PCB Trace Width and Spacing 

Rush PCB UK fabricates all types of PCBs. We also design some boards, where it is necessary to route the traces. However, as designs grow in complexity and size, routing traces can be highly challenging. It requires judicious decisions on the part of the designer to optimize the trace width and spacing.

This is because the designer may have to address each net with its unique routing characteristics for it to function as intended. Additionally, nets may require routing in different layers apart from the specific areas that the designer can use. This calls for different PCB trace widths and spacing that the designer must manage.

PCB Trace Width and Spacing 

Challenges in Routing

For simple circuit boards, routing traces is typically a straightforward procedure, with the designer assigning all traces with a default width and spacing. This works fine, except for power and ground connections that may require wider tracks or a copper area. However, with growing complexity of circuit boards, the requirements for trace width and spacing become more complicated. For instance, a circuit board may have some or all of the following:

  • Specified widths and spacing for controlled impedance routing.
  • Wider spacing of sensitive high-speed traces to prevent cross-talk.
  • Wider traces but close spacing for power and ground connections.
  • Multiple trace widths to handle different currents in power supplies.
  • Wider spacing between analog and digital traces to isolate them from each other.

In addition, the functionality of a circuit may dictate the various widths and spacing requirements. This requirement may also come from the location of the traces. For instance:

  • Traces passing in between closely spaced pins of a connector may require using a smaller width.
  • Escape routing from fine-pitch components like small outline packages or quad flat packages may need trace width reduction.
  • Traces routing in and around pins and vias of ball grid arrays may require shrinking widths.
  • Vias may also dictate the width and spacing of traces. Apart from regular vias, a designer may also use microvias for high density interconnect designs that require thinner traces and spacing.

Signal Integrity in Digital Circuits

Although most trace routing for digital circuits is adequate with default trace widths and spacing, some high-speed signals may require specific trace width and spacing to meet controlled impedance. Usually, the board layer stackup design factors in the calculations for specific trace widths. To avoid cross-talk in high-speed but sensitive traces, designers may have to increase the spacing.

Analog Circuits

Analog circuits also require varying trace width and spacing depending on the purpose of the circuitry. Some constricted areas may require thin traces closely spaced, but the designer must also take care that PCB fabrication will not compromise the connections.

Ground and Power Routing

Depending on the current flow in the track, traces for routing ground and power may need to be wider. Wider tracks have lower resistance that allows them to remain cool while passing high currents. Tracks handing ground and power in the inner layers may need to be wider to enable them to disperse heat more effectively, as compared to traces on the top and bottom surfaces of the board, where they are exposed to air.

Wider traces also help to reduce inductance, thereby improving noise sensitivity of the board. High voltage operation may require more spacing between traces to prevent arcing between them.

Ease of Fabrication

It is easier to fabricate wider traces. As making the traces requires chemical etching, it is more effective for wider traces and larger separation. Making the trace as wide as possible and keeping them far apart is advantageous for PCB fabrication. However, this may not always be possible, considering the circuit functionality requirements.

Circuit Board Assembly

Assembling components on very wide traces may be troublesome, as the extra width may conduct heat away from the joint very quickly, resulting in a dry solder joint. During reflow, the presence of a large ground or power track may result in uneven heating, leading to several poor solder joints, tombstoning, and other effects that call for manual touch up activities.

Conclusion

Designers need complete control over rules defining trace routing, their widths, and spacing. Most PCB CAD Design software packages allow defining multiple width and spacing for traces, along with the necessary simulation for inductance and signal integrity verifications.