1. Understand the Factors Involved
Current Carrying Capacity: The amount of current the trace needs to safely carry.Copper Thickness: Typical copper thicknesses in PCBs range from 1 oz/ft² (about 35 µm) to 2 oz/ft² (about 70 µm).
Temperature Rise: The allowable increase in temperature of the trace above ambient temperature.
Trace Length: Longer traces have higher resistance and may require a wider width to carry the same current.
2. Use the IPC-2221 Standard
The IPC-2221 standard provides a widely accepted formula to calculate the trace width based on the current carrying capacity and temperature rise. The formula derived from this standard is commonly implemented in PCB design software and trace width calculators.3. Formula for Trace Width
The formula according to the IPC-2221 for external traces (exposed to air) is:$$
W = \frac{k \cdot I^{0.44}}{\Delta T^{0.725}}
$$
where:
- \( W \) is the trace width in mils (1 mil = 0.001 inches),
- \( I \) is the current in amperes,
- \( \Delta T \) is the temperature rise in degrees Celsius,
- \( k \) is a constant that depends on the copper thickness (typical value for 1 oz/ft² copper is 0.048).
4. Adjustments for Internal Traces
Internal traces (buried within the PCB) carry less heat away than external traces. They require a different constant in the formula:$$
W = \frac{k' \cdot I^{0.44}}{\Delta T^{0.725}}
$$
where \( k' \) is typically higher than \( k \) due to reduced heat dissipation (a common value for \( k' \) with 1 oz/ft² copper might be 0.024).
5. Utilize Online Calculators or Software
PCB Trace Width Calculators: There are several online tools available for calculating trace width. These calculators typically ask for input parameters like current, copper thickness, and temperature rise, and calculate the required trace width using IPC-2221 formulas.PCB Design Software: Most PCB design software has built-in tools to calculate trace width as part of the layout process, ensuring compliance with electrical and thermal requirements.
6. Consider Practical Constraints
Manufacturing Capabilities: Check with your PCB manufacturer about their capabilities and minimum trace width requirements to ensure your design can be manufactured without issues.Space Limitations: In highly dense PCBs, balancing trace width with space availability can be challenging. Prioritize critical high-current paths for wider traces.