Wireless Charging PCB Design, Lead Time and Cost

A Board That Is Partly an Antenna

A wireless charging board is unusual because part of it is not a circuit at all. The spiral coil that couples energy magnetically is etched into the same copper as the driver electronics, and its geometry decides the coupling coefficient, the quality factor and how much of the transferred power turns into heat instead of charge. Everything that would be a minor detail on a conventional board, such as trace width, copper weight and the material underneath the coil, becomes a first order design variable here.

The boards appear in phones and accessories, in automotive consoles, in industrial equipment that has to be sealed, and in medical devices where a connector is a reliability risk. This guide covers the design, the manufacturing implications and the commercial questions of minimum quantity and delivery.

How the System Works

• The transmitter board drives an alternating current through its coil and produces an alternating magnetic field.
• The receiver coil converts that field back into electrical energy on the device side.
• The control electronics manage the power level, the communication between the two sides, foreign object detection and the protection functions.

Inductive systems operate over a short range with a strong coupling and are the basis of the common consumer charging standard. Resonant systems tune the coils to work over a slightly larger distance and with more tolerance to alignment, at the cost of more careful tuning.

The board itself carries four elements: the spiral copper coil, the ferrite shield behind it, the pads for the power management devices, and the grounding and thermal via structure that keeps everything cool.

The Coil

The coil is etched from the board copper, typically 1 to 3 ounces thick. Thicker copper lowers the resistance, which reduces the I squared R loss and improves the quality factor, and it also spreads heat. The trade is that heavy copper etches less precisely, so the achievable pitch and the minimum gap between turns are more limited, and the cost rises with each step up in weight.

Four coil parameters matter in the design:

• Number of turns, which sets the inductance and therefore the resonant frequency of the tuned circuit.
• Trace width and spacing, which set the resistance and the inter-turn capacitance.
• The spiral geometry, whether the turns stay on one layer or move between layers through vias. A two layer coil reduces the board area for a given inductance but adds via transitions and the associated loss.
• Copper weight, which is the main lever on resistance and on thermal spreading.

The quality factor combines the inductance and the loss, and it is the parameter that most directly decides the efficiency. A coil with more turns and a higher inductance but a much higher resistance can easily perform worse than a lower inductance coil with wide, thick turns.

The Ferrite Shield

Behind the coil sits a ferrite layer whose job is to direct the magnetic field and stop it coupling into the metal parts of the product, which would otherwise absorb energy and generate eddy currents. The shield determines how much of the field reaches the other coil and how much is wasted heating the chassis. Its placement, its thickness and the adhesive used to attach it all affect performance and reliability, and it is one of the reasons the mechanical drawing and the board design have to be developed together.

Stackup and Loss Control

• Keep the coil on the outer layer facing the other unit, with as little material as possible between them.
• Keep a ground or shield plane behind the ferrite, not directly under the coil, where it would act as a shorted turn.
• Design the thermal path deliberately. The coil and the switching devices both dissipate, and the enclosing housing usually traps the heat. Thermal vias and copper spreading are part of the coil layout rather than a separate exercise.
• Control EMI at the source, because the system is a magnetic field generator by design and has to meet emission limits anyway. Switching loop area and the layout of the driver stage are the two controls.
• Match the dielectric and copper thickness to the tuning, since the board material sits inside the magnetic path and its thickness affects the coupling.

Where the coil also carries a significant current, the thermal and current considerations overlap with the design rules for heavy copper PCB fabrication, and the heat path has to be planned with the same discipline as any other thermal management problem in a sealed enclosure.

Manufacturing Considerations

• Copper weight and etching. A 2 or 3 ounce coil needs an etch process that holds the turn spacing without undercutting. This is the step that most limits how fine the coil can be.
• Coil uniformity. The turns should be consistent across the board, because a variation in width changes the resistance locally and produces a hot spot.
• Layer registration where the coil runs on two layers and the turns connect through vias, since misalignment changes the inductance from unit to unit.
• Ferrite attachment. The shield is usually bonded on, and the adhesive and the placement tolerance affect both the magnetic performance and the mechanical reliability.
• Board flatness, because the coil has to sit at a controlled distance from the mating surface and a warp changes the coupling.

