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Thermal

PCB Trace Temperature Rise

Calculate PCB copper trace temperature rise under load current using IPC-2152

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Formula

ΔT=(I/(k×Wb))(1/c)—IPC−2152ΔT = (I / (k × W^b))^(1/c) — IPC-2152

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)

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.

Per IPC-2152 (supersedes IPC-2221 outdated 1950s data), temperature rise follows empirical formula: deltaT = (I / (k x A^b))^(1/c), where k=0.048 for external traces, 0.024 for internal, A is cross-sectional area in mils^2, and b=0.44, c=0.725. 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) internal trace carrying 4A continuous on a 4-layer board at 55C ambient. Maximum allowed temperature is 105C.

Solution per IPC-2152:

  1. Cross-sectional area: A = 1.5mm x 70um = 105,000 um^2 = 163 mils^2
  2. Internal layer constant: k = 0.024
  3. Temperature rise: deltaT = (4 / (0.024 x 163^0.44))^(1/0.725)
  4. Calculate: 163^0.44 = 9.1; 0.024 x 9.1 = 0.218; 4/0.218 = 18.3; 18.3^1.38 = 46.5C
  5. Actual temperature: T = 55C + 46.5C = 101.5C
  6. Margin: 105C - 101.5C = 3.5C — insufficient margin!

Solution

Either (1) widen trace to 2mm (reduces rise to 35C), (2) use 3oz copper (reduces rise to 32C), or (3) move trace to external layer (reduces rise to 23C due to convective cooling).

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

  • ✗Using IPC-2221 charts — based on 1950s military data, underestimates current capacity by 20-40%. IPC-2152 (2009) uses modern thermal modeling validated by testing and is industry standard.
  • ✗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.
Per IPC-2152, 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 — IPC-2152 formulas use 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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