Determining Minimum Trace Width for High-Power Motor Driver PCBs
Designing custom motor driver hardware introduces significant thermal challenges that standard logic-level PCB design never encounters. Stepper motors, brushless DC (BLDC) motors, and heavy-duty brushed motors often require continuous drive currents ranging from 5 to 20 Amps, punctuated by aggressive inductive stall spikes. If the copper traces feeding the H-bridge ICs are undersized, $I^2R$ resistive heating will delaminate the FR4 fiberglass substrate and instantly destroy the board.
To safely route high-amperage power across a PCB, engineers rely on the IPC-2152 standard. This standard correlates the cross-sectional area of the copper trace to a specific, safe temperature rise above ambient. By properly calculating your continuous load and utilizing heavy copper weights, you can ensure your motor driver survives peak operational loads.
Calculate IPC-2152 Trace Sizing Instantly
Prevent board delamination by determining the precise trace width required for your motor’s continuous current.
In This Guide:
IPC-2152 Math and Temperature Rise
Unlike standard hook-up wire in free air, PCB traces are adhered to a thermal insulator (FR4). When passing current, the heat must be dissipated into the surrounding air or thermal planes. The IPC-2152 standard allows engineers to design for an acceptable temperature rise ($Delta T$)—typically 10°C or 20°C above the ambient temperature.
For a continuous 10 Amp load targeting a conservative 10°C rise on an external (top or bottom) layer, standard 1 oz/ft² copper dictates an incredibly wide trace. If the trace is internal, the width requirement balloons even further due to the trapped heat.
1 oz vs 2 oz Copper for Motor Drivers
Standard PCB manufacturing defaults to 1 oz/ft² copper thickness (approx 1.37 mils or 35 $mu m$). For motor drivers, upgrading the manufacturing spec to 2 oz/ft² copper is standard practice. Doubling the thickness effectively halves the required trace width, saving critical board real estate.
| Continuous Motor Load | Required Width (1 oz Copper) | Required Width (2 oz Copper) | Target $Delta T$ |
|---|---|---|---|
| 2 Amps | 30 mils (0.76mm) | 15 mils (0.38mm) | 10°C |
| 5 Amps | 118 mils (3.0mm) | 59 mils (1.5mm) | 10°C |
| 10 Amps | 355 mils (9.0mm) | 175 mils (4.4mm) | 10°C |
| 15 Amps | 650 mils (16.5mm) | 320 mils (8.1mm) | 10°C |
When a trace requires 355 mils of width, it is no longer a “trace” but a polygon pour. Utilizing 2 oz copper allows a 10A feed to route cleanly at 175 mils, leaving room for gate drive signals and thermal vias.
Frequently Asked Questions
Q: Should I size traces for the motor’s running current or stall current?
A: Traces should be sized based on the continuous running current of the motor, plus a 20% margin. Short stall spikes (milliseconds to a few seconds) rely on the thermal mass of the copper and do not have enough duration to reach steady-state thermal failure, provided the continuous rating is respected.
Q: Does exposing the copper and adding solder help with ampacity?
A: Removing the solder mask and adding a thick layer of solder over the trace decreases resistance and increases thermal mass. However, solder is a poor conductor compared to copper (roughly 10% the conductivity). It is a helpful hack for prototypes but should not replace proper copper weight sizing in production.
Q: Why are thermal vias necessary on motor driver boards?
A: Motor driver ICs dissipate significant heat through their exposed thermal pads. Arrays of thermal vias transfer this heat directly to large copper ground planes on the opposite side of the PCB, effectively turning the entire board into a convective heat sink.
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