Introduction
PCB trace is the copper trace on the PCB board that connects different electronic components. A trace that is too narrow for the current it carries can generate excessive heat, cause an unwanted voltage drop, reduce reliability, and, in severe cases, lead to PCB damage. On the other hand, a thick trace can increase PCB size, routing complexity, and manufacturing cost.
Therefore, selecting the correct PCB trace width is more than simply choosing a value from a standard trace width chart. The required width depends on several factors, including the current flowing through the trace, copper thickness, the signal being routed, allowable temperature rise, and trace location.
Industry standards such as IPC-2221 and IPC-2152 provide important guidance for determining conductor dimensions and current-carrying capability. IPC-2152, in particular, provides a more comprehensive approach to understanding the thermal behavior of PCB conductors and the factors that influence their temperature rise.
This technical tutorial will cover a comprehensive guide on how to determine PCB trace width, how current and copper thickness affect trace capacity, the differences between internal and external PCB traces, and how IPC standards can be applied to practical PCB designs.
What is PCB Trace Width?
PCB trace width is the physical width of the copper conductor used to electrically connect components, vias, and pads on PCB boards. It is normally specified in mils (thousandths of an inch) or millimeters (mm). For example, a PCB design may use a 6 mil (0.152 mm) signal trace for a low-current connection and a much wider 50 mil (1.27 mm) trace for a power connection.
The width of a trace is one of the factors that determines how much electrical current it can safely carry. A wider trace provides a larger cross-sectional area for current flow, which generally reduces electrical resistance and heat generation.
Trace Width vs. Copper Thickness
Before we dive into the technicalities, it is important to know the difference between trace width and copper thickness. Trace width refers to the horizontal dimension of the copper conductor on the PCB surface, while copper thickness refers to the vertical thickness of that copper layer.
However, both copper thickness and trace width cumulatively determine the cross-sectional area of the conductor.
Conductor Cross-Sectional Area = Trace Width × Copper Thickness
For example, increasing a trace from 10 mil to 20 mil doubles its width. Similarly, increasing the copper from 1 oz to 2 oz increases the copper thickness and therefore increases the available cross-sectional area. This is why a designer can often achieve a higher current-carrying capability either by making a trace wider, using thicker copper, or combining both approaches.
Why Trace Width Matters
Trace width directly affects several important PCB design parameters:
| Factor | Effect of Wider Trace |
|---|---|
| Current-Carrying Capacity | Wider conductors can generally carry more current. |
| Temperature Rise | A narrow trace carrying excessive current can become significantly hotter. |
| Resistance | Wider traces have lower resistance for the same copper thickness and length. |
| Voltage Drop | Lower trace resistance reduces voltage loss along power paths. |
| Power Dissipation | Trace resistance causes electrical power to be converted into heat. |
| PCB Reliability | Excessive heating can damage the PCB or reduce long-term reliability. |
PCB Trace Width and Current Carrying Capacity
One cannot randomly choose the PCB trace width for their PCB design. Because different components, signals, and applications require choosing different trace widths. For instance, for a 12V/2A power, the selected trace width should be thick. On the other hand, routing an ESP32 GPIO signal will only require a trace width of 8 mil. Therefore, trace width is entirely dependent on the type of signal being routed.
When current flows through a copper trace, the trace has some electrical resistance. This resistance causes power to be dissipated as heat, and the amount of heat generated depends on the current, resistance, and physical dimensions of the conductor. As the current increases, the amount of heat generated by a PCB trace can increase significantly. Therefore, a trace designed for a few hundred milliamps may not be suitable for several amperes of current. Increasing the trace width provides a larger cross-sectional area for current flow, which reduces resistance and helps distribute the generated heat over a larger area.
How Current Affects PCB Trace Width
The amount of current a PCB trace can safely carry is not determined by trace width alone. Several parameters must be considered when determining the appropriate conductor size:
| Design Factor | Description |
|---|---|
| Maximum Current | The continuous or peak current that the trace must carry. |
| Trace Width | A wider conductor provides a greater cross-sectional area. |
| Copper Thickness | Thicker copper increases the conductor’s cross-sectional area without necessarily increasing its width. |
| Allowable Temperature Rise | The maximum temperature increase permitted during operation. |
| Trace Location | External traces can dissipate heat differently from traces located on internal PCB layers. |
| Trace Length | Longer traces have greater resistance and can produce greater voltage drop and power dissipation. |
| PCB Construction | Board thickness, nearby copper areas, planes, and other construction characteristics can influence thermal performance. |
IPC Standards for PCB Trace Width
In this section, we will discuss the IPC standards for selecting the right trace width for the PCB board, instead of selecting the generic trace width rule. There are two particular IPC standards for trace width guide known as IPC-2221 and IPC-2152.
