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Thermal

PCB Trace Temperature Rise

Calculate PCB copper trace temperature rise under load current with the IPC-2221 external-layer fit. Enter trace width, thickness, and current to get ΔT above ambient. Essential for thermal PCB design and reliability engineering. Free, instant results.

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Formula

ΔT=(Ik⋅A0.725)1/0.44,k=0.048 (external), A in mil2\Delta T = \left(\frac{I}{k \cdot A^{0.725}}\right)^{1/0.44},\quad k = 0.048 \text{ (external)},\ A \text{ in mil}^2

Reference: IPC-2221B Appendix B (external layers)

ΔT— Temperature rise above ambient (°C)
I— Trace current (A)
k— IPC-2221 constant (external: 0.048)
b— IPC-2221 exponent (0.44)
c— IPC-2221 cross-section exponent (0.725)
A— Conductor cross-section (width × thickness) (mil²)

How It Works

The PCB Trace Temperature Calculator computes steady-state temperature rise for current-carrying traces — essential for power electronics, motor drivers, and LED circuits where trace overheating causes solder joint failure and PCB delamination. Thermal engineers use this to verify designs stay below FR4's glass transition temperature (Tg = 130-180C) with appropriate safety margins.

This calculator uses the IPC-2221 external-layer fit to the conductor charts: I = k x deltaT^0.44 x A^0.725, so deltaT = (I / (k x A^0.725))^(1/0.44), where k = 0.048 for external traces and A is the cross-sectional area in mils^2. IPC-2221 gives k = 0.024 for internal traces, which this calculator does not apply. IPC-2152 (2009) replaces those charts with newer measurements and corrections for board thickness and nearby copper planes, published as charts rather than as this closed form. Internal traces run 40-50% hotter than external at same current because convection cooling is blocked by surrounding dielectric.

Actual temperature = ambient + deltaT. A 20C rise design at 25C ambient reaches 45C; at 85C automotive ambient reaches 105C — approaching solder reflow temperature (183-220C) and risking long-term reliability. Per IPC-9701A, each 10C temperature increase halves solder joint lifetime due to thermal cycling fatigue.

Copper resistivity increases 0.393%/C per ASTM B193. A trace at 75C (50C above 25C reference) has 20% higher resistance than calculated at room temperature, creating positive feedback that can lead to thermal runaway at high currents. Design calculations should use worst-case temperature for resistance.

Worked Example

Problem: Verify a 1.5mm wide, 2oz copper (70um), 100mm long external trace carrying 4A continuous at 55C ambient. Maximum allowed temperature is 105C.

Solution per the IPC-2221 external-layer fit:

  1. Cross-sectional area: A = 1.5mm x 0.070mm = 0.105 mm^2 = 162.75 mils^2 (1 mm^2 = 1550 mils^2)
  2. External layer constant: k = 0.048
  3. Temperature rise: deltaT = (4 / (0.048 x 162.75^0.725))^(1/0.44)
  4. Calculate: 162.75^0.725 = 40.12; 0.048 x 40.12 = 1.926; 4/1.926 = 2.077; 2.077^(1/0.44) = 5.27C
  5. Actual temperature: T = 55C + 5.27C = 60.3C
  6. Margin: 105C - 60.3C = 44.7C — comfortable; the same trace carries 5.30A at a 10C rise

Resistance and loss: R = 0.01724 / 0.105 x 0.1 m = 16.42 mOhm, so P = 4^2 x 0.01642 = 263 mW.

On an internal layer IPC-2221 uses k = 0.024, which gives about 25C for the same trace; this calculator computes external layers only.

Practical Tips

  • ✓Target 10C rise for conservative design, 20C for compact boards, 30C maximum for cost-optimized consumer products — per IPC-2152 Table 6-1 recommendations.
  • ✓Add copper pour around power traces — thermal spreading improves effective cooling by 15-25% per thermal simulation studies, reducing temperature rise at same current.
  • ✓For automotive (85C ambient): use external layers with 2oz copper for power traces — provides 2x current capacity versus 1oz internal at same temperature rise.

Common Mistakes

  • ✗Treating the IPC-2221 fit this calculator uses as exact: it comes from 1950s data and does not account for board thickness or nearby copper planes. IPC-2152 (2009) replaces those charts with newer measurements that do; check critical traces against its charts, which this calculator does not implement.
  • ✗Calculating at 25C ambient when product operates at 55-85C — per IPC-9701A, high operating temperature dramatically accelerates solder fatigue. Always add actual ambient to calculated temperature rise.
  • ✗Ignoring internal layer thermal penalty — internal traces run 40-50% hotter than external per IPC-2152 because heat must conduct through dielectric rather than convect to air. Size internal power traces 50-100% wider.

Frequently Asked Questions

Depends on application per IPC-2152: consumer electronics typically 20-30C rise; industrial 10-20C; automotive/aerospace 10C maximum due to reliability requirements. Critical constraint is solder joint: each 10C cycling range doubles fatigue damage per IPC-9701A. Keep total temperature (ambient + rise) below 105C for long-term reliability.
In the IPC-2221 fit this calculator uses, current capacity scales as A^0.725 where A is cross-sectional area. Doubling width (same thickness) increases capacity by 2^0.725 = 1.65x (65%), not 2x, because wider traces also have larger surface area for cooling. For same temperature rise: 1mm trace at 2A; 2mm trace at 3.3A; 3mm trace at 4.5A.
Yes — the IPC-2221 fit this calculator uses takes cross-sectional area directly. 1oz copper (35um) at 1mm width has A = 35,000 um^2; 2oz (70um) at same width has A = 70,000 um^2, increasing current capacity by 1.65x. Thicker copper also improves thermal spreading, providing additional 5-10% capacity bonus per thermal modeling.
Per IPC-2152: (1) Ambient temperature — adds directly to calculated rise; (2) Adjacent traces — thermal coupling adds 5-15C; (3) Copper pours — improve heat spreading 15-25%; (4) Solder mask — traps heat, adds 5-10C; (5) Board material — FR4 conducts heat better than polyimide. Include 20-30% margin for these factors.
Per IPC-2152 design guidelines: (1) During initial design — size traces for expected current; (2) After layout — verify actual trace lengths and copper distribution; (3) After any current increase; (4) For production — measure actual temperature on prototypes using IR camera or thermocouples. Calculate at worst-case operating conditions.

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