Minimum Quantity and Lead Time

The commercial questions are more settled than the technical ones. Custom wireless charging boards are usually available from a small prototype quantity, commonly five to ten pieces, with the understanding that the setup cost dominates the price at that level. Small batch production typically starts from a few tens of pieces and volume production runs from several hundred upward.

Lead times follow the construction rather than the quantity: roughly one to two weeks for a straightforward two layer board with a single layer coil, adding a few days where the coil runs on two layers with vias or where the board carries a heavy copper weight. Where the ferrite is attached by the fabricator rather than the assembly house, that adds a process step and a day or two. Production quantities run from two to four weeks depending on the total surface area involved.

Samples take the longest in proportion, because the coil geometry usually needs one or two iterations to hit the target inductance, and each iteration costs a fabrication cycle. That is worth budgeting for at the start rather than discovering on the second revision.

Cost Structure

• Base board: a two layer FR-4 panel with 1 ounce copper, priced like any other small board of its size.
• Copper weight: moving to 2 or 3 ounce copper for the coil adds both material and yield cost, and the etching difficulty rises with the weight.
• Two layer coil: adds vias, a second etched layer and tighter registration.
• Ferrite shield: a separate material with its own cost, and it is often supplied and attached by the assembly house rather than the board fabricator.
• Finish and mask: a flat finish where fine pitch driver devices are placed, chosen with the assembly process in mind.
• Quantity: as always, the largest single lever, with the prototype price dominated by setup.

A useful way to compare two quotations on this product is to fix the coil geometry, the copper weight and whether the ferrite is included in the board price or supplied separately. Those three items account for most of the difference between quotes that look far apart. Comparing designs against a written specification through a formal custom PCB pricing request avoids that confusion.

Assembly and Test

Assembling a wireless charging board is conventional surface mount work with two special considerations. The ferrite and any magnetic components are sensitive to mechanical stress, so handling and board depanelisation have to avoid flexing the area around the coil. And the finished unit has to be measured as a power transfer system, not only as a board: the efficiency, the temperature rise at the worst case load and the detection of a foreign object are functional properties that no continuity test can confirm. Building that test into the flow from the first prototype is far cheaper than adding it once the product is in production, which is why the same reasoning applies here as in any PCB assembly program where the end function is measurable.

Selecting a Supplier

Five points to confirm: experience with heavy copper etching at the coil geometry requested, the ability to hold the registration tolerance where the coil runs on two layers, flatness control, a documented process for the finish and the mask, and a willingness to iterate on the coil geometry with the designer. The last point is the most important and the hardest to assess from a capability list. A board that meets the drawing but misses the target inductance has failed, and the supplier who can explain why the measured value differs from the simulation is worth more than the one who simply ships what was drawn. The underlying process requirements are those described under PCB manufacturing, and where the board is part of a wireless power product the same measurements should carry into the assembly test plan and the program’s PCBA testing records.

FAQ

What copper weight is used for a wireless charging coil? Typically 1 to 3 ounces, with the weight chosen against the resistance, the thermal requirement and the achievable turn spacing.

Why is a ferrite shield needed? It directs the magnetic field and prevents it from coupling into the metal parts of the product, where it would be absorbed and wasted as heat.

How fast can a wireless charging board be made? One to two weeks for a single layer coil on a standard construction, adding a few days for a two layer coil, vias or heavy copper.

What is the minimum order quantity? Prototypes from five to ten pieces are common, with volume pricing starting in the hundreds.

How is efficiency verified? By measuring power transfer at the finished assembly, including the temperature rise at full load, not by a board level electrical test.

Summary

A wireless charging board combines a coil, a ferrite shield, driver electronics and a thermal path on one substrate. The coil geometry and the copper weight set the resistance, the quality factor and the efficiency, the ferrite directs the field away from the product’s metalwork, and the stackup determines how much of the field is available to couple. Design the coil with the mechanical assembly, keep the ground plane out of the magnetic path, plan the heat path deliberately and verify the efficiency on the finished unit rather than on the bare board. Prototypes take one to two weeks and start from five to ten pieces, with the cost driven by the copper weight, the coil layers and the quantity.