PCB trace current, voltage drop, and temperature rise
Trace current capacity is not just a single amp rating. The practical check includes resistance, voltage drop, copper width/thickness, heat spreading, and allowable temperature rise.
What should you check when a PCB trace carries current?
Check voltage drop and power loss from trace resistance, then check whether the resulting heat and temperature rise are acceptable for the layout, copper, stackup, and environment.
Model summary
- Trace voltage drop: Vdrop = I * Rtrace.
- Trace power loss: P = I^2 * Rtrace.
- Temperature rise depends on copper geometry, board construction, neighbouring copper, airflow, and environment.
Worked example
A trace carrying 2 A with 50 mOhm resistance drops 100 mV.
Power loss is 2^2 * 50 mOhm = 0.2 W in the copper path.
That loss may be acceptable or unacceptable depending on width, copper weight, length, planes, and local thermal environment.
Calculator status
A dedicated trace-current calculator is a future workflow candidate. Until then, use the explanation to frame what the future calculator must include.
Common mistakes
- Checking current alone and ignoring voltage drop.
- Using an external-layer assumption for an internal trace.
- Ignoring connector, via, polygon-neckdown, and solder-joint resistance in the same current path.
When the approximation breaks down
- Trace temperature-rise estimates are empirical and layout-dependent; high-current designs need margin and sometimes thermal simulation or measurement.
- IPC-style equations are not a substitute for verifying the actual stackup and copper geometry.