Wireless Power PCB Design for Qi and RF Harvesting

What Is a Wireless Power PCB?

A wireless power PCB carries the transmit or receive coil, rectifier, and power-management circuitry needed to transfer energy without galvanic contact. The two dominant approaches are near-field inductive coupling (Qi standard, typically 100–205 kHz, up to 15 W for consumer devices) and far-field RF energy harvesting (900 MHz–5.8 GHz, microwatts to low milliwatts). Both place unique demands on copper weight, dielectric loss, EMI shielding, and thermal management that differ sharply from ordinary digital or even conventional RF PCB applications.

Qi Inductive Charging: Coil and Board Layout

Planar Coil Geometry

Qi transmitter and receiver coils are usually wound from litz wire and placed against a ferrite shield, but an increasing number of compact designs use planar spiral coils etched directly into the PCB. A typical Qi receiver coil occupies 40–50 mm diameter, uses 2 oz (70 µm) or heavier copper on one or two layers, and targets a series resistance below 200 mΩ to keep I²R losses manageable at the 1–2 A RMS currents common in 5–15 W profiles.

Key layout rules for a planar Qi coil:

• Trace width and spacing — 0.3–0.5 mm traces with 0.15–0.25 mm gaps balance inductance per turn against copper loss. Wider traces lower DC resistance but reduce the number of turns that fit, cutting inductance.
• Turn count — 8–15 turns is typical for a receiver at 100–205 kHz; simulate with the Wheeler or Mohan inductance formula and verify with an impedance analyzer.
• Symmetry — Route the coil concentrically; asymmetric spirals skew the magnetic field and degrade coupling factor k.
• No copper fill under the coil — A ground plane directly beneath the spiral acts as a shorted turn, inducing eddy currents that slash efficiency by 20–40 %. Keep at least one dielectric layer (≥ 0.2 mm) clear, or better, cut a keep-out in the reference plane beneath the coil footprint.

Ferrite Shielding

A flexible ferrite sheet (µ_r ≈ 100–200, thickness 0.1–0.3 mm) bonded to the back of the coil redirects flux away from the battery and metal chassis. Without it, eddy-current heating in a phone’s aluminum midframe can waste 30 % or more of the transferred power. The Wireless Power Consortium’s shielding guidance details test methods for evaluating ferrite performance in Qi-certified designs.

Power Plane IR Drop in the Rectifier Path

After the receive coil, current flows through a full-bridge rectifier into a DC bus that feeds the battery charger IC. At 5 W / 5 V that is 1 A DC; at 15 W Extended Power Profile the bus may carry 1.5–2 A through the power plane before the buck converter. Even modest plane resistance creates voltage drop that the charger IC must compensate, reducing headroom and efficiency. Use the calculator below to check whether your power plane copper weight and geometry keep IR drop under 50 mV—a practical ceiling for most Qi receiver designs.

For example, a 35 µm (1 oz) copper plane carrying 1.5 A over a 20 mm path that is 8 mm wide shows roughly 12 mV of drop—comfortable. Narrow that path to 2 mm (common when routing around a connector cutout) and the drop jumps to about 48 mV, right at the limit. If your result exceeds 50 mV, widen the plane, add stitching vias to a parallel inner layer, or step up to 2 oz copper. This analysis is equally relevant when designing EV wireless charging boards, where currents can be tens of amps and IR drop budgets are even tighter.

RF Energy Harvesting: Rectenna PCB Design

How RF Harvesting Differs from Qi

RF energy harvesting captures ambient or dedicated RF signals at frequencies from 900 MHz to 5.8 GHz and rectifies them with a Schottky-diode voltage doubler or charge pump. Available power densities are tiny—on the order of 1–10 µW/cm² from ambient WiFi at 3 m—so every fraction of a dB in the RF path matters. The PCB doubles as the antenna substrate and the rectifier carrier, making dielectric loss tangent (Df) a first-order design variable.

Material Selection

Standard FR-4 (Df ≈ 0.020 at 1 GHz) is acceptable for a 915 MHz ISM-band rectenna where simplicity and cost matter more than the last 0.5 dB. For 2.45 GHz and above, a low-loss laminate such as Rogers RO4350B (Dk 3.48, Df 0.0037) or Isola Astra MT77 (Dk 3.00, Df 0.0017) preserves antenna efficiency and matching-network Q. A detailed comparison of rectenna and booster board design covers material trade-offs for multi-band harvesters.

Antenna and Rectifier Integration

A patch or slot antenna is etched on the top copper layer with a continuous ground plane on layer 2. The rectifier diodes (typically SMS7630 or HSMS-285x series) connect directly at the antenna feed point to minimize transmission-line loss. Layout tips:

• Keep the diode-to-antenna-feed trace under 2 mm; every millimeter adds parasitic inductance that detunes the match at 2.45 GHz.
• Place the DC output filter capacitor (100 pF–1 nF) within 1 mm of the diode pads to suppress re-radiation of harmonics.
• Use via-fenced ground stitching around the antenna edge to suppress surface-wave leakage—0.5 mm diameter vias on λ/20 pitch is a solid starting rule at 2.45 GHz (about 2.5 mm center-to-center).

