Advanced PCB Routing Strategies

PCB routing is pivotal in ensuring the functionality, signal integrity, manufacturability, and reliability of electronic designs. Conductive traces are more than mere component interconnections or power distribution paths, and through effective routing, engineers can minimize signal degradation, crosstalk, and electromagnetic interference (EMI). Exact impedance control is crucial in maintaining signal integrity, as high-impedance traces are more noise-sensitive.

Thoughtful component placement and routing aid fault diagnosis and testing procedures, expediting debugging and reducing time-to-market and development costs. Furthermore, strategic routing decisions can facilitate future adaptations and changes to the design. By leaving room for expansion, using modular routing techniques, and adhering to design standards, PCB layouts become more flexible for future modifications or upgrades.

Traces and Transmission Lines in PCB Design

Traces are conductive pathways that serve varying purposes like signal transmission, power distribution, or ground connections. A transmission line is a specific type of trace that follows certain design guidelines and characteristics to ensure proper signal transmission, especially at high frequencies. These guidelines may include controlling parameters such as impedance, length, and routing topology to meet the transmitted signals' requirements.

For example, in high-speed serial communication standards like USB or PCIe, signal transmission lines often need to be precisely impedance-controlled and length-matched to ensure signals arrive at their destinations simultaneously, minimizing skew and ensuring reliable data transmission.

General Considerations in PCB Routing

Designers generally should strive to minimize the length of traces to maintain signal integrity by reducing interference, crosstalk, and signal decay. Since voltage drops over distance, longer traces are more prone to causing these problems. Similar to narrow traces, long ones exhibit higher resistance — leading to increased signal attenuation, potentially resulting in a weakened signal at the receiving end. In addition, long traces can act as an antenna, especially in susceptible designs that utilize components sensitive to picking up noise, such as RF applications.

Similar to thickness and length, designers must route traces in a way that leaves enough space between them. Too narrow spacing can lead to substantial cross-coupling between neighboring traces, ultimately resulting in signal decay. As a rule of thumb, three times the dielectric thickness is often a suitable general lower bound for spacing traces. For similar reasons, traces should not run in parallel over large distances without enough separation. Instead, designers should fan out traces that run in parallel to space them apart and keep parallel stretches as short as possible.

The trace width is crucial in ensuring signal integrity and controlling impedance. Designers should keep the trace width consistent to avoid changing the trace’s resistance per unit length, which leads to a larger voltage drop the narrower a trace becomes due to increased resistance. Additionally, trace width changes affect a transmission line's distributed capacitance. Capacitance per unit length is inversely proportional to trace width, so thinner traces have higher capacitance. Variations in capacitance along the transmission line can impact the characteristic impedance and introduce signal distortions.

Routing Order and Differential Pair Routing Techniques

While there’s no one-size-fits-all approach to routing, laying out the traces of the most critical signals first and then working their way to power wires and planes is typically a proper approach most designers can follow. Doing so allows them to group similar signals and those originating or terminating in the same subcircuit. Running traces underneath sensitive components, such as oscillators and RF modules, should also be avoided to reduce the risk of signal interference and noise coupling. Similarly, signal-carrying traces should always run over a suitable reference plane.

Differential pairs should always be routed together to ensure they have the same length. Designers must choose a suitable routing technique, acknowledging each method’s potential benefits, drawbacks, and the target application. Parallel routing maintains equal spacing between traces, ensuring both traces experience similar environmental conditions and balanced impedance. Edge-coupled routing minimizes crosstalk by utilizing adjacent edges of a reference plane. Microstrip routing places traces on the PCB surface with a ground plane underneath, while stripline routing puts traces between two ground planes for superior signal integrity and EMI shielding.

Ultimately, traces should be kept as short as possible while ensuring sufficient spacing between them to maintain signal integrity and mitigate problems such as crosstalk. Modern routing tools and PCB design software usually allow for the automatic calculation and simulation of various physical characteristics to match a design’s impedance requirements.

Vias and Their Role in PCB Routing

Vias play a crucial role in impedance control, ensuring consistent signal integrity by transitioning signals between layers with different characteristic impedances. Properly placed vias help minimize signal path lengths, reduce reflections, and optimize signal transmission quality. Vias come in pairs (signal and ground vias) and should be placed close to where they are needed to minimize signal path lengths and reduce EMI.

