It is crucial to understand the resistance of PCB traces. This directly affects power supply stability, heat generation, and even signal quality.
However, many engineers have found that it is difficult to accurately measure the resistance of fine traces using conventional methods.
Why Are Conventional Methods Inaccurate?
Limitations of Multimeters
The lowest range on a standard multimeter is typically 0.1 Ω (100 milliohms).
However, a trace that is 10 cm long and 0.2 mm wide may have a resistance as low as 50 milliohms.
In this case, taking a measurement is like trying to weigh a feather on a bathroom scale—it’s simply impossible to obtain an accurate reading.
Challenges of the Voltage Drop Method
Although resistance can be calculated using the formula
Resistance = Voltage ÷ Current
in practice:
- At low currents (such as 10 mA), a voltage drop of 0.05 V requires precision instruments to measure accurately
- High currents may burn out the circuit
- Interference signals can cause fluctuations in the measurement results
The Most Practical Calculation Method: The Resistive Block Method
Core Principle: Break the trace down into multiple “blocks,” each with a fixed resistance.
Steps:
1. Counting Squares: Trace Length ÷ Width = Number of Squares
Example: A trace 200 mm long and 0.2 mm wide → 200/0.2 = 1,000 squares
2. Look up the resistance value per square:
- 1 ounce copper thickness (common thickness): ≈0.5 mΩ per square
- 2 ounce copper thickness: ≈0.25 mΩ per square
3. Calculate total resistance: Number of squares × Resistance per square
Example: 1,000 squares × 0.5 mΩ = 500 mΩ (0.5 Ω)
Why does this work?
The resistance of copper foil is directly proportional to its length and inversely proportional to its width.
Just as a water pipe becomes thinner the longer it is, the greater the resistance to water flow.
Adjustments in Actual Design
Effect of Copper Thickness
1 oz copper thickness ≈ 0.035 mm; 2 oz is double that
Doubling the copper thickness halves the resistance
Example: Changing the power trace from 1 oz to 2 oz directly reduces resistance by 50%
Selection of Trace Width
0.1mm wide trace → Resistance per cm ≈ 50 mΩ
0.5mm wide trace → Resistance per cm ≈ 10 mΩ
Design Tip: For high-current circuits (such as power supplies), widen the traces as much as possible to significantly reduce heat generation.
Effect of Temperature
The resistance of copper increases with rising temperature, increasing by approximately 0.4% for every 1°C rise.
Example: A 0.5Ω trace, after a 50°C temperature rise → 0.5Ω × (1+0.004×50) = 0.6Ω
Quick Estimation Tools
Online Calculators
Recommended tool: Saturn PCB Toolkit (free)
Instructions: Enter the length, width, and copper thickness to display the resistance value immediately.
Design Software Assistance
Software such as Altium Designer and KiCad includes built-in impedance calculation features that can automatically estimate trace resistance.
Special Considerations for High-Frequency Signals
When the signal frequency exceeds 1 MHz, the current becomes concentrated on the surface of the conductor (skin effect), increasing effective resistance. In this case:
Resistance ≈ DC resistance × (trace thickness ÷ skin depth)
Example: For a 1 MHz signal on 1-ounce copper foil, the skin depth is approximately 0.066 mm, and the resistance will be twice that of a DC signal.
Practical Advice for Engineers
Design Phase
Trace width ≥ current (A) × 0.3 mm (1 oz copper)
Allow a 20% margin to prevent excessive resistance due to temperature rise
Measurement Techniques
Use the four-wire method (to eliminate the effect of contact resistance)
Apply a constant current of 1A to the trace and measure the voltage across its ends (1mV = 1 milliohm)
Common Misconceptions
Assuming that “shorter traces always have lower resistance” → Thin, short traces may actually have higher resistance than longer, wider ones Ignoring the resistance of pads and vias → A single via can add 0.5–2 milliohms
Mastering the calculation of PCB trace resistance can effectively prevent issues such as insufficient power supply and abnormal heating.
For everyday design, simply remember these three key points:
1. Count the squares: Length ÷ Width equals the number of squares
2. Look up the table value: 0.5 mΩ per square for 1 oz copper
3. Allow for margin: Account for temperature rise and manufacturing tolerances
Conclusion
In practical PCB engineering, trace resistance is not a secondary parameter—it is a first-order design constraint that directly governs power integrity, thermal behavior, and signal reliability.
Conventional measurement approaches often fail at milliohm levels, making analytical estimation not just convenient but necessary.
The resistive block (square-counting) method provides a robust, scalable, and design-friendly framework.
By reducing geometry to a normalized unit (squares) and combining it with known material properties, engineers can quickly obtain resistance values with sufficient accuracy for most applications.
More importantly, this method aligns directly with layout decisions—trace width, copper thickness, and routing length—making it highly actionable during design.
However, real-world performance is never purely theoretical. Temperature rise, manufacturing tolerances, vias, and high-frequency effects such as skin effect all introduce deviations that must be anticipated.
Ignoring these factors leads to underestimation of resistance, which in turn risks voltage drops, localized heating, and long-term reliability issues.
A disciplined approach is therefore essential: estimate early using the square method, validate critical paths with four-wire measurement, and incorporate conservative margins.
When combined with modern EDA tools and calculators, this workflow ensures both efficiency and accuracy.
Ultimately, mastering PCB trace resistance is less about complex mathematics and more about consistent engineering judgment—understanding how geometry, material, and operating conditions interact, and designing with those interactions in mind.