For readers working on the broader ecosystem of low-power wireless nodes, the layout principles here overlap with industrial wireless sensor PCB design, where harvested energy often supplements a coin cell.

Thermal Management for Wireless Power PCBs

Qi transmitters at 15 W dissipate 2–4 W as heat in the coil, MOSFETs, and ferrite. Without thermal relief, board temperature can exceed 60 °C and trigger the Qi thermal fold-back, cutting power. Proven countermeasures:

• Thermal vias under the driver FETs — a 5 × 5 array of 0.3 mm vias on 1.0 mm pitch, filled and capped, drops junction-to-board thermal resistance by roughly 40 %.
• Exposed copper pads on the bottom layer — allow contact with a heat spreader or metal chassis through a thermal interface material.
• Copper weight ≥ 2 oz on inner planes — spreads heat laterally and reduces the thermal gradient across the board. The trade-off is increased lamination cost and minimum trace/space (typically 5/5 mil at 2 oz versus 3/3 mil at 1 oz).

These techniques overlap heavily with those used in RF power amplifier thermal design, where similar power densities appear in a similarly constrained footprint.

EMI and Regulatory Compliance

Qi operates in the 100–205 kHz band but generates harmonics well into the MHz range. FCC Part 18 (ISM equipment) and the Qi specification itself set radiated-emission limits. RF harvesters must meet Part 15 limits for unintentional radiators. Board-level practices that help both cases:

• Solid, unbroken ground planes on all non-coil layers to provide a low-impedance return path.
• π-filter or ferrite bead on every power and signal line leaving the wireless-power section.
• Guard ring of grounded vias around the coil area to contain stray flux coupling into adjacent analog or digital circuits.

Stackup Examples

4-Layer Qi Receiver

1. Layer 1 — Planar spiral coil (2 oz Cu), no ground fill.
2. Layer 2 — Partial ground plane with keep-out under coil; full ground elsewhere.
3. Layer 3 — Power plane for rectified DC bus (1–2 oz Cu).
4. Layer 4 — Signal and charger IC routing, full ground pour.

Core material: standard mid-Tg FR-4 (Tg ≥ 150 °C) is adequate at 100–205 kHz; dielectric loss is negligible at these frequencies.

2-Layer RF Harvester at 2.45 GHz

1. Layer 1 — Patch antenna and rectifier circuit (1 oz Cu).
2. Layer 2 — Continuous ground plane (1 oz Cu).

Substrate: 0.762 mm (30 mil) RO4350B gives a 50 Ω microstrip width of approximately 1.7 mm—wide enough for standard etching tolerances.

Design Checklist

• ☐ Coil inductance and Q verified by simulation and impedance measurement.
• ☐ No unbroken ground plane directly beneath the inductive coil.
• ☐ IR drop across the DC power plane calculated and under budget (≤ 50 mV for Qi).
• ☐ Ferrite shield specified with correct µ_r and thickness.
• ☐ Thermal vias and copper weight adequate for worst-case dissipation.
• ☐ EMI filters on all lines crossing the wireless-power boundary.
• ☐ For RF harvesters: Df of substrate verified at operating frequency; via fencing pitch ≤ λ/20.
• ☐ Regulatory pre-scan (FCC Part 15 or Part 18, IEC 63028 for Qi) completed before certification submission.

Frequently Asked Questions

Can I use FR-4 for a wireless power PCB?

Yes, for Qi at 100–205 kHz, FR-4 is the standard choice because dielectric loss at those frequencies is negligible. For RF harvesting above 1 GHz, switch to a low-Df laminate such as RO4350B to preserve antenna and matching-network efficiency.

What copper weight do I need for a Qi coil?

2 oz (70 µm) is the practical minimum for a planar PCB coil carrying 1–2 A RMS. Heavier copper (3 oz) reduces resistance further but increases cost and limits minimum trace width.

How much power can RF harvesting deliver?

From ambient WiFi at a few meters, expect 1–50 µW. A dedicated 915 MHz transmitter at 1 W EIRP can deliver roughly 100 µW at 2 m with a well-matched rectenna. This is enough to trickle-charge a supercapacitor for periodic sensor readings, not to charge a phone.

Do I need a separate shielding layer for EMI?

A dedicated EMI shield layer is not always necessary. A solid inner ground plane combined with via-fenced boundaries and proper filtering usually meets Qi and FCC limits. If pre-scan testing shows margin issues, add a stamped metal shield can over the power stage.

How do I transition a wireless power PCB from prototype to production?

Lock down the coil geometry and ferrite specification early, because both directly affect coupling factor k and tuning. Validate IR drop, thermal performance, and EMI at prototype stage so that the prototype-to-production transfer does not introduce surprises when copper weight tolerances or laminate lots shift slightly.