PCB Trace Resistance Calculator

Calculate the electrical resistance of PCB traces based on dimensions, material, and temperature factors.

Calculate Your PCB Trace Resistance Calculator

Trace Length

Trace Width

Copper Weight

Temperature (°C)

Understanding PCB Trace Resistance

Trace resistance is an important consideration in PCB design that affects power distribution, signal integrity, and thermal management. Understanding how to calculate and manage trace resistance helps engineers create more reliable and efficient circuit boards.

Factors Affecting Trace Resistance

Trace Length

Resistance is directly proportional to trace length. Longer traces have higher resistance, so critical signal paths and power distribution traces should be kept as short as possible.

Trace Width

Resistance is inversely proportional to trace width. Wider traces have lower resistance, which is particularly important for power distribution networks where voltage drop must be minimized.

Copper Thickness

Resistance is inversely proportional to copper thickness. PCB copper thickness is typically measured in ounces per square foot (oz/ft²), where 1 oz/ft² equals approximately 35 μm thickness. Heavier copper reduces resistance but increases manufacturing costs.

Temperature

The resistance of copper increases with temperature at a rate of approximately 0.393% per °C. This means that a trace at 100°C will have about 31% higher resistance than the same trace at 25°C, which can be significant in high-power or high-temperature applications.

The Mathematics of Trace Resistance

The resistance of a PCB trace can be calculated using the resistivity formula:

R = ρ × L / (W × T)

Where:

R is the resistance in ohms
ρ (rho) is the resistivity of copper (1.68 × 10⁻⁸ Ω⋅m at 20°C)
L is the trace length
W is the trace width
T is the trace thickness

Sheet Resistance

In PCB design, it's often convenient to work with sheet resistance, which is the resistance of a square sheet of conducting material of uniform thickness. For copper traces, sheet resistance (Rs) is calculated as:

Rs = ρ / T

For standard 1 oz/ft² copper (35 μm thickness), the sheet resistance is approximately 0.5 mΩ/square.

Once you know the sheet resistance, you can calculate the trace resistance by multiplying it by the number of squares in the trace:

R = Rs × (L / W)

Where L/W gives the number of "squares" in the trace.

Practical Implications

Voltage Drop

Voltage drop across a trace can be calculated using Ohm's Law (V = I × R). Excessive voltage drop can cause:

Unreliable operation of ICs due to supply voltage falling below minimum requirements
Reduced noise margins in digital circuits
Inaccurate measurements in analog circuits

Power Dissipation

Power dissipated in a trace is calculated as P = I² × R. This power appears as heat, which can:

Raise the temperature of the PCB, potentially affecting nearby components
In extreme cases, cause PCB delamination or damage
Reduce overall system efficiency

Design Recommendations

For Power Distribution: Design traces wide enough to keep voltage drop under 2% of the supply voltage.
For High-Current Paths: Consider using copper pours, multiple layers connected with vias, or thicker copper weights.
For Sensitive Signals: Account for trace resistance in impedance calculations and signal integrity analyses.
For High-Precision Analog Circuits: Use 4-wire (Kelvin) connections for critical measurements to eliminate the effects of trace resistance.
For Temperature-Critical Applications: Account for the increase in trace resistance as temperature rises.

Related Calculators

PCB Trace Current Calculator

Calculate maximum current capacity for PCB traces based on width, thickness, and material.

PCB Trace Width Calculator

Calculate optimal PCB trace width based on current requirements and temperature rise.

PCB Impedance Calculator

Calculate the characteristic impedance of PCB traces based on dimensions, materials, and layout.

3D Printing Cost Calculator

Calculate the cost of 3D printing materials, time, and electricity.

Frequently Asked Questions

PCB trace resistance is calculated using the formula: R = ρ × L / (W × T), where R is resistance in ohms, ρ (rho) is the resistivity of the conductor material (usually copper), L is the trace length, W is the trace width, and T is the trace thickness. For copper at room temperature, ρ is approximately 1.68 × 10^-8 ohm-meters. In PCB calculations, this is often simplified to use sheet resistance based on copper weight (oz/ft²).

Sheet resistance is the resistance of a square sheet of conducting material with uniform thickness. For PCB copper, it's typically measured in milliohms per square (mΩ/sq). For standard 1 oz/ft² copper (35 μm thickness), the sheet resistance is approximately 0.5 mΩ/sq. The total resistance of a trace is the sheet resistance multiplied by the number of squares (length divided by width).

Temperature has a significant effect on trace resistance. Copper's resistivity increases with temperature at a rate of approximately 0.393% per °C. This means that a copper trace at 100°C has about 31% higher resistance than the same trace at 25°C. This temperature coefficient must be considered in designs where significant temperature fluctuations are expected or where traces carry enough current to self-heat.

Yes, copper weight directly affects trace resistance. The standard unit for PCB copper thickness is ounces per square foot (oz/ft²), where 1 oz/ft² equals approximately 35 μm thickness. Doubling the copper weight (e.g., from 1 oz to 2 oz) halves the trace resistance for the same width and length. Common copper weights are 0.5 oz, 1 oz, 2 oz, and 3 oz.

Trace resistance is important for several reasons: 1) Voltage drop across traces can affect circuit performance, especially in power distribution networks, 2) Resistive losses generate heat that can impact thermal management, 3) In high-speed digital circuits, resistance contributes to signal integrity issues, 4) In precision analog circuits, even small resistances can cause measurement errors, and 5) For high-current applications, resistance directly affects power efficiency.

To reduce PCB trace resistance: 1) Increase trace width, 2) Use heavier copper weights (2 oz or 3 oz instead of 1 oz), 3) Keep traces as short as possible, 4) Consider using multiple layers connected with vias for critical power paths, 5) For extreme cases, use copper pour areas instead of traces, or 6) Add additional copper through plating processes. Each method has trade-offs in terms of board space, manufacturing cost, and design flexibility.

Share This Calculator

Found this calculator helpful? Share it with your friends and colleagues!