Understanding the PCB trace width-current relationship
Printed Circuit Boards (PCBs) are the foundation of modern electronics. They provide mechanical support and electrical connections for components using conductive copper traces. One critical aspect of PCB design is determining the appropriate trace width for a given current. The relationship between trace width, copper thickness, and current is governed by several factors, including:
- Copper foil thickness
- Allowable temperature rise
- Ambient temperature
- Length of the trace
In this article, we will explore these factors in depth and provide guidelines for selecting appropriate trace widths based on the current requirements of your design.
Copper Foil Thickness and Its Impact on Current Capacity
The thickness of the copper foil used in a PCB has a significant impact on its current carrying capacity. Thicker copper allows for higher currents, while thinner copper limits the maximum current a trace can handle without excessive heating.
Common copper foil thicknesses used in PCBs include:
| Copper Weight (oz) | Thickness (mm) | Thickness (mils) |
|---|---|---|
| 0.5 | 0.018 | 0.7 |
| 1 | 0.035 | 1.4 |
| 2 | 0.070 | 2.8 |
| 3 | 0.105 | 4.2 |
| 4 | 0.140 | 5.6 |
As a general rule, doubling the copper thickness allows a trace to carry 41% more current for a given temperature rise and trace width.
Allowable Temperature Rise and Ambient Temperature Considerations
The amount of current a trace can safely carry is directly related to the allowable temperature rise and ambient temperature. As current flows through a trace, it generates heat due to the trace’s resistance. This heat must be dissipated to prevent damage to the PCB and its components.
The allowable temperature rise depends on several factors, including:
- The maximum operating temperature of the PCB and its components
- The ambient temperature in which the PCB will be used
- The thermal conductivity of the PCB substrate material
IPC-2221, a standard for PCB design, recommends a maximum temperature rise of 10°C for external traces and 20°C for internal traces. However, these values may need to be adjusted based on the specific requirements of your design.
Determining Trace Width Based on Current and Copper Thickness
To determine the appropriate trace width for a given current and copper thickness, you can use the IPC-2221 charts or online calculators. These resources provide recommended trace widths based on the following inputs:
- Current in amps
- Copper thickness in oz or mm
- Allowable temperature rise in °C
- Ambient temperature in °C
- Trace length in mm or mils
Here’s an example IPC-2221 chart for external traces with 10°C rise:
| Current (A) | 0.5 oz (0.018 mm) | 1 oz (0.035 mm) | 2 oz (0.070 mm) | 3 oz (0.105 mm) | 4 oz (0.140 mm) |
|---|---|---|---|---|---|
| 0.5 | 0.15 mm | 0.08 mm | 0.04 mm | 0.03 mm | 0.02 mm |
| 1 | 0.30 mm | 0.15 mm | 0.08 mm | 0.05 mm | 0.04 mm |
| 2 | 0.60 mm | 0.30 mm | 0.15 mm | 0.10 mm | 0.08 mm |
| 3 | 0.90 mm | 0.45 mm | 0.23 mm | 0.15 mm | 0.11 mm |
| 4 | 1.20 mm | 0.60 mm | 0.30 mm | 0.20 mm | 0.15 mm |
| 5 | 1.50 mm | 0.75 mm | 0.38 mm | 0.25 mm | 0.19 mm |
To use this chart, find your desired current in the left column and trace along the row to the column corresponding to your chosen copper thickness. The value at the intersection is the recommended minimum trace width.
For example, if you need to carry 3A on a 1 oz copper trace with a 10°C temperature rise, the recommended minimum trace width is 0.45 mm.
If your design requires a different temperature rise or ambient temperature, you can use an online calculator to determine the appropriate trace width. Many PCB design software packages also include trace width calculators as part of their toolset.
Additional Factors Affecting Trace Current Capacity
While copper thickness and trace width are the primary factors determining a trace’s current capacity, there are several other considerations to keep in mind:
Trace Length
Longer traces have higher resistance and generate more heat, reducing their current carrying capacity. When using the IPC-2221 charts or online calculators, be sure to input the correct trace length for accurate results.
Thermal Relief Pads
Components with large thermal mass, such as connectors and power transistors, can sink heat away from traces, effectively increasing their current capacity. To take advantage of this effect, you can use thermal relief pads, which are enlarged copper areas connected to the component pads. Thermal relief pads help distribute heat and improve the trace’s current handling capability.
Copper Pour and Plane Layers
Using copper pour or plane layers can significantly improve a PCB’s current carrying capacity and heat dissipation. By filling unused areas of the PCB with copper connected to ground or power planes, you create a low-resistance path for current and a large surface area for heat dissipation. This technique is especially useful for high-current designs or when space constraints limit trace widths.
Parallel Traces
If a single trace is not sufficient to carry the required current, you can use parallel traces to increase the effective width and current capacity. To ensure proper current sharing between parallel traces, keep them closely spaced and of equal length. It’s also important to consider the potential impact on signal integrity when using parallel traces for high-speed signals.
FAQ
What happens if I use a trace width that’s too small for my current?
Using a trace width that’s too small for your current can lead to excessive heating, potentially damaging the PCB and its components. In extreme cases, the trace may even melt, causing an open circuit. Always use the recommended trace width for your current and copper thickness to ensure reliable operation.
Can I use thicker copper to reduce trace width?
Yes, using thicker copper allows you to use narrower traces for a given current. This can be helpful when space is limited on the PCB. However, keep in mind that thicker copper is more expensive and may require adjustments to your manufacturing process.
How do I account for temperature variations in my design?
When determining trace widths, consider the maximum ambient temperature your PCB will experience and adjust the allowable temperature rise accordingly. For example, if your PCB will operate in a 60°C environment and you want to limit the temperature rise to 10°C, use the calculations for a 50°C rise at 25°C ambient.
What if my required current exceeds the chart’s values?
If your current requirement exceeds the values in the IPC-2221 charts, you can either use a wider trace, thicker copper, or a combination of both. You can also consider using parallel traces or copper pour to increase current capacity. In extreme cases, you may need to use an external heatsink or active cooling to dissipate the generated heat.
Can I use these guidelines for flexible PCBs?
The IPC-2221 guidelines and most online calculators are designed for rigid PCBs. Flexible PCBs have different thermal properties and may require additional considerations when determining trace widths. Consult with your flexible PCB manufacturer for specific guidelines on trace width and current capacity.
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” alt=”” class=”wp-image-136″ >Conclusion
Selecting the appropriate trace width based on your current requirements and copper thickness is crucial for designing reliable and efficient PCBs. By understanding the relationship between these factors and using the IPC-2221 guidelines or online calculators, you can ensure your traces can handle the required current without excessive heating or damage.
Remember to consider additional factors such as trace length, thermal relief pads, copper pour, and parallel traces when optimizing your design. By taking a comprehensive approach to PCB trace width selection, you can create designs that are both robust and cost-effective.