Generic Standard for PCB Design: IPC-2221
IPC-2221 is a generic PCB design standard that includes requirements related to conductor width and thickness. IPC-2221 is a generic standard covering a broad range of printed-board design requirements. Its scope includes conductor characteristics such as conductor width and thickness, along with other aspects of PCB construction and layout.
For trace-width design, IPC-2221 provides a traditional foundation for determining conductor dimensions based on electrical and thermal requirements. However, designers should recognize that it is a generic design standard, rather than a dedicated thermal model for every PCB construction. IPC-2221 is particularly useful when establishing general PCB design requirements, but current-carrying capacity should be evaluated with the more specific guidance provided by IPC-2152.
Standard for Determining Current-Carrying Capacity: IPC-2152
IPC-2152 is specifically focused on determining the current-carrying capacity of conductors on printed boards. The standard describes the relationship between current, conductor size, and temperature, and provides guidance for selecting conductor dimensions based on the required current and acceptable conductor temperature rise.
Instead of considering trace width as an isolated parameter, the standard helps designers evaluate the thermal behavior of the conductor and the conditions surrounding it.
IPC-2221 vs. IPC-2152
Although both standards are relevant to PCB trace design, they serve different purposes, which are distinguished in the table below.
| Feature | IPC-2221 | IPC-2152 |
|---|---|---|
| Primary Purpose | Generic PCB design requirements | Current-carrying capacity |
| Trace/Conductor Width | Covered | Used in current-capacity evaluation |
| Conductor Thickness | Covered | Important design parameter |
| Temperature Rise | Considered | Central to the methodology |
| Current Capacity | General design consideration | Primary focus |
| Application | Broad PCB design | Thermal/current conductor sizing |
PCB Trace Width Calculation
Once you understand both the IPC-2221 and IPC-2152 standards for trace width guidance with respect to current, temperature rise, and thermal performance. It is now time to understand and learn how to calculate the trace width for your design. Therefore, in this section, you will learn how to calculate the trace width.
Calculating the correct PCB trace width is essential when a copper trace must carry a specified amount of current without exceeding the allowable temperature rise. The calculation is not based on current alone; it also depends on the copper cross-sectional area, copper thickness, PCB layer, and the maximum temperature rise permitted by the design.
Basic PCB Trace Width Calculation
To determine the trace width required, first determine the maximum current that the trace must carry. Next, define the maximum temperature rise that can be tolerated. The required copper cross-sectional area can then be determined and converted into a physical trace width based on the available copper thickness.
As per IPC-2221 standard, the required conductor cross-sectional area can be calculated using:
Where:
- A = required copper cross-sectional area in mil²
- I = current in amperes
- ΔT = allowable temperature rise in °C
- k = constant based on conductor location
- k = 0.048 for an external conductor
- k = 0.024 for an internal conductor
Converting Copper Cross-Section to Trace Width
A PCB trace can be approximated as a rectangular copper conductor. Therefore:
Where:
- A = copper cross-sectional area
- W = trace width
- T = copper thickness
For standard PCB copper, 1 oz copper is approximately 35 µm (1.378 mil) thick. Therefore, once the required copper area is known, the required physical trace width can be calculated from the actual finished copper thickness.
How Copper Thickness Changes the Required Width
Copper thickness has a major effect on the physical width required for a given current. Increasing copper thickness increases the conductor’s cross-sectional area, allowing a designer to carry the same current with a narrower trace.
PCB Trace Width Calculator: Practical Design Method
In this section, we will use an online available trace width calculator to determine the trace width. Using an online calculator can make the design process faster and easier.
WELLER PCB trace width calculator is an online and freely available calculator, and it uses the IPC-2221 standard to estimate the required copper trace width.
Step 1: Open the PCB Trace Width Calculator
Open the WELLER PCB Trace Width Calculator and locate the input fields for the trace design. The calculator provides separate results for internal layers and external layers in air, which is important because the thermal behavior of an internal trace differs from that of a trace exposed on the PCB surface.
Step 2: Enter the Required Current
The first important parameter is Current (I), specified in amperes (A). Enter the maximum current that the PCB trace is expected to carry. For a power trace, use the maximum expected continuous operating current rather than simply using the nominal load current. For example, if a 12 V power rail normally carries 1.5 A but the load can continuously reach 2 A, use 2 A as the design current.
