Introduction to PCB Trace Width-Current Relationship

Printed Circuit Boards (PCBs) are essential components in modern electronics, providing a platform for electrical connections and mechanical support for various components. One of the critical aspects of PCB design is determining the appropriate trace width for a given current carrying capacity. The relationship between trace width and current is influenced by several factors, including the copper foil thickness, temperature rise, and the maximum allowable current density.

In this article, we will explore the PCB trace width-current relationship in detail, discussing the factors that influence this relationship and providing guidelines for designing PCBs with optimal trace widths.

Factors Affecting PCB Trace Width-Current Relationship

Copper Foil Thickness

The thickness of the copper foil used in PCB manufacturing plays a significant role in determining the current carrying capacity of traces. Thicker copper foils allow for higher current carrying capacity, as they provide a larger cross-sectional area for current flow. Standard copper foil thicknesses used in PCB manufacturing include:

Copper Foil Thickness (oz)	Thickness (mm)	Thickness (µm)
0.5 oz	0.017	17.5
1 oz	0.035	35
2 oz	0.070	70
3 oz	0.105	105
4 oz	0.140	140

As a general rule, doubling the copper foil thickness allows for a 41% increase in current carrying capacity for a given trace width.

Temperature Rise

The current flowing through a PCB trace generates heat due to the resistance of the copper. This heat causes a temperature rise in the trace, which can lead to performance issues and potential damage if not properly managed. The maximum allowable temperature rise is typically determined by the PCB material, components, and the operating environment.

The temperature rise in a PCB trace can be calculated using the following equation:

ΔT = (I^2 × R × L) / (k × A)

Where:
– ΔT is the temperature rise (°C)
– I is the current (A)
– R is the resistance of the copper (Ω/m)
– L is the length of the trace (m)
– k is the thermal conductivity of the PCB material (W/m·K)
– A is the cross-sectional area of the trace (m^2)

To minimize temperature rise, designers can increase the trace width, use thicker copper foils, or improve the thermal conductivity of the PCB material.

Maximum Allowable Current Density

Current density is the amount of current flowing through a given cross-sectional area of a conductor. In PCB design, the maximum allowable current density is determined by the copper foil thickness, trace width, and the acceptable temperature rise.

The current density can be calculated using the following equation:

J = I / A

Where:
– J is the current density (A/m^2)
– I is the current (A)
– A is the cross-sectional area of the trace (m^2)

Typical maximum allowable current densities for PCB traces range from 30 to 50 A/mm^2, depending on the application and the acceptable temperature rise.

PCB Trace Width Calculation

To determine the appropriate trace width for a given current carrying capacity, designers can use the following equation:

W = (I × ρ × L) / (ΔT × k × t)

Where:
– W is the trace width (m)
– I is the current (A)
– ρ is the resistivity of the copper (Ω·m)
– L is the length of the trace (m)
– ΔT is the acceptable temperature rise (°C)
– k is the thermal conductivity of the PCB material (W/m·K)
– t is the copper foil thickness (m)

This equation takes into account the current, resistivity of the copper, trace length, acceptable temperature rise, thermal conductivity of the PCB material, and the copper foil thickness to provide an optimal trace width.

PCB Trace Width-Current Tables

To simplify the process of determining the appropriate trace width for a given current, designers often refer to trace width-current tables. These tables provide a quick reference for the minimum trace width required for various current levels and copper foil thicknesses.

Here is an example trace width-current table for a temperature rise of 10°C and a trace length of 25.4 mm (1 inch):

Current (A)	0.5 oz (0.017 mm)	1 oz (0.035 mm)	2 oz (0.070 mm)	3 oz (0.105 mm)	4 oz (0.140 mm)
0.5	0.20 mm	0.10 mm	0.05 mm	0.03 mm	0.03 mm
1.0	0.41 mm	0.20 mm	0.10 mm	0.07 mm	0.05 mm
1.5	0.61 mm	0.31 mm	0.15 mm	0.10 mm	0.08 mm
2.0	0.81 mm	0.41 mm	0.20 mm	0.14 mm	0.10 mm
2.5	1.02 mm	0.51 mm	0.25 mm	0.17 mm	0.13 mm
3.0	1.22 mm	0.61 mm	0.31 mm	0.20 mm	0.15 mm
3.5	1.42 mm	0.71 mm	0.36 mm	0.24 mm	0.18 mm
4.0	1.63 mm	0.81 mm	0.41 mm	0.27 mm	0.20 mm

It is important to note that these values are based on specific assumptions and may vary depending on the PCB material, operating environment, and other factors. Designers should always verify trace widths using the appropriate calculations and simulations.

Frequently Asked Questions (FAQ)

1. What is the relationship between PCB trace width and current carrying capacity?

The current carrying capacity of a PCB trace increases with wider trace widths. This is because wider traces have a larger cross-sectional area, allowing for more current to flow through the trace without causing excessive temperature rise.

2. How does copper foil thickness affect the current carrying capacity of PCB traces?

Thicker copper foils provide a larger cross-sectional area for current flow, allowing for higher current carrying capacity. As a general rule, doubling the copper foil thickness allows for a 41% increase in current carrying capacity for a given trace width.

3. What is the maximum allowable temperature rise for PCB traces?

The maximum allowable temperature rise for PCB traces depends on the PCB material, components, and operating environment. Typically, a temperature rise of 10°C to 20°C is considered acceptable for most applications. However, designers should always verify the specific requirements for their project.

4. How can I minimize temperature rise in PCB traces?

To minimize temperature rise in PCB traces, designers can:
– Increase the trace width to provide a larger cross-sectional area for current flow
– Use thicker copper foils to increase the current carrying capacity
– Improve the thermal conductivity of the PCB material to dissipate heat more efficiently
– Reduce the current flowing through the trace by optimizing the circuit design

5. Are trace width-current tables accurate for all PCB designs?

Trace width-current tables provide a quick reference for determining the appropriate trace width for a given current. However, these tables are based on specific assumptions and may not be accurate for all PCB designs. Designers should always verify trace widths using the appropriate calculations and simulations, taking into account factors such as the PCB material, operating environment, and acceptable temperature rise.

Conclusion

Understanding the relationship between PCB trace width and current carrying capacity is crucial for designing reliable and efficient PCBs. By considering factors such as copper foil thickness, temperature rise, and maximum allowable current density, designers can determine the optimal trace widths for their projects.

Using the guidelines and equations provided in this article, along with trace width-current tables as a reference, PCB designers can ensure that their traces are capable of handling the required current without causing excessive temperature rise or potential damage to the PCB and its components.