Track Width v/s Current Capacity: PCB Layout Tips for Power Routing

In the intricate art of PCB design, power routing is the unsung hero carrying the current that breathes life into circuits. While signal traces get all the attention for their impedance matching and noise control, power tracks quietly shoulder the responsibility of delivering current without frying themselves or the components they feed. But how wide should these tracks be? That's where the relationship between track width and current capacity comes into play.  If you think 0.1mm tracks are enough for 10A, you're just designing a PCB-shaped fuse. To know more about PCBs, see our ultimate guide on PCB manufacturing.

Why Track Width Matters in Power Routing

Power routing is all about efficiently delivering current while managing heat, voltage drops, and signal integrity. Get the track width wrong, and you might as well invite heat buildup, voltage sag, or, in extreme cases, a trace meltdown. And yes, melted traces may look cool in sci-fi movies, but in real life, they're just a design disaster.

IPC has developed standards relating the appropriate methodologies to test and calculate temperature rise in PCB traces for specific input currents. These standards are IPC-2221 and IPC-2152, and they both contain a large amount of information on these topics. Obviously, these standards are quite extensive and most designers do not have time to parse through all of the data to determine a trace width vs. current table.

What is Circuit Board Trace Resistance?

Every substance in nature has a certain resistance to current, so both conductors and insulators will produce varying degrees of obstruction to the passage of current. In the fabrication of PCB tracks, copper is a common conductor material, but due to the composition and properties of copper itself, it will still cause resistance to the current, which is PCB track resistance.

Resistance in a circuit, measured in ohms, indicates the opposition to current flow. In PCBs, trace resistance is crucial for assessing signal transmission losses and power handling. Resistance depends on the trace's thickness, width, and length and must remain low to allow efficient current flow. High resistance leads to power loss, overheating, and reduced conductivity. To minimize resistance, designers can increase trace width, use thicker copper layers, or expand the copper area. These methods effectively manage heat and improve PCB performance, especially in thick copper designs.

Factors Affecting Current Capacity

When a single-layer trace cannot support the required current, the trace can be extended across multiple layers via stitching to link these layers, thereby enhancing the current-carrying capacity when the trace thickness remains constant across the layers. While using thicker copper traces of 2 oz or 3 oz increases the current capacity due to reduced resistivity, it also escalates the processing costs. The current-carrying capacity of a PCB trace isn’t just about its width. Several factors influence it and here is the proper breakdown of all the factors:

Copper Thickness: Copper thickness, measured in ounces per square foot, defines how much copper is laminated onto the PCB.

• 1 oz/ft² (35 µm): Standard for most PCBs.
• 2 oz/ft² (70 µm): Used for higher current applications.
• 3 oz/ft² or more: For power-hungry designs.

Ambient and Operating Temperature: Higher temperatures reduce the current capacity of traces. Ambient conditions and heat from components both matter. Design for the worst-case temperature scenario because your PCB won’t operate in a utopia of air-conditioned bliss.

Trace Length: Longer traces create higher resistance, leading to more heat and voltage drop. Keep power tracks as short as possible. If your PCB looks like a squiggly maze, start over.

For high-power applications like electric automobiles and inverters, where currents exceed 100A, traditional copper traces are often inadequate. In such scenarios, copper bus bars, which are thicker than standard traces, can be employed. These busbars are soldered onto the PCB’s pads and are capable of handling high currents without overheating.

Calculating Track Width for Current Capacity

You can use an online calculator to determine various trace components, such as trace temperature, maximum current, resistance, voltage drop and power dissipation. You can better understand your calculator results by becoming familiar with the following formulas.

Use online trace width calculators or tools in PCB design software. Why memorize math when you can outsource it to a calculator?

7 Tips to Optimize Power Routing in PCB designs:

Many things drive trace characteristics in the PCB world so make sure to follow best practices when routing your next PCB and you’ll find that balance between PCB fab cost, circuit density, and overall performance.

1. Design for Adequate Trace Width: Ensure traces can handle the expected current without overheating. Example: For a 2A load on a 1 oz copper PCB, a width of approximately 1mm (40 mils) is typical for external layers.

2. Use Wider Traces for Higher Currents: When in doubt, make the trace wider. A wider trace reduces resistance, improves current-carrying capacity and minimizes voltage drops.

3. Employ Power Planes: Power planes are large copper areas that distribute current efficiently. For designs with high power consumption, use dedicated power and ground planes instead of relying on traces.

4. Thermal Management: Heat dissipation is critical for high-current traces. Add thermal reliefs or vias to spread heat across layers. Use thicker copper for better heat conduction.

5. Minimize Voltage Drop: Voltage drop along a trace can lead to component malfunctions, especially in low-voltage designs. Keep traces short, wide, and direct to reduce resistance.

6. Avoid Sharp Bends: Sharp angles create hotspots and increase inductance. Use 45° or curved bends for power traces.

7. Parallel Traces for High Currents: For extremely high currents, divide the load between parallel traces or layers. More than one via’s is recommended when routing power trace over different layer. The current carrying limit for one via is around 500mA.

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Common Mistakes and How to Avoid Them

1. Ignoring Thermal Effects: Using thin traces on inner layers for high-current paths. Opt for wider traces or thermal vias to manage heat better and route the power traces on the top or bottom layer.

2. Overcrowded Layouts: Routing power traces through congested areas is not generally recommended. Plan routing early and prioritize power paths.

3. Relying Solely on Outer Layers: Overloading outer traces and neglecting inner-layer routing. Distribute current through multiple layers when needed, route the high current through main layers and distribute the signal/digital current over different layers.

Why To Remove Solder Mask In Power Routing:

Removing the welding mask allows the track to carry a larger capacity of current, which is an economical method. When the solder mask is removed from the board, the copper material at the bottom is exposed and can be repaired with another solder material to increase the thickness of the copper and reduce the total resistance in the PCB components. In this case, the PCB can adapt to the higher power bearing capacity without increasing the PCB trace width.

Conclusion

Power routing isn't just about connecting components; it's about managing currents, minimizing heat, and ensuring reliability. By balancing track width, copper thickness, and thermal management, you can design PCBs that deliver power without breaking a sweat or the traces. See the full design process of a PCB keeping tracks calculations in mind.

• Trace width and copper layer thickness are crucial for managing the current-carrying capacity of PCBs, with standards provided by IPC-2152 and IPC-2221.

• Location of a trace external or internal, significantly affects its heat dissipation capabilities, influencing the choice of trace for safe temperature management.

• Copper thickness is often measured in ounces per square foot, correlating directly to weight and providing a straightforward metric for determining necessary trace thickness.

Remember: a little extra effort in the design phase can save you from melted traces and fried components later. So, go wide, go cool, and avoid the fiery disasters of underestimating your current-carrying needs!