How to calculate the trace width for a rigid PCB?

Calculating the trace width for a rigid PCB is a crucial step in the design process, as it directly impacts the performance, reliability, and safety of the printed circuit board. As a rigid PCB supplier, I understand the importance of getting this calculation right. In this blog post, I will share with you the key factors to consider and the methods to calculate the appropriate trace width for your rigid PCB.

Importance of Trace Width in Rigid PCBs

The trace width on a rigid PCB serves as the electrical pathway for current flow. If the trace width is too narrow, it can lead to excessive heat generation due to high resistance, which may cause the trace to burn out, damage components, or even pose a fire hazard. On the other hand, if the trace width is too wide, it can waste valuable board space, increase the cost of production, and potentially cause issues with signal integrity. Therefore, finding the optimal trace width is essential for ensuring the proper functioning of the PCB.

Factors Affecting Trace Width Calculation

Several factors need to be taken into account when calculating the trace width for a rigid PCB:

1. Current Carrying Capacity

The amount of current that will flow through the trace is one of the most critical factors. Higher currents require wider traces to minimize resistance and heat generation. The current carrying capacity of a trace is influenced by the material of the trace (usually copper), the thickness of the copper layer, and the ambient temperature.

2. Copper Thickness

The thickness of the copper layer on the PCB affects the trace's ability to carry current. Thicker copper layers can handle higher currents with less resistance. Common copper thicknesses for rigid PCBs include 1 oz (35 µm), 2 oz (70 µm), and 3 oz (105 µm).

3. Allowable Temperature Rise

The maximum temperature rise that the trace can withstand without causing damage to the PCB or its components is another important consideration. A lower allowable temperature rise will require a wider trace to dissipate heat more effectively.

4. PCB Material

The type of PCB material used can impact the heat dissipation properties of the trace. Different materials have different thermal conductivities, which affect how quickly heat can be transferred away from the trace.

5. Trace Length

Longer traces have higher resistance, which can lead to more significant voltage drops and heat generation. Therefore, longer traces may require wider widths to compensate for the increased resistance.

Methods for Calculating Trace Width

1. IPC-2221 Standard

The IPC-2221 standard is a widely recognized guideline for printed circuit board design. It provides formulas and tables for calculating the trace width based on the current, copper thickness, and allowable temperature rise. The formula for calculating the trace width (W) in inches is as follows:

[ W=\frac{I}{k\times(T_{max}-T_{amb})^b\times D^c} ]

Where:

(I) is the current in amperes
(k), (b), and (c) are constants based on the copper thickness and the type of PCB (internal or external layer)
(T_{max}) is the maximum allowable temperature of the trace
(T_{amb}) is the ambient temperature
(D) is the copper weight in ounces per square foot

The IPC-2221 standard also provides pre-calculated tables that can be used to quickly determine the trace width for different current levels and copper thicknesses.

2. Online Trace Width Calculators

There are many online trace width calculators available that simplify the calculation process. These calculators typically ask for input values such as the current, copper thickness, allowable temperature rise, and PCB material, and then provide the recommended trace width. Some popular online calculators include the Saturn PCB Design Calculator and the Eagle PCB Trace Width Calculator.

3. Rule of Thumb

A simple rule of thumb for calculating trace width is to use 10 mils (0.254 mm) of trace width for every 1 ampere of current for a 1 oz copper layer. However, this rule is only a rough estimate and may not be suitable for all applications, especially those with high currents or strict temperature requirements.

Example Calculation

Let's say we have a rigid PCB with a 1 oz copper layer, a maximum allowable temperature rise of 20°C, and a current of 2 amperes. Using the IPC-2221 standard or an online calculator, we can determine the appropriate trace width.

Assuming we use an online calculator and input the values:

Current ((I)) = 2 A
Copper thickness = 1 oz
Allowable temperature rise ((\Delta T)) = 20°C

The calculator may recommend a trace width of approximately 20 mils (0.508 mm).

Considerations for Different Types of Rigid PCBs

1. Single Side Rigid PCB

Single side rigid PCBs have components and traces on only one side of the board. When calculating the trace width for single side PCBs, it's important to consider the limited space available. You may need to optimize the trace layout to fit all the necessary traces while ensuring they have the appropriate width.

Single Side Rigid PCB double rigid (2)

2. Double Sided Rigid PCB

Double sided rigid PCBs have components and traces on both sides of the board, which provides more flexibility in trace routing. However, you still need to ensure that the traces on both sides have the correct width to handle the current. Additionally, you need to consider the vias that connect the traces on the two sides, as they can also affect the overall electrical performance.

3. Metal Core Rigid PCB

Metal core rigid PCBs have a metal core (usually aluminum or copper) that provides excellent heat dissipation properties. This allows for higher current densities and potentially narrower trace widths compared to traditional PCBs. When calculating the trace width for metal core PCBs, you can take advantage of the improved thermal performance to optimize the design.

Conclusion

Calculating the trace width for a rigid PCB is a complex process that requires careful consideration of multiple factors. By understanding the importance of trace width, the factors that affect it, and the methods for calculation, you can ensure that your PCB design is reliable, efficient, and safe.

As a rigid PCB supplier, we have the expertise and experience to help you with your trace width calculations and PCB design. If you have any questions or need assistance with your next PCB project, please feel free to contact us for a consultation. We look forward to working with you to create high-quality rigid PCBs that meet your specific requirements.