Engineering GuideApril 17, 2026• 10 min read
How to Size Copper for Motor Driver Boards
Quick Answer
For most motor driver boards, start with 1oz outer-layer copper for prototypes and move to 2oz when continuous path current is above about 8-10A, routing space is tight, or voltage drop and thermal rise are too high with practical 1oz pours.
Key Takeaways
- •Size motor-driver copper from RMS or sustained current, not short marketing peak current alone.
- •Battery input, half-bridge outputs, shunt paths, and return loops deserve the most copper and the shortest routes.
- •2oz copper becomes the better default when 1oz widths get awkward, enclosure temperature is high, or voltage-drop margin is tight.
- •Via arrays, connector pads, shunts, and neck-downs often fail before the long straight trace does.
For most motor driver boards, start with 1oz outer-layer copper for prototypes, 2oz when continuous phase current is above about 8-10A per path or routing space is tight, and size traces from the real RMS current, allowed temperature rise, and voltage-drop budget instead of from peak current alone.
A workable default for compact BLDC, stepper, and brushed DC controllers is to keep battery input, half-bridge outputs, current-sense returns, and regeneration paths on external layers with short loops, stitched copper pours, and enough via count to match the trace cross-section. Use the Trace Width Calculator, Via Current Calculator, and FR4 trace calculator together, because motor driver reliability is usually limited by heat, bottlenecks at layer changes, and layout symmetry more than by one straight trace segment.
What Copper Size Should You Start With?
Motor driver boards are not routed like small-signal control PCBs. The critical copper has to carry phase current, survive regenerative current spikes, and keep voltage drop low enough that the MOSFETs, shunts, connectors, and supply all behave predictably under load.
For buyers and engineers comparing stackups, the first decision is usually not the exact trace width. It is whether 1oz copper with wider pours is still practical, or whether 2oz copper is the cleaner way to hit ampacity and thermal targets without turning the board into a routing compromise.
| Board Situation | Recommended Start | Why |
|---|---|---|
| Prototype or low-current controller up to about 5A continuous per path | 1oz outer copper with wide pours | Lowest cost and easiest fabrication; routing density stays reasonable. |
| Compact 12V to 48V motor driver at 5A to 10A continuous | 1oz or 2oz depending on board area | If space is available, 1oz can work. If the board is crowded, 2oz reduces required width. |
| Phase, battery, or brake path above roughly 8A to 10A continuous | 2oz outer copper | Usually the safer default for temperature rise and voltage-drop margin. |
| Sustained high-current inverter, robotics, or automotive power stage | 2oz outer copper plus planes/pours and parallel vias | High current rarely fits well into narrow traces; spreading current lowers hot spots. |
If copper weight is still open, review the 0.5oz vs 1oz vs 2oz copper guide before locking the fabrication stackup.
Size From RMS Current, Not Marketing Peak Current
One of the most common motor-driver mistakes is sizing copper from a short burst current number on the product sheet. Copper heating tracks RMS current and duty cycle, while component stress and protection events may be set by peak current. You need both numbers, but the trace and pour geometry should usually start from the sustained case.
A board that survives 20A for 200 ms can still overheat if it carries 8A RMS for minutes inside a sealed enclosure. This is why current profile, ambient temperature, airflow, and allowable temperature rise must be defined before you freeze the copper.
- Use RMS or worst-case continuous current for trace and pour sizing.
- Check peak current separately for short bottlenecks such as shunts, connectors, neck-downs, and vias.
- Include regenerative current paths from the motor back to bulk capacitance or supply input.
- Budget voltage drop early; low-voltage motor systems often feel copper loss before they hit absolute thermal limits.
Recommendation: If the design is below 24V, keep voltage-drop targets explicit. A few tens of millivolts across a battery feed, phase path, or current-sense return can materially change startup torque, current measurement accuracy, and thermal balance.
Which Paths Need the Most Copper?
Not every net on a motor driver board needs the same treatment. The priority is the high-current loop, not every trace connected to the power stage. Focus copper budget where heating, voltage drop, and switching current actually concentrate.
| Path | Priority | Layout Guidance |
|---|---|---|
| Battery or DC bus input | Very high | Use short, wide external pours; keep bulk capacitors and MOSFET bridge tightly coupled. |
| Half-bridge to motor phase output | Very high | Prefer broad pours over long traces; keep the three phases geometrically similar. |
| Current-sense shunt path | High | Avoid neck-downs near the shunt and separate force current from Kelvin sense routing. |
| Ground return between bridge, shunt, and input capacitors | Very high | This loop is often the real thermal and EMI bottleneck; keep it compact and low impedance. |
| Gate-drive and logic power | Low to medium | Route cleanly, but do not waste high-current copper budget on control nets. |
For automotive and robotics layouts, the automotive PCB calculator and robotics control PCB design guide are useful companion pages because they frame reliability, transient loading, and return-path discipline around real control hardware.
