology of PCB routing is the premise of ensuring signal integrity, reducing electromagnetic interference, and optimizing board performance. This article sorts out the most essential routing terms, from basic concepts such as trace width to advanced technical terms such as impedance matching, and explains them in combination with practical application scenarios.

I. Basic Routing Terminology: Foundation of PCB Layout

1. Trace/ Track

A trace (also referred to as track) is the conductive path on a PCB that transmits electrical signals or supplies power between components. Traces are typically made of copper foil, with their shape and size directly affecting current-carrying capacity and signal transmission efficiency.

Practical note: In routing, traces are divided into signal traces and power traces. Power traces require larger widths to carry higher currents.

2. Trace Width

Trace width refers to the width of the copper foil trace. It is one of the most critical parameters in PCB routing, determined by two core factors: current carrying capacity and board space constraints.

Calculation logic: Under the same temperature rise condition, wider traces can carry larger currents. For example, a 1oz copper trace with a width of 0.5mm can carry about 1A of current in a typical FR-4 PCB.

Practical application: Power traces (VCC, GND) usually use wider widths (e.g., 1–2mm), while low-current signal traces can use narrower widths (e.g., 0.2–0.3mm).

3. Trace Spacing

Trace spacing is the distance between adjacent traces. It is mainly used to prevent electrical breakdown and signal crosstalk.

Key requirement: The spacing must meet the insulation voltage standard. In high-voltage PCB design, the spacing needs to be significantly increased to avoid arc discharge between traces.

Crosstalk correlation: Smaller spacing will intensify electromagnetic coupling between traces, leading to crosstalk. High-speed signal traces require larger spacing to reduce interference.

4. Via

A via is a conductive hole that connects traces on different layers of a multilayer PCB. It consists of three parts: the pad, the conductive hole wall, and the solder mask (optional).

Classification: Vias are divided into through-hole vias (penetrating the entire PCB), blind vias (connecting the surface layer and the inner layer), and buried vias (completely located inside the PCB, not exposed on the surface).

Routing note: Blind vias and buried vias can save board space and reduce signal interference, but they increase manufacturing costs.

5. Pad

A pad is the metal area on the PCB used to solder component pins. It is closely connected to traces and vias, acting as a bridge between components and the routing system.

Types: There are through-hole pads (for plug-in components) and surface-mount device (SMD) pads (for SMD components).

Routing consideration: The size of the pad should match the component pins to ensure firm soldering and stable electrical connection.

II. Intermediate Routing Terminology: Signal Transmission and Layout Design

1. Single-Ended Routing

Single-ended routing is the most common routing method, where a single signal trace transmits signals with the ground (GND) as the reference plane. It is suitable for low-speed signal transmission (e.g., UART, I2C signals in low-speed scenarios).

Advantages: Simple design, saving board space.

Disadvantages: Poor anti-interference ability, vulnerable to external electromagnetic interference.

2. Differential Pair Routing

Differential pair routing refers to routing two signal traces with equal width, equal length, and close spacing. Signals transmitted on the two traces are equal in amplitude and opposite in phase, and the final signal is obtained by detecting the voltage difference between the two traces.

Core advantages: Strong anti-interference ability. Common-mode interference can be effectively suppressed, making it the preferred routing method for high-speed signals (e.g., USB 3.0, HDMI, Ethernet).

Routing rules: Strictly ensure equal length of the two traces (length difference should be less than 5mil for high-speed signals), equal width, and consistent spacing, avoiding crossing or branching.

3. Ground Plane

A ground plane is a complete copper foil layer in a multilayer PCB dedicated to grounding. It is not a simple "ground wire", but a reference plane for signal traces and a path for current return.

Key functions: Reduce signal loop area, suppress electromagnetic interference, improve signal integrity, and provide a low-impedance current path for power supply.

Routing application: High-speed signal traces should be routed directly above or below the ground plane to minimize signal radiation.

4. Power Plane

A power plane is a copper foil layer dedicated to supplying power (e.g., 3.3V, 5V). Similar to the ground plane, it is widely used in multilayer PCBs.

Advantages: Compared with using wide power traces, the power plane can provide a more stable voltage and lower impedance, reducing voltage drop and noise.

Layout note: The power plane should be paired with the ground plane to form a "power-ground pair", which can effectively suppress electromagnetic interference.

III. Advanced Routing Terminology: Signal Integrity and Impedance Control

1. Impedance Matching

Impedance matching means making the output impedance of the signal source, the characteristic impedance of the transmission line (trace), and the input impedance of the load equal. It is the core requirement for high-speed PCB routing.

Core purpose: Eliminate signal reflection. When impedance is mismatched, part of the signal will be reflected back to the source end, causing signal distortion, overshoot, or undershoot.

Practical methods: Use matching resistors, adjust trace width and spacing, or optimize the reference plane structure to achieve the target characteristic impedance (common values: 50Ω for RF signals, 100Ω for differential pairs).

2. Characteristic Impedance

Characteristic impedance is the inherent impedance of a transmission line (PCB trace) when signals propagate along it. It is determined by the trace width, trace thickness, dielectric constant of the substrate, and the distance from the reference plane—not the length of the trace.

Calculation basis: For microstrip lines (traces on the surface layer with a ground plane as the reference), the characteristic impedance decreases as the trace width increases or the distance from the reference plane decreases.

Industry standard: High-speed signal traces usually require precise control of characteristic impedance, with a tolerance of ±10% or even ±5%.

3. Crosstalk

Crosstalk refers to the electromagnetic interference between adjacent traces. When a signal propagates on one trace, it will induce an unwanted signal on the adjacent trace through capacitive and inductive coupling.

Classification: Crosstalk is divided into forward crosstalk (interference signal propagating in the same direction as the original signal) and backward crosstalk (interference signal propagating in the opposite direction).

Suppression methods: Increase trace spacing, use ground planes for isolation, adopt differential pair routing, and shorten the parallel length of traces.

4. Signal Integrity (SI)

Signal Integrity refers to the ability of a signal to maintain its original waveform and quality during transmission on the PCB. Poor signal integrity is manifested as signal reflection, crosstalk, delay, and distortion, which can lead to system malfunctions.

Key influencing factors: Trace impedance, routing topology, via design, and the quality of the reference plane.

Optimization direction: Impedance matching, minimizing trace length, reducing via usage, and avoiding right-angle turns in traces.

IV. Routing Rule Terminology: Standardization of Design

1. Design Rule Check (DRC)

Design Rule Check (DRC) is a function provided by PCB design software (e.g., Altium Designer, Cadence Allegro) to verify whether the routing meets the preset rules (e.g., trace width, spacing, via size).

Core role: Automatically detect potential routing errors, such as insufficient trace spacing, too narrow power traces, and mismatched via sizes, avoiding design defects before manufacturing.

2. Right-Angle Trace

A right-angle trace is a trace with a 90° turning angle. In low-speed PCB design, it has little impact, but in high-speed design, the right-angle part will cause impedance discontinuity, leading to signal reflection and radiation.

Optimization scheme: Replace right-angle turns with 45° angles or rounded corners to ensure smooth signal transmission and reduce impedance mutation.