Designing a coil on PCB—often called a planar inductor or trace coil—replaces bulky wire-wound magnetics with etched copper spirals. You see them in LLC resonant converters, Qi wireless charging pads, RFID antennas, and metal detector search coils. The immediate advantage is a low profile and automated assembly; the immediate trap is thermal failure from under-sizing the traces. To carry 2A continuous on a standard 1 oz outer copper layer with a 10°C temperature rise, you need a minimum trace width of roughly 35 mils (0.89 mm). However, coil geometries force tight spacing, making thermal management and high-frequency skin effect your primary design constraints.
Sizing PCB Coils: Trace Width, Copper Weight, and Current Capacity
When routing a planar coil, you cannot rely on the same trace widths you use for standard DC power routing. The continuous loops concentrate heat, and high-frequency switching currents trigger the skin effect, forcing electrons to the outer edges of the trace. This makes the trace perimeter more critical than the raw cross-sectional area.
The table below provides baseline trace widths for external layers based on IPC-2221 standards, assuming a 10°C temperature rise above ambient. For internal layers, you must increase these widths by approximately 50% to 100% due to the insulating properties of the FR4 core trapping heat.
| Continuous Current (A) | 1 oz Copper (35 µm) Width | 2 oz Copper (70 µm) Width | Estimated DCR (1 oz, 100mm trace) |
|---|---|---|---|
| 1.0 A | 10 mils (0.25 mm) | 5 mils (0.13 mm) | ~0.02 Ω |
| 2.0 A | 35 mils (0.89 mm) | 15 mils (0.38 mm) | ~0.06 Ω |
| 3.0 A | 70 mils (1.78 mm) | 30 mils (0.76 mm) | ~0.03 Ω |
| 5.0 A | 150 mils (3.81 mm) | 65 mils (1.65 mm) | ~0.014 Ω |
| 10.0 A | 450 mils (11.4 mm) | 190 mils (4.83 mm) | ~0.004 Ω |
Source data derived from IPC-2221 generic standard charts. For precise geometries, use the Saturn PCB Toolkit to calculate exact thermal relief requirements.
If your coil requires 3A but physical board space limits your trace width to 20 mils on 1 oz copper, the trace will overheat and delaminate. The fix is to specify 2 oz or 3 oz copper weight from your fab house (like JLCPCB or PCBWay), or to use a multi-layer coil where you stack identical spiral traces on layers 1, 2, and 3, connecting them with vias at the center and outer pads to parallel the current path and halve the DC resistance (DCR).
Breadboard to PCB: Migrating Coil Circuits Without Losing Performance
Prototyping a coil circuit on a solderless breadboard is a necessary first step, but it masks parasitic behaviors that will immediately break your design once etched into FR4. Which mistakes survive the transition? The most common is ignoring parasitic capacitance to the ground plane.
On a breadboard, your hand-wound 22 AWG magnet wire coil sits in free air. The parasitic capacitance between the coil and the bench is negligible. On a PCB, if you place a solid copper ground plane on Layer 2 directly beneath your Layer 1 coil, you inadvertently create a parallel-plate capacitor. This capacitance (often 15pF to 40pF depending on coil diameter) lowers the Self-Resonant Frequency (SRF) of your inductor. If your SRF drops below your switching frequency, the coil stops acting like an inductor and becomes a capacitor, destroying your Q-factor and causing massive switching losses.
The Breadboard-to-PCB Migration Checklist
- Check SRF Margin: Calculate the parasitic capacitance of your coil over the ground plane. Ensure the SRF is at least 5x to 10x higher than your operating frequency.
- Hatch the Ground Plane: If SRF is too low, remove the ground plane directly under the coil, or use a hatched (grid) copper pour to reduce the capacitive surface area while maintaining shielding.
- Verify Wire-to-Trace Equivalence: 22 AWG magnet wire handles roughly 1.5A to 2A easily. Do not route a 10-mil PCB trace expecting it to handle the same current; refer to Table 1.
- Account for Via Inductance: If routing the coil center tap to an inner layer, standard 0.3mm vias add ~0.5nH of inductance each. Use multiple parallel vias to reduce this parasitic spike.
- Clearance Rules: Set your EDA software (KiCad, Altium) clearance rules to at least 8 mils (0.2mm) between coil turns for standard 1 oz copper to prevent etching micro-bridges that cause shorted turns.
Workshop Safety, Soldering, and Testing Your First-Spin Board
Soldering Heavy Copper Coil Pads
PCB coil pads often connect to massive ground or power pours. These pours act as giant heatsinks, pulling thermal energy away from your soldering iron tip and resulting in cold, grainy solder joints. Do not compensate by cranking your iron to 450°C; this oxidizes the tip and burns the flux instantly.
Instead, increase your thermal mass. Use a wide chisel tip (e.g., Weller RT4 or Hakko T18-D24) rather than a fine conical tip. For lead-free SAC305 alloy, set your station to 350°C–360°C. For traditional Sn63/Pb37 eutectic, 320°C–330°C is ideal. Apply a generous blob of fresh, high-activity flux (like Amtech NC-559 or MG Chemicals 8341) to the pad before applying heat. The flux breaks down the surface oxidation and dramatically improves thermal transfer from the tip to the heavy copper pad.
How to Test a First-Spin PCB Coil Board
When your first-spin boards arrive from the fab house, do not immediately apply power. Follow this sequential test protocol to catch manufacturing defects and verify your electromagnetic models.
- Visual and DMM Short Check: Inspect the tight inner turns of the coil under a 10x loupe. Fab houses sometimes leave micro-bridges of copper between tightly spaced traces. Use a digital multimeter (DMM) in continuity mode. Measure between the coil's start pad and end pad. You should read a low resistance (e.g., 0.2 Ω to 2.0 Ω), but never a dead short (0.0 Ω). If it reads dead short, you have an etching defect or a solder bridge.
- LCR Meter Verification: Connect an LCR meter (such as a DER EE DE-5000 or Keysight E4980A) across the coil pads. Set the test frequency to your intended operating frequency (e.g., 100 kHz for a switching supply, 13.56 MHz for RFID). Record the inductance (L) and the equivalent series resistance (ESR). If your measured inductance is more than 15% off from your EDA calculations, check for an incorrect core air-gap or unexpected magnetic coupling from nearby components.
- Thermal Load Testing: Once the circuit is populated and powered, inject the maximum expected continuous current. Use a thermal camera (like a FLIR ONE or Seek Thermal) to scan the board after 10 minutes of steady-state operation. The coil traces should not exceed 70°C in a standard 25°C ambient environment. If the inner turns are significantly hotter than the outer turns, your current density is too high, and you must increase the copper weight or widen the traces on the next revision.
Designing a reliable coil on PCB is an exercise in balancing electromagnetic theory with physical thermal limits. By respecting trace ampacity, managing parasitic capacitance during layout, and rigorously testing your first spin with an LCR meter and thermal imaging, you can successfully replace wire-wound magnetics with robust, low-profile planar designs.