Different types of vias offer varying electrical characteristics, degrees of routing flexibility, and manufacturing complexity. Through-hole vias traverse the entire PCB, providing electrical connections between all layers. Blind vias connect an outer layer to one or more inner layers without spanning the board's whole thickness, offering more routing flexibility and reducing signal path lengths. Buried vias connect inner layers without penetrating the outer layers, allowing for denser PCB layouts. Staggered vias, also known as offset vias, are formed by drilling overlapping holes on different layers, reducing signal reflections and improving impedance control. Microvias are small-diameter vias used in high-density interconnect (HDI) PCB designs, enabling finer pitch routing and increased routing density. They are typically laser-drilled and offer improved signal integrity and manufacturability in compact electronic devices.

The physical characteristics of vias, including barrel size, pad size, and anti-pad size, influence signal integrity. The barrel refers to the conductive cylinder formed within the drilled hole. The pad is the exposed copper surface surrounding the hole, and the anti-pad is a copper-less clearance area around the via hole that prevents unintended electrical connections or short circuits. A larger barrel and pad size can reduce impedance and signal losses, while a smaller anti-pad size helps minimize signal distortion and electromagnetic interference.

Additionally, vias play a crucial role in providing return current paths for signals, completing the loop for the current flow. Adequately designed and carefully placed vias help ensure that return currents have a low-impedance path to their source, minimizing electromagnetic interference and maintaining signal integrity. By strategically placing vias near signal traces and ground planes, designers can optimize return paths and mitigate potential noise and signal distortion issues.

Routing With Manufacturability in Mind

The task of routing PCB traces also includes ensuring adherence to manufacturability guidelines. Designers should maintain minimum trace widths and spacings to prevent fabrication issues like acid traps. Sharp bends should be avoided, as they can result in signal integrity problems and fabrication challenges. Additionally, designers should pay attention to clearance requirements to prevent short circuits and ensure proper solder mask coverage.

Finally, more complex designs are typically more expensive to manufacture and often have longer lead times. Therefore, designers must balance their project’s cost and time-to-market limitations and the design's technical, electrical, and physical requirements.

Application-Dependent PCB Routing Requirements, Standards, and Limitations

The classification of PCBs as class 2 or class 3 significantly influences PCB routing guidelines and standards. Class 3 PCBs are commonly used in aerospace and defense (A&D) applications and require stricter design rules than class 2 PCBs, which are dominant in commercial applications.

For example, A&D applications require adherence to standards such as MIL-PRF-31032 and MIL-STD-275, which outline performance requirements and design criteria for PCBs used in military and aerospace systems. These standards cover various aspects of PCB design, including material selection, trace routing, via placement, and standardized testing procedures.

Adherence to these standards ensures reliability, performance, and compatibility with often rigorous requirements in A&D applications. Designers must consider the classification of PCBs and relevant standards to effectively implement routing strategies that meet the intended application's specific reliability and performance requirements.

Summary

Transmission lines, a specialized type of trace, require careful design to maintain impedance and minimize signal degradation, especially in high-frequency applications like USB or PCIe. Exact impedance control is vital, as impedance mismatches can lead to signal distortion and loss. Generally, traces should be kept as short as possible, and their width should be sufficient and consistent over the entire length to prevent fluctuations in physical properties over unit length.

Vias play a crucial role in transitioning signals between layers, controlling impedance, and providing return paths, as adequately designed vias minimize signal path lengths, reduce reflections, and optimize signal transmission. Different via types offer varying flexibility and electrical characteristics. Aside from the type, each via's physical characteristics further influence signal integrity. A larger barrel and pad size can generally reduce impedance and signal losses, while a smaller anti-pad size helps minimize signal distortion and EMI.

Routing should start with the most vital signals and progress toward power lines. Differential pairs should always be routed together to ensure they have the same length. Designers must choose a suitable routing technique, acknowledging each method's potential benefits, drawbacks, and the target application.

Finally, routing PCB traces also includes ensuring adherence to manufacturability guidelines. Designers should maintain minimum trace widths and spacings to prevent fabrication issues like acid traps.