Step 3: Enter the Ambient Temperature
The next input is Ambient Temperature, specified in °C. Ambient temperature represents the approximate temperature surrounding the PCB during operation. For a simple example, we can use ambient temperature of 25°C. Therefore, if the ambient temperature is 25°C and the allowable temperature rise is 10°C, the approximate trace temperature under the specified conditions would be:
Step 4: Select the Copper Thickness
The next parameter is Thickness (t), specified in oz/ft² . A common PCB design uses 1 oz copper, which corresponds to approximately 35 µm of copper thickness. Other common choices include 0.5 oz, 2 oz, and 3 oz copper. For example 1oz copper thickness.
Step 5: Enter the Trace Length
The WELLER calculator also includes Trace Length, specified in inches. For understanding purpose, we take the trace length to be 4 inches. Trace length does not directly determine the minimum thermal trace width in the IPC-2221 calculation. However, it is important when calculating the electrical performance of the trace.
Step 6: Set the Allowable Temperature Rise
The next parameter is Temperature Rise (TRise). This specifies how much hotter the trace is allowed to become compared with the surrounding ambient temperature. For example allowable temperature rise is 10°C.
Step 7: Read the Calculated Trace Width
After entering the required parameters, the WELLER calculator provides the Required Trace Width (W) separately for internal layers and external layers in air.
PCB Trace Width for Power vs Signal Traces
Not all PCB traces need to have the same width. We have already discussed the trace width IPC standards and its calculation method. Now, there are different types of traces commonly used in PCB design, known as power traces, and signal traces. Power traces are primarily concerned with current-carrying capacity, temperature rise, voltage drop, and power dissipation. Signal traces, on the other hand, are usually governed by signal integrity, impedance, noise, crosstalk, and timing requirements rather than current-carrying capacity.
Power Traces
Power traces distribute electrical power between the power source, regulators, ICs, connectors, and loads.
- Battery power connections
- DC input and output rails
- Voltage regulator paths
- Motor-driver power connections
- LED power connections
- Heater connections
- High-current ground returns
- Battery charging paths
The primary concern with a power trace is its ability to carry the required current without excessive temperature rise or voltage drop.
Signal Traces
Signal traces are used to transfer information rather than significant electrical power. Common examples include:
- UART
- I²C
- SPI
- CAN
- USB
- Ethernet
- Analog sensor signals
- Clock signals
Many of these traces carry relatively little current, so their width is usually not determined by the same current-capacity calculation used for power traces.
Power Traces vs. Signal Traces
The key differences between the power traces and the signal traces are shown in the table below for quick understanding.
| Design Consideration | Power Traces | Signal Traces |
|---|---|---|
| Primary Concern | Current and heating | Signal integrity |
| Typical Current | Medium to high | Low |
| Trace Width | Often wider | Often narrower |
| Temperature Rise | Important | Usually less critical |
| Voltage Drop | Important | Usually less critical |
| Resistance | Important | Can be important for sensitive signals |
| Impedance Control | Usually not required | Important for high-speed signals |
| Crosstalk | Secondary | Can be critical |
| Return Path | Important | Very important |
| Copper Pours/Planes | Frequently useful | Often used for return paths |
| IPC Current Calculation | Often applicable | Usually unnecessary for low-current signals |
Conclusion
To sum up, choosing the correct PCB trace width is essential for building a reliable and thermally safe printed circuit board. Trace width should not be selected based on a single standard value because the required width depends on current, copper thickness, allowable temperature rise, trace location, trace length, and PCB construction.
The IPC-2221 provides a widely used method for calculating trace width based on current and temperature rise, while
provides more comprehensive guidance for understanding the thermal behavior and current-carrying capacity of PCB conductors. For quick calculations, Most trace width calculators can be used for PCB design.
Frequently Asked Questions (FAQ)
How do I calculate PCB trace width?
PCB trace width can be calculated by considering the required current, allowable temperature rise, copper thickness, and whether the conductor is located on an external or internal PCB layer.
What is the standard PCB trace width?
There is no single standard PCB trace width that applies to every design. Low-current signal traces may be relatively narrow, while high-current power traces require greater width or thicker copper
What PCB trace width should I use for 1 A?
The required width for a 1 A trace depends on copper thickness, allowable temperature rise, and whether the trace is on an external or internal layer. For example, a 1 oz external trace designed for a low temperature rise will generally require a different width than a 2 oz internal-layer trace.
Is IPC-2221 still used for PCB trace width?
Yes. IPC-2221 remains an important generic PCB design standard and provides an empirical method commonly used for conductor sizing.
Should power traces be wider than signal traces?
yes, when the power trace carries significantly more current. Power traces are primarily sized according to current-carrying capacity, temperature rise, resistance, and voltage drop.
How much current can a 10 mil PCB trace carry?
There is no universal current rating for a 10 mil trace. Its current-carrying capacity depends on copper thickness, allowable temperature rise, whether it is an internal or external trace, and the surrounding PCB construction.
