PCB Track Width Graphic : Thickness, Current and Design

When designing printed circuit boards (PCBs), one important consideration is determining the appropriate track width for the copper traces on the board. The track width impacts current carrying capacity, voltage drop, and heat dissipation. This article provides graphics and tables to help PCB designers quickly determine the optimal track width based on thickness, current, and other factors.

Track Width Basics

The track width refers to the width of the copper traces on a PCB that carry electrical signals and power between components. Wider tracks can handle more current, while narrower tracks may be required for routing or to control impedance. Here are some key guidelines on track width:

• Track width is measured in mm or mils (1 mil = 0.001 inches).
• Typical track widths range from 0.2mm to 2mm.
• Wider tracks reduce resistance and voltage drop.
• Narrower tracks are needed for high density routing.
• Match track width to expected current to avoid overheating.
• Follow any specified trace width rules from PCB manufacturers.

Other factors like copper thickness, ambient temperature, and allowed temperature rise also impact the track width selection.

Track Width vs. Current

One of the most important considerations in selecting PCB track width is the required current carrying capacity. Wider traces allow more current without overheating. The graphics and table below provide guidelines on track width vs. current for different copper thicknesses.

Track Width vs. Current Graphic

Track Width vs. Current Table

Track Width (mm)Current (Amps) - 35um CopperCurrent (Amps) - 70um Copper0.21.42.20.251.72.80.32.23.40.53.55.50.75581.07101.510162.01422

Table 1. Approximate current carrying capacity for common track widths and copper thicknesses.

These values assume a temperature rise of 20°C and must be derated for higher ambient temps. They provide a good starting point for estimating required track width.

Track Width Calculator

To precisely calculate the minimum track width, you need to consider:

• Desired current (I)
• Copper thickness (t)
• Maximum allowable temperature rise (ΔT)
• Copper resistivity (ρ)
• Ambient temperature (Ta)

The full formula is:

Track Width = (I x t x ΔT x ρ) / (0.024 x (Tmax - Ta)^^0.44))

Where:

• I is in amps
• t is in mm
• ΔT is in °C
• ρ is 0.0178 ohm.mm/m at 20°C
• Tmax is max temperature (°C)
• Ta is ambient temp (°C)

This can be simplified to the following track width calculator:

Track Width Calculator

Current (A): 10

Copper Thickness (mm): 0.035

Temp Rise (°C): 20

Ambient Temp (°C): 25

Minimum Track Width = 1.2 mm

An online calculator can also be used to determine the track width.

Other Track Width Considerations

While current capacity is critical, other factors also impact the choice of track width:

Voltage Drop

• Thinner tracks have higher resistance, increasing voltage drop.
• Make power traces wider to minimize unwanted voltage drop.
• For signal traces, impedance requirements limit width.

Routing Density

• Dense designs require narrower traces to route between components.
• Use wider tracks where space allows, narrower tracks where space is limited.

Manufacturing Capabilities

• Work with board house on their design rules and capabilities.
• Have a mix of track widths within their tolerances.
• Very thin tracks (<0.1mm) may require special processes.

Impedance Control

• Match track width to required impedance for controlled impedance routing.
• Vary gap distance between differential pairs to tune impedance.

Heat Dissipation

• Wider tracks spread heat over larger surface area.
• Ensure enough copper is present for required power handling.

EMC Performance

• Tuning track width along with spacing helps control EMI.
• Follow good high speed layout practices for sensitive signals.

Cost

• Too much copper increases material costs.
• Optimize track widths to balance cost and performance.

Track Width Design Rules and Guidelines

Here are some typical design rules and guidelines to follow for track widths on a PCB:

• Minimum track width: 0.2mm (8mils)
• Minimum spacing between tracks: 0.2mm (8mils)
• Preferred track width for signals: 0.25mm (10mils)
• Track width for power traces: 0.5mm to 2mm
• Match track width to expected current (see table)
• Avoid using the minimum track width where possible
• Allow for +/- 10% manufacturing tolerance
• Follow any specific rules from PCB manufacturer

Rules may vary across different board houses, processes, and materials. Discuss requirements with your fabricator early in the design process.

Example Track Width Applications

To illustrate the principles covered, here are some examples of selecting an appropriate PCB track width:

Processor Power

• 120W processor with 1.2V supply
• 35um copper thickness
• Max temp rise 40°C
• Ambient temp 35°C

Current = Power / Voltage = 120W / 1.2V = 100A

Using the calculator:

• Current: 100A
• Thickness: 0.035mm
• Rise: 40°C
• Ambient: 35°C

Minimum track width = 2.8mm

Use 3mm width for power trace to processor.

Differential USB 2.0

• USB 2.0 differential pair
• 12 Mbps data rate
• 70um copper thickness
• Max ambient temp 55°C

Track width = 0.3mm for 90Ω differential impedance

Current = 0.5A per USB 2.0 spec

From table, 0.3mm width supports 3.4A in 70um copper.

Use 0.3mm track width for USB 2.0 traces.

Analog Signal Routing

• Low level analog signal
• Track between ADC and amplifier
• 35um copper thickness
• Low density area available

Track width = 0.5mm for lower resistance and noise

Current = 10mA based on circuit

From table, 0.5mm width supports 3.5A, much more than needed.

Use 0.5mm track for better analog performance.

Conclusion

Determining the optimal PCB track width requires balancing current capacity, voltage drop, impedance, heat, layout space, manufacturability, and cost constraints. This article provides graphics, tables, calculators and guidelines to help PCB designers select an appropriate trace width for their specific design requirements. Some key tips:

• Match track width to expected current
• Use wider tracks for power, narrower for signals
• Calculate minimum width based on current, thickness and temperature
• Follow manufacturer design rules and capabilities
• Allow for manufacturing tolerances
• Adjust width to control impedance or EMI as needed

With the right track width, your PCB layout will perform reliably under all expected operating conditions.

Frequently Asked Questions

What is the standard PCB track width?

There is no single standard PCB track width. Typical widths range from 0.2mm to 2mm. For signals, 0.25mm (10mil) is a common default, while power traces are often 0.5mm or wider. However, width should be based on current, impedance, and other requirements.

How thin can a PCB track be?

The thinnest PCB tracks are around 0.1mm (4mils), but this requires specialized PCB processes. Most fabricators support down to 0.15-0.2mm (6-8mils) for regular PCBs. Extremely thin tracks have higher resistance and are challenging to manufacture.

What is a good track width for power supply?

For power, start with a width of 0.5mm and adjust as needed based on expected current. Use the track width vs. current tables to determine suitable width. Wider tracks will allow more current and minimize voltage drop along power traces.

How do I calculate track width impedance?

Use an online calculator or formula to determine track width and spacing needed to achieve target impedance, based on copper thickness and dielectric material. For differential pairs, you can tune impedance by altering track-to-track spacing for a given width.

Should track widths be matched on a PCB?

It generally best to match track widths for signals in the same bus or bundle. Some key exceptions are wider power traces, narrow sections to route through tight spaces, and differential pairs with tuned gaps. Matching trace widths ensures similar current capacity and impedance.