Picking the Right Trace Width

Stephen Kraig |   |  Engineering, PCB Design, Trace Width

When picking trace widths to route a PCB, there are a few rules and guidelines that will help inform the value of the trace width. With the exception of special cases such as very high frequency signaling and high power applications these rules can be applied to every PCB.

The important characteristics to consider when selecting a trace width include:

  • The current capacity of the trace (how much current will flow through it)
  • The allowable spacing between traces
  • The size and pitch of the pads that the trace will be connecting to

Trace Ampere Capacity

For any given trace on a PCB there is a maximum amount of current it can handle before failure. Passing large currents through a trace will cause it to dissipate heat, and given enough current (and time) the trace will be destroyed by either burning through, or delaminating from the PCB and breaking the trace. Often we think of traces as zero resistance connecting wires between two components, but this is certainly not the case. All traces will have resistance and it is important to consider this when selecting widths. Knowing the resistance of a trace and the maximum current that will be passed through it will help inform which width to use.

Calculating the resistance of a trace is not trivial and can involve a lot of work. Luckily, there are a handful of trace width calculators available online that will help guide the choice of trace width when considering ampere capacity.

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Figure. PCB Trace Width Calculator from www.circuitcalculator.com

 

These trace width calculators will prompt you to enter design specifications such as the thickness of copper, the maximum amperage that will pass through the trace, the length of the trace, and the acceptable increase in temperature due to the resistance of the trace dissipating power.  After entering these values you will be presented with a calculated trace width. It is important to note that this value is a minimum width required to meet the design criteria inputs.

Determining the width of a trace based on the current demands is important for most power traces and high power signals, however, most traces on PCBs pass signals which draw negligible current. For these low power signal traces, we must look at other characteristics of the PCB to determine the width.

Trace Spacing

The size of a PCB is directly connected to the cost of the PCB, so in general PCBs are kept as small as possible. The downside of reducing board size is that it can limit the available space to route traces. For low power signal traces it is generally advised to keep traces small to increase the available space available for routing. Excessively large traces consume valuable PCB space while offering highly diminished returns. 6 to 30 mils is typical for most signal trace widths.

MacroFab offers a minimum of 5mil traces as a standard and if smaller traces are required, the extended manufacturing option will allow traces all the way down to 3mil.

 

Trace Termination

The point at which a trace meets a pad can also inform the width of a trace. In most cases, trace widths are set to the same width as the pad they terminate to. This will help aid with routing traces away from the component they connect to and will avoid violating spacing between adjacent traces. The image below demonstrates this:

spacing
Figure 2 Traces stemming off from a component pad of similar width

 

In the image we can see two traces stemming off of the SOIC style footprint. The width of these traces are both slightly smaller than the width of the component pads. This allows for plenty of clearance between adjacent pads while routing away from the chip.

Selecting trace widths for BGA components can be more tricky. For more information on routing these footprints check out our posts here and here.