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Several issues need to be addressed when designing PCBs, including material selection, trace length, PCB trace width and impedance matching. The copper pathways on a Printed Circuit Board, also known as traces, serve as electrical connectors.
The width of a PCB trace determines how much current it can safely carry without causing an excessive rise in temperature. This article explains how to calculate PCB trace width in reference to the IPC-2152 standard. It also compares internal and external layers and provides real-world design examples and details of common issues.
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Why PCB Trace Width Matters?
PCB trace width refers to the physical width of the copper conductor etched onto a printed circuit board. Together with copper foil thickness, trace width determines the trace’s cross-sectional area, which directly affects resistance, current-carrying capacity and heat dissipation performance.
Trace width influences several important electrical and thermal parameters of a PCB:
- Current carrying capacity
- Temperature rise
- Voltage drop
- Manufacturing yield
PCB Trace Width vs Current (ΔT = 10°C, External Layer)
This PCB trace width vs current table provides a quick reference for common copper thicknesses.
| Trace Width (mm) | 35 µm Copper (≈1 oz) | 50 µm Copper | 70 µm Copper |
| 0.2 mm | ~0.55 A | ~0.7 A | ~0.9 A |
| 0.3 mm | ~0.8 A | ~1.1 A | ~1.3 A |
| 0.5 mm | ~1.35 A | ~1.7 A | ~2.0 A |
| 0.8 mm | ~2.0 A | ~2.4 A | ~2.8 A |
| 1.0 mm | ~2.3 A | ~2.6 A | ~3.2 A |
| 1.5 mm | ~3.2 A | ~3.5 A | ~4.2 A |
| 2.0 mm | ~4.0 A | ~4.3 A | ~5.1 A |
The broader traces are more effective at dissipating heat, and narrower traces provide more resistance to the passage of electricity, causing an increase in temperature and a decrease in voltage. This ensures that the electrical operation of the printed circuit board is within the established parameters of safe operation and performance.
How to Calculate PCB Trace Width
Calculating the appropriate trace width ensures that your circuit board can handle the required current without overheating or sustaining damage. For hobbyist projects or basic designs, you can use online calculators or standard guidelines like IPC-2221 to determine trace width.
PCB Trace Width Quick Design Rules (IPC-2152)
Before diving into calculations, the following quick rules are often used during early layout:
- 1 oz copper, external layer: ~20 mil per 1 A (≈10°C temperature rise)
- Internal layers: Require 50–70% wider traces than external layers
- Higher ambient temperature: Increase trace width or copper thickness
- Long power traces: Check voltage drop in addition to temperature rise
PCB Trace Width Calculation Formulas
To calculate PCB trace width, the following parameters must be defined:
- Current (A)
- Copper thickness (oz/ft2 or um)
- Limit of temperature increase (degC)
- Trace position (external or internal layer)
According to the research by Ghosh (2021), the formula that is most in use is based on IPC standards:
I = k × (ΔT)^0.44 × A^0.725
Where:
I = current (A)
ΔT = allowable temperature rise (°C)
A = trace cross-sectional area (mil²)
k = 0.048 (external layers), 0.024 (internal layers)
The two examples of External and internal Layer traces are shown below.
Example 1: External Layer Trace
- Current: 2 A
- Copper thickness: 1 oz
- Allowable temperature rise: 10°C
- Layer: External
Result: Based on IPC-2152 charts or calculators, a 2 A external-layer trace typically requires a trace width of approximately 40–45 mil.
Example 2: Internal Layer Trace
At the same parameters, though on an internal layer:
Result: Internal traces are surrounded by dielectric material and dissipate heat less effectively. The required trace width increases to approximately 65–70 mil to maintain the same temperature rise.
Factors Affecting Trace Width
The following variables need to be understood to develop safe, efficient and manufacturable printed circuit board designs.
- Current Carrying Capacity: The important fact in the calculation of trace width is the amount of current that the trace is capable of carrying. The increased current density causes longer traces to be used to eliminate the heating resistive nature that may cause circuit breakage.
The IPC-2152 specifications show that the current-carrying capacity increases exponentially with trace width – doubling the trace width does not automatically increase the current-carrying capacity, be it thermal or electromagnetic. At the design stage, the peak current load and the continuous current load should be taken into consideration, such that the delivery of power traces can be made.
