PCB Trace Resistance Calculator
Calculate PCB copper trace DC resistance from width, length, thickness, and temperature. Includes sheet resistance and temperature coefficient.
Formula
R = \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
How It Works
PCB trace resistance calculation is a critical aspect of electronic design, involving understanding how electrical conductivity changes with material properties and temperature. The resistance of a conductive trace depends on four primary factors: resistivity (ρ), length (L), width (W), and thickness (T) of the trace. Copper, the most common PCB conductor, has a base resistivity of 1.724e-8 Ω·m at 25°C. As temperature increases, the material's atomic lattice becomes more energetic, causing increased electron scattering and consequently higher resistance. The temperature coefficient (α) quantifies this resistance change, which for copper is approximately 0.00393/°C. Engineers must account for these factors to ensure precise power delivery, minimize voltage drops, and prevent potential thermal-related performance degradation in high-current or precision circuits.
Worked Example
Consider a 50mm long, 0.5mm wide copper trace with 35µm thickness on a standard FR4 PCB. Using ρ(copper) = 1.724e-8 Ω·m and α = 0.00393/°C, calculate the trace resistance at 75°C. First, compute the base resistance at 25°C: R = (1.724e-8 · 0.050) / (0.0005 · 0.000035) = 0.0246 Ω. Then, apply the temperature correction: R(75°C) = 0.0246 · [1 + 0.00393 · (75-25)] = 0.0318 Ω. This demonstrates how even modest temperature changes can impact trace resistance.
Practical Tips
- ✓Use wider traces for high-current paths to minimize resistance
- ✓Consider temperature effects in critical power or precision signal circuits
- ✓Validate trace resistance calculations with simulation tools
Common Mistakes
- ✗Neglecting trace width and thickness variations when estimating resistance
- ✗Assuming constant resistivity across all temperatures without correction factor
- ✗Overlooking current density limitations in narrow or thin traces
Frequently Asked Questions
How does trace width affect resistance?
Wider traces have lower resistance due to increased cross-sectional area. Trace width is inversely proportional to resistance.
Can trace resistance cause significant voltage drop?
Yes, especially in high-current circuits or long traces. Even small resistances can create meaningful voltage drops.
Does copper type impact trace resistance?
Different copper grades have slightly varying resistivity. Electrolytic and electrodeposited copper have minor resistance differences.
How accurate are PCB trace resistance calculations?
Typical calculations are accurate within ±10%, depending on manufacturing tolerances and material variations.
What's the maximum current for a standard PCB trace?
Current capacity depends on trace width, thickness, and temperature rise. IPC-2221 provides comprehensive current-carrying guidelines.
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