IPC-2221 Formula
Cross-sectional area (sq mil) = (I / (k × ΔT^0.44))^(1/0.725). For external traces k = 0.048, internal k = 0.024. Copper thickness (mil) = oz × 1.37. Width (mil) = area / thickness. Resistance = (ρ × L) / (W × T), where ρ = resistivity of copper. All results should be treated as minimums — add 20% margin for production tolerance.
Frequently Asked Questions
The IPC-2221 formula: cross-sectional area = (current / (k × ΔT^0.44))^(1/0.725), where k = 0.048 for internal traces and 0.048 × 1.5 for external. Width = area / (copper thickness in mils). Area is in square mils, copper thickness depends on the copper weight (1 oz/ft² = 1.37 mils).
Copper weight (oz/ft²) specifies how thick the copper layer is. 1 oz/ft² = approximately 1.37 mils (34.8µm) thick. Standard PCBs use 1 oz copper. High-power designs may use 2 oz (2.74 mils) or 3 oz (4.11 mils). Heavier copper allows narrower traces for the same current capacity.
IPC-2221 uses a temperature rise above ambient as the design criterion. A 10°C rise is the typical default for most designs. 20°C rise is sometimes allowed for non-critical traces. Lower temperature rise = wider trace = more conservative design. The temperature rise adds to ambient temperature to give the actual copper temperature.
IPC-2221 includes tables for standard cases, but the formula handles the full range of currents, copper weights, and temperature rises continuously. Online calculators also account for the difference between external (top/bottom) and internal (buried) traces, which have different thermal dissipation characteristics.
Trace resistance = (resistivity × length) / (width × thickness). Resistivity of copper at 20°C = 1.72 × 10⁻⁸ Ω·m. Resistance increases with temperature: approximately 0.393% per °C. Long, thin traces with light copper have the highest resistance and greatest voltage drop.
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