- Copper Thickness and Weight: Copper thickness in ounces per square foot (oz/ft2) may have a major impact on the current capacity. Conventional PCB is 1 oz copper (about 1.4 mils or 35 microns), but 0.5 oz, 2 oz and 4 oz copper could be used in unusual applications. Copper is relatively harder to etch than finer and more expensive and difficult to make due to decreasing trace width and weight increases.
- Ambient Conditions and Temperature Rise: The accepted range above the ambient temperature should be known. In IPC-2152, different cases of temperature rise curves exist, where the majority lies between 10 °C and 30 °C. Situations where high ambient temperatures are found, such as industrial equipment or operation under the hood, require a smaller increase in temperature limit. The cooling pathways, airflow conditions, and heating elements should be addressed in thermal models.
- Location Layer Internal vs External: PCB stackup is very significant to thermal performance. The advantage of the external layer traces is that they are directly cooled by the air and can have up to 50-100 percent more current carried than internal traces of the same size, as they are enclosed with a dielectric insulating material.
Internal power planes need to have additional traces or vias to the adjacent stages to provide sufficient current distribution. Multi-layer boards require thermal analysis that is intensive depending on the layout and is founded on IPC-2152.
- Trace Length and Voltage Drop: Traces with longer lengths will have a resistive voltage drop, making it less efficient, which can cause malfunctions. The voltage drop for the signals and power traces can be calculated using the formula (V drop = R × I), where resistance can be measured by the width of the copper trace and the length of the trace.
Common Problems in PCB Trace
- Designing PCB Trace Width Based on Current Only: One of the mistakes PCB designers commit is merely looking at the current value and not considering the increase in temperature. The existing current can be at a reasonable level, but heat may spoil traces over time and cause failures. The traces can damage copper when it is incredibly hot, and when there is too much heat.
- Using the Same Solder Mask Rules for Power and Signal Traces: Signal traces conduct little current, and the power traces are to carry loads much higher. In this type of case, since they are being handled in the same manner, the routing of power is usually undersized.
- Insufficient Trace Width on Internal PCB Layers: The designers must remember that the internal trace layers are not identical to the external ones. Dielectric traces can be found inside, and they are not supposed to be disclosed. To cool this, they will have to be wider than external traces when intended to carry larger quantities of current.
- Trace Width Reduction Caused by PCB Etching: Impacts of production are at times overlooked. In the etching process, narrow traces can be reduced slightly in width; hence, this is an issue. This can create a risky design in comparison to what is evident during the designing stage of the view of the layout.
- Excessive Trace Length Causing Voltage Drop: The longer the length of the trace, the higher the resistance, and the thinner the designed trace, the higher the voltage drop. This can provide a diversion to the current and could lead to unexplained failures in the power distribution.
Conclusion
Selecting the correct width for PCB traces is essential for ensuring the electrical reliability, thermal safety and long-term performance of printed circuit boards. Well-designed traces must strike a balance between current-carrying capacity, permissible temperature rise, copper thickness and layer position, rather than relying only on current values.
Successful PCB design utilizes IPC-2152 standard compliance to balance DFM and thermal performance. Adopting these best practices in the initial design phase helps to reduce the need for costly modifications, improve yield rates and ensure consistent performance of the PCB throughout the product’s entire lifecycle.
FAQs
A1: Yes, it is possible it can be due to several reasons such as high ambient temperature, insufficient trace width, or low copper trace.
A2: For less critical designs, calculators based on standards such as IPC-2152 can help for preliminary designs, but for more critical designs, it is better to seek a specialist to evaluate the design.
A3: An increase in ambient temperature will further reduce the thermal margin so it calls for more border traces to help with heat dissipation.
A4: Not necessarily. With some designs, it can lead to a high level of congestion, and with high frequency designs it can also lead to impedance issues.
It can. A solder mask increases heat retention and this could mean that the outer layers need a wider trace.
Reference:
- Ghosh, P. (2021, July 20). IPC-2221 Standards in PCB Design. Sierra Circuits. https://www.protoexpress.com/blog/ipc-2221-circuit-board-design/
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