How Do You Calculate PCB Trace Width?

Introduction

When designing printed circuit boards (PCBs), one important consideration is determining the appropriate width for the copper traces that connect components. The width of a trace affects the amount of current it can carry without overheating. Trace width also impacts signal integrity for high-speed signals. This article provides an overview of how to calculate the optimal trace width for different design scenarios.

Trace Width Factors

Several key factors go into calculating appropriate PCB trace widths:

Current Load

The amount of current a trace will carry is the primary determinant of trace width. Wider traces allow more current without overheating. Trace width must be calculated based on the expected current through the trace.

Temperature Rise

As current flows through a conductor, resistive heating occurs. The temperature rise must be limited to avoid damaging the PCB or components. Common limits are 10°C or 20°C above ambient. The allowable temperature rise influences the minimum trace width.

Conductor Thickness

Trace width calculations depend on the thickness of the copper foil, usually expressed in weight per area such as ounces per square foot. The most common thicknesses for PCBs are 1 oz (34 μm) and 2 oz (68 μm). Thicker copper has lower resistance, allowing narrower widths.

Ambient Temperature

A higher ambient temperature reduces how much the trace can heat up without exceeding temperature limits. Ambient temperature should be considered in trace width calculations.

Heat Sinking

Traces routed over a continuous ground plane act as heat sinks, allowing the use of narrower traces thanks to the additional cooling. Ground planes dissipate and spread out heat.

Duty Cycle

If a trace carries pulsed power rather than continuous loads, the duty cycle of the power pulses affects heating. Shorter or less frequent pulses allow the use of narrower traces compared to constant current.

Number of Traces

When there are multiple paths to carry a given overall current, dividing the current over parallel traces reduces heating effects. Each trace can be narrower than if carrying the full current alone.

Calculating Trace Width

There are two main methods for calculating appropriate trace widths:

1. Using a trace width calculator
2. Manual calculations based on standards and equations

Trace Width Calculator

The easiest approach is to use an online PCB trace width calculator. You enter the required parameters and the calculator outputs the minimum width needed. Some key parameters are:

• Desired current through the trace
• Conductor thickness (1 oz, 2 oz, etc.)
• Maximum allowable temperature rise over ambient
• Presence/absence of ground plane for heat sinking

Output trace widths are often shown for both internal and external traces since internal layers have better heat sinking from surrounding foil areas. Here is an example using a common online calculator:

Current: 3 A Copper thickness: 1 oz Temp rise: 20°C External trace No heat sink

Output trace width: 0.36 mm

This calculator result provides a quick method to determine trace widths for a given set of parameters. However, understanding the calculations behind the outputs is also helpful.

Manual Calculations

You can manually calculate trace widths using standards like IPC-2221 and equations based on conductor heating and geometry. This helps provide insight behind the required widths.

IPC-2221 Trace Width Tables

IPC-2221 provides trace width tables based on:

• Current
• Temperature rise
• Copper thickness
• Heat sinking (interior layer or exterior)

You look up the desired current and temperature rise to find the minimum width. IPC-2221 assumes a maximum ambient temperature of 55°C when generating the tables.

This table provides a starting point for common trace width needs, but you may need to perform additional calculations to account for higher ambient temperatures or other requirements not assumed in the IPC tables.

Trace Width Equation

The IPC tables are generated using this standard equation for determining trace widths:

Where:

• W = Trace width (mils)
• ΔT = Temperature rise (°C)
• θ = Temperature coefficient of resistivity for copper (234.5 °C)^-1
• ρ = Resistivity of copper (1.678 x 10^-8 Ωm)
• I = Current (A)
• δ = Thickness of copper foil (mils)
• k = Constants based on heat sinking

This equation calculates the trace width needed to limit temperature rise based on resistive heating in the trace. The k-factors account for heat dissipation into the PCB, with lower k-factors allowing narrower widths. Common k-factors are:

• k = 0.048 for an external trace with no heat sinking
• k = 0.024 for an external trace with heat sinking to a ground plane
• k = 0.048/2 = 0.024 for an internal trace surrounded by foil

Let's walk through an example manual calculation using this equation to match the online calculator result from earlier:

• Current (I) = 3 A
• Temperature Rise (ΔT) = 20°C
• Copper thickness (δ) = 1 oz = 1.4 mils
• k = 0.048 (external no heat sink)

Plugging these values into the equation gives:

Rounding up gives:

W = 0.36 mm

This matches the output from the online calculator, confirming the validity of both methods. The manual calculation provides more insight into the thermal and resistive factors that determine appropriate trace widths.

Trace Width Guidelines

Here are some general guidelines for PCB trace widths:

• High current traces (> 3A) should be ≥ 0.4 mm
• Typical signal traces can be 0.2 to 0.25 mm
• Thinner traces down to 0.1 mm can be used for low current applications
• Minimize length of thinnest sections of traces
• Use larger traces whenever space allows, even if not required
• Avoid using excessive trace widths larger than required
• Adjust width for grouped traces carrying higher combined current

Narrower traces have higher resistance and are more susceptible to damage, so use larger widths when possible. But overly large widths waste board space and provide no benefit.

