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PCB Trace Resistance Calculator

Calculate PCB copper trace DC resistance from width, length, thickness, and temperature. Get sheet resistance and temp coefficient. Free, instant results.

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Formula

R=ρ(T)⋅LW⋅TcR = \rho(T) \cdot \frac{L}{W \cdot T_c}

Reference: IPC-2221B; copper ρ₂₀ = 1.72×10⁻⁸ Ω·m, α = 3.93×10⁻³ /°C

ρ(T)— Resistivity at temperature T (Ω·m)
L— Trace length (m)
W— Trace width (m)
Tc— Copper thickness (m)

How It Works

The PCB Trace Resistance Calculator computes DC and temperature-corrected resistance for copper traces — essential for power integrity analysis, voltage drop budgeting, and thermal management. Power electronics and analog engineers use this to ensure voltage drops stay below 1-2% of supply rails, as required by most IC datasheets.

Per IPC-2221B Appendix A, trace resistance follows R = rho x L / (W x T), where rho is copper resistivity (1.724e-8 ohm-m at 25C), L is length, W is width, and T is thickness. Temperature coefficient alpha = 0.00393/C (per ASTM B193) means resistance increases 39.3% per 100C rise. A trace designed for 50 mohm at 25C measures 70 mohm at 75C — critical for precision current sensing.

Copper thickness varies with manufacturing: 1oz copper nominally 35um becomes 30-32um after etching, increasing resistance 10-15% versus calculation. Per IPC-6012D Class 2, minimum copper thickness is 80% of nominal, so design margins must account for this. Surface roughness (Rz = 2-5um per IPC-4562) further increases effective resistance by 3-8% at high frequencies due to skin effect.

For power distribution networks (PDN), trace resistance sets DC drop but inductance dominates above ~1 MHz. A 100mm trace at 1mm width has approximately 100 nH inductance — at 10 MHz this presents 6.3 ohm reactance versus 50 mohm DC resistance, explaining why decoupling capacitors must be placed close to ICs.

Worked Example

Problem: Calculate resistance of a 50mm long, 0.5mm wide, 1oz copper trace at 25C and 75C for a 3.3V power rail carrying 500mA.

Solution per IPC-2221B:

  1. Copper parameters: rho = 1.724e-8 ohm-m, T = 35um (1oz), alpha = 0.00393/C
  2. R at 25C: R = 1.724e-8 x 0.050 / (0.0005 x 35e-6) = 8.62e-10 / 1.75e-8 = 49.3 mohm
  3. R at 75C: R(75) = R(25) x [1 + 0.00393 x (75-25)] = 49.3 x 1.197 = 59.0 mohm
  4. Voltage drop at 500mA: V = 0.5 x 0.059 = 29.5mV (0.9% of 3.3V)
  5. Power dissipation: P = 0.5^2 x 0.059 = 14.8mW
Verification: 0.9% drop is within typical 2% budget. For 1A current, drop doubles to 59mV (1.8%) — still acceptable. For 2A, drop = 118mV (3.6%) — exceeds budget, need wider trace or 2oz copper.

Practical Tips

  • ✓Use 2oz copper for power traces to halve resistance — per IPC-2221B, cost increase is only 10-15% for significant reliability improvement.
  • ✓Add resistance measurement test points (Kelvin sense pads) on critical power traces — enables production verification per IPC-9252 test methods.
  • ✓For precision analog: derate copper resistivity 15% in calculations to account for etching variation and surface roughness per IPC-4562.

Common Mistakes

  • ✗Using nominal 35um for 1oz copper — actual post-etch thickness is 30-32um per IPC-6012D, increasing resistance 10-15%. Use 32um for conservative calculations.
  • ✗Ignoring temperature coefficient — 50C operating temperature rise increases resistance 20%, causing unexpected voltage drops that may violate IC supply tolerances (+/-5% typical).
  • ✗Calculating DC resistance for high-frequency currents — skin effect confines current to surface layer (skin depth = 21um at 10 MHz), effectively doubling resistance above 10 MHz per Pozar.

Frequently Asked Questions

Resistance is inversely proportional to width: doubling width halves resistance. Per IPC-2221B, a 1mm wide 1oz copper trace has 49 mohm/100mm; at 2mm width, 24.5 mohm/100mm. For high-current paths (>1A), minimum 1mm width is recommended to keep voltage drop below 50mV/100mm.
Yes — a 100mm trace at 1mm width (1oz copper) drops 49mV at 1A. For 3.3V supply with +/-5% tolerance (165mV), this single trace consumes 30% of the tolerance budget. Multi-amp supplies (5A+) require 2-3mm traces or 2oz copper to stay within budget per IPC-2152 PDN guidelines.
Yes — electrodeposited (ED) copper has 5-10% higher resistivity than rolled annealed (RA) copper due to grain structure per IPC-4562. ED copper: approximately 1.8e-8 ohm-m; RA copper: approximately 1.72e-8 ohm-m. Most PCB fabs use ED copper; specify RA for precision current sensing applications.
Theoretical calculations are +/-5% accurate for standard geometries. Real-world variation adds +/-15% due to: (1) copper thickness variation per IPC-6012D; (2) width variation from etching (+/-10%); (3) surface roughness; (4) temperature uncertainty. For production, measure actual resistance on representative samples.
Per IPC-2152: 1oz external copper, 10C rise, 1mm width carries approximately 2A; 2mm width carries approximately 3.5A. For 20C rise: multiply by 1.4x. Internal traces carry 40-50% less due to reduced cooling. Maximum practical current for standard 2oz copper: 10-15A in 5mm+ traces with proper thermal management.

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