A Practical Sizing Workflow for Engineers and Buyers
- Define the sustained current per path, not just the driver IC peak rating.
- Set a voltage-drop budget for battery input, phase path, and return path based on system voltage and torque sensitivity.
- Choose external-layer routing for the highest-current copper whenever possible.
- Select 1oz or 2oz copper based on available board area, current density, and fab limits.
- Calculate trace or pour width with the trace width calculator using realistic ambient and temperature-rise assumptions.
- Check every layer transition with the via current calculator; the via field must match the current capacity of the trace or pour feeding it.
- Confirm that neck-downs at shunts, connectors, fuse pads, and test points do not become the new bottleneck.
- Review manufacturability: heavier copper raises minimum trace/space and can increase cost and etch variation.
Buyer checkpoint: If a supplier says the board is 2oz copper but the quote also promises fine-pitch routing and low-cost standard fabrication, verify the actual minimum trace/space and annular-ring rules. Heavy copper and dense routing often collide.
When 1oz Is Enough and When 2oz Is the Better Answer
1oz Still Makes Sense When
- Continuous current per path is modest and the board has room for wider pours.
- The project is in prototype or cost-sensitive volume and you want simpler fabrication.
- Fine-pitch gate-driver, MCU, or sensing escape routing dominates the layout.
- The thermal strategy depends more on copper area, vias, airflow, and heat sinking than on copper thickness alone.
Move to 2oz When
- You keep fighting width constraints around MOSFETs, shunts, connectors, or board-edge terminals.
- Continuous current is high enough that 1oz geometry becomes awkward or forces long detours.
- The enclosure is hot, sealed, or vibration-heavy and you need more thermal and mechanical margin.
- You want lower resistive loss without making every power path dramatically wider.
If you are deciding between thinner and thicker copper on the same stackup, compare the routing and fabrication tradeoffs with the internal vs external layers guide and the copper weight comparison article.
Common Failure Modes to Catch Before Release
Mistake 1: Sizing the straight trace but ignoring the bottlenecks. Motor driver boards usually fail at connector pads, fuse lands, shunts, vias, and MOSFET escape regions before they fail on the long easy section of copper.
Mistake 2: Routing the outgoing path generously but starving the return path. Current loops heat as a system. If only one side gets the copper area, the real temperature rise and EMI can still be poor.
Mistake 3: Treating vias as free. A wide top-layer pour that dives through too few vias into an inner plane creates a current choke point. Always size the via field with the via calculator.
Mistake 4: Choosing 2oz copper to fix a thermal problem that is really a layout problem. Better capacitor placement, shorter loops, broader pours, and more copper sharing often matter more than jumping straight to heavy copper.
Quick Checklist Before You Send the Board Out
| Checkpoint | Pass Target | Reason |
|---|---|---|
| Continuous current defined | RMS or sustained current documented for each high-current path | Prevents sizing from unrealistic burst numbers. |
| Voltage-drop budget defined | Input and return losses reviewed, especially below 24V | Protects torque and current-sense accuracy. |
| Highest-current paths on outer layers | Yes where practical | Improves cooling and allows wider copper. |
| Via transitions checked | Via array capacity matches copper path capacity | Avoids hidden current chokepoints. |
| Shunt routing reviewed | Force current and Kelvin sense separated | Reduces measurement error and local heating. |
| Copper weight confirmed with fab | Stackup and minimum rules match the quote | Avoids last-minute DFM surprises. |
Final Recommendation
For most motor driver boards, choose copper based on continuous path current, voltage-drop budget, and available routing area. Start with 1oz on outer layers for low-to-moderate current designs, but move to 2oz once continuous current, enclosure temperature, or space pressure makes 1oz pours awkward.
The best result is usually not one oversized trace. It is a balanced power path: short loops, wide pours, enough parallel vias, controlled bottlenecks, and realistic calculator inputs. Use the trace width calculator, via current calculator, and FR4 calculator together before you release the board.
Tags
Motor Driver PCBCopper WeightHigh Current PCBPower ElectronicsPCB Layout
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Quick FAQ
Should I use 1oz or 2oz copper on a motor driver PCB?
Use 1oz when continuous current is modest and the board has room for wider pours. Move to 2oz when continuous path current is roughly above 8-10A, board area is tight, or you need lower loss and more thermal margin without excessive width.
Do I size motor driver traces from peak current or continuous current?
Start from RMS or worst-case continuous current for copper heating, then verify peak current separately for short bottlenecks such as shunts, connectors, vias, and fuse pads.
What areas of a motor driver board need the widest copper?
Prioritize the battery or DC bus input, half-bridge phase outputs, shunt current path, and the return loop between the bridge and bulk capacitors. Those paths dominate heating, loss, and switching current stress.
Why are vias so important on high-current motor driver boards?
A wide pour can still bottleneck through too few vias at a layer change. The via field must carry the same current as the copper path feeding it, or local heating and voltage drop will concentrate there.
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