Other Trace Width Considerations

While current capacity primarily determines trace width, other factors may influence width selection:

Mechanical Robustness

For traces exposed to mechanical stress and vibration, make traces wider to increase durability. This helps prevent cracked traces.

Etching Consistency

Narrow traces close to the limit of the PCB fabrication process result in lower quality etching. Wider traces produce more consistent etching results.

High Frequency Signals

Wider traces help maintain signal integrity at high frequencies by reducing resistive losses. But widths much over 50 mils can cause unwanted impedance effects.

Manufacturing Tolerances

Avoid trace widths close to the resolution limits of the PCB fabrication process. Stay at least a few mils above minimum capabilities.

Environmental Concerns

In harsh environment applications, slightly thicker traces improve corrosion and contamination resistance.

Trace Width Design Tips

Here are some helpful tips when designing PCB trace widths:

• Clearly designate required widths on layout drawings, don't just default to minimums
• Indicate higher width segments of traces that must handle greater currents
• Watch for neck-down sections along traces that may limit current
• Use larger vias where traces change layers to avoid current bottlenecks
• Verify any high-current traces manually with calculations or modeling
• Adjust width as traces heat up going through holes in ground planes
• Account for seasonal temperature variations that influence width needs
• Document assumptions used when calculating trace widths
• Leave notes on design drawings indicating how widths were determined

Carefully planning trace widths during layout helps avoid overheated traces or unnecessary width expansions. Clearly document trace sizes to support design review and reliability analysis.

Example Trace Layouts

Here are some example PCB trace layouts illustrating the application of appropriate trace widths:

High current power traces

For distributing high current from input filters or power supplies, substantial trace widths are required, such as 1 mm or larger. Widths increase as traces merge and carry higher combined currents.

Signal routing traces

Signal traces for routing data between components can use typical widths between 0.2 to 0.3 mm. Routing traces should use consistent widths to balance etch quality.

Component power pins

The traces connecting to component power pins should widen to provide lower resistance current flows into the pins. Short widened sections are used adjacent to the pins.

Thermal relief spokes

The thin traces radiating from pads prone to heating provide thermal relief by reducing conductive heat transfer into the pad.

Thermal Modeling

For critical high-current traces, thermal modeling using finite element analysis tools can provide additional verification of safe trace temperatures. Models account for heat spreading in the PCB and adjacent traces for increased accuracy. Models help:

• Confirm trace widths meet temperature requirements
• Evaluate transient and thermal cycling effects
• Optimize trace geometries to balance heating and thermal gradients
• Identify locations prone to overheating

Physics-based thermal models provide more precise analysis compared to more simplified calculations. This improves reliability for traces handling high power levels.

Summary

Determining appropriate PCB trace widths requires:

• Calculating width based on current flows and temperature rise
• Allowing for ambient temperature and heat sinking
• Selecting widths based on IPC-2221 tables or equations
• Accounting for mechanical, signal integrity and manufacturability factors
• Laying out traces with clearly designated widths
• Confirming widths for critical traces via modeling

Carefully planning copper trace widths results in:

• Reliable PCB performance without trace overheating
• Optimized use of board space and routing channels
• Reduced manufacturing rework due to insufficient widths
• Documentation to support the design process

Considering trace width requirements during layout and analysis helps produce robust and effective printed circuit board designs. Proper trace sizing avoids common issues like intermittent faults and damaged boards caused by excessive trace heating.

Frequently Asked Questions

What is a good minimum trace width?

For typical signal traces, 0.2 to 0.25 mm provides a good balance of manufacturability, resistance, and durability. High current traces should be at least 0.4 mm or wider based on current levels. Thinner traces down to 0.1 mm can be used in non-critical areas to conserve space.

How accurate do my calculations need to be?

It is usually sufficient to round calculated widths to the nearest 0.05 or 0.1 mm. Excess precision simply clutters documentation. Rounding up also adds margin to account for inaccuracies in modeling parameters.

Can I use a constant trace width for all traces?

Using a single standardized width for all traces can simplify layout, but results in traces that are wider than necessary in many places. Optimize widths individually for more efficient use of space and to minimize etching issues from overly narrow traces.

Should I double trace widths for external layers?

Doubling width for external layers is often unnecessary. Use external k-factors in calculations. Increasing external widths by 20% typically accounts for the reduced heat sinking compared to internal traces.

Can I adjust trace widths to control impedance?

Yes, increased widths help reduce impedance for matched transmission lines. But avoid widths much over 50 mils to minimize effects on signal propagation and timing. Use thinner dielectrics or ground planes to control impedance without excessively wide traces.