PCB coil manufacturing and use guide

Coil PCB refers to an inductor-like component created by directly routing traces into a coil shape on a PCB. This technology offers excellent voltage resistance, high Q factor, strong consistency, flexible routing, and high current-carrying capacity, making it widely used in antenna applications. A PCB coil is a coil directly fabricated on a printed circuit board using conductor traces on the circuit board layers.

In this article, we will provide a detailed introduction to PCB coils, covering their concept, advantages, applications, manufacturing processes, and other key characteristics to help users gain a comprehensive understanding of PCB coils.

What is a PCB Coil?

A PCB coil is a device that uses precisely designed metal traces on a printed circuit board to form a coil of a selected shape. Interestingly, this process can be implemented across multiple layers, depending on different shapes and models.
Additionally, PCBs consist of alternating insulating layers and conductors that interweave to form grouped PCB coils.
Moreover, this component includes a conductor layer with tracks, allowing it to fit an appropriate arc shape while being divided into multiple conductive sections.

Basic Structure of PCB Coils

PCB coils mainly consist of the following components:

Coil Conductor: Typically made of copper foil, formed into spiral or other special shapes through PCB routing processes.
Insulation Layer: Commonly made of FR4, polyimide (PI), or ceramic substrates to ensure proper insulation performance.
Vias: Used to connect coil traces in multilayer PCBs, enhancing current-carrying capacity or increasing inductance.
Surface Treatment: Includes ENIG (electroless nickel immersion gold), OSP (organic solderability preservative), leaded/unleaded solder plating, etc., to improve soldering reliability and conductivity.

Types of PCB Coils

(1) Classification by Layer Count

Single-Layer PCB Coil: The coil structure is laid out only on one side of the PCB, featuring a simple design suitable for low-power applications.
Multilayer PCB Coil: Uses multiple PCB layers stacked and connected via vias to enhance inductance and current-carrying capacity.
Flexible PCB Coil (FPC Coil): Made from flexible materials like PI, suitable for bendable and ultra-thin applications, such as wireless charging coils.

(2) Classification by Shape

Spiral Coil: The most common structure, with the coil spirally distributed along the PCB surface. Widely used in wireless charging and RFID antennas.
Serpentine Coil: Designed for sensor and high-frequency applications, reducing parasitic capacitance.
Rectangular/Ring Coil: Used in specialized structural designs, such as electromagnetic shielding and transformer applications.

How Does a PCB Coil Work?

The electromagnetic principles of PCB coils are identical to those of wire-wound coils or any other inductors:

Conductive Element: Copper traces act as conductive components, replacing the wire windings in discrete inductors.
Induced Magnetic Field: When an alternating or pulsed current flows through the traces, it generates an expanding and contracting magnetic field.
Energy Storage: Due to the coil’s inductance, the magnetic field temporarily stores energy in each cycle of the AC current.
Induced Voltage: Any change in the applied current induces a voltage across the coil proportional to the rate of change, due to inductance.
Impedance: The coil presents an impedance that varies with frequency, primarily due to inductive reactance.

Thus, a PCB coil essentially functions as a standard inductor, with its parameters determined by its structure and materials.

Key Design Considerations for PCB Coils

(1) Coil Parameter Calculations

The design of a PCB coil primarily involves the following key parameters:

Inductance (L): Depends on the number of turns, trace width, spacing, and substrate material.
Resistance (R): Determined by copper foil thickness, trace width, and length, directly affecting energy losses.
Q Factor (Quality Factor): A higher Q factor indicates lower losses, making it suitable for high-efficiency applications.
Resonant Frequency (f₀): The resonant point must be considered to avoid signal interference or losses.

(2) Coil Layout Considerations

Trace Width and Spacing: Should be selected based on current levels and frequency to prevent overheating or electromagnetic interference (EMI).
Interlayer Connections (Via Design): Via diameter and copper filling should be optimized to reduce impedance and enhance reliability.
Shielding and Ground Planes: For high-frequency applications, ground planes or shielding layers can be used to minimize external interference.

PCB Coil Manufacturing Process and Precision Control

The manufacturing of PCB coils involves photolithography, etching, and electroplating processes. The typical workflow is as follows:

Photolithography Process

Steps: Photoresist coating → UV exposure (using film or LDI direct imaging) → Development → Etching → Resist stripping.
Precision: LDI (Laser Direct Imaging) technology enables line width/spacing ≤ 25μm, meeting the requirements of high-frequency coils.

Copper Layer Thickening Technology

Electroplated Copper: Electrolytic deposition increases the copper layer from 1oz to 10oz, significantly reducing resistance. (For example, in a 1mm wide, 10mm long coil, 1oz copper has ~5mΩ resistance, while 10oz copper reduces it to ~0.5mΩ.)
Applications: High-power inductors or transformers require a balance between cost and heat dissipation.

Multilayer PCB Process

Lamination & Vias: Multilayer copper foils are laminated using prepreg sheets, with vias formed via laser or mechanical drilling to establish electrical connections between layers.
Advantages: Planar transformers (e.g., a 4-layer board with primary and secondary coils vertically coupled) enhance magnetic coupling efficiency.

Laser Cutting Technology

Suitable for: Flexible PCB coils, high-frequency meander lines, eliminating the need for etching by directly cutting the copper layer.
Precision: CO₂ or UV lasers achieve ±10μm cutting accuracy.

Applications of PCB Coils

(1) Wireless Charging

Used in Qi wireless charging transmitters (TX) and receivers (RX).
Multilayer spiral coil designs improve energy transfer efficiency.

(2) RF and NFC Communication

Used in RFID, NFC applications such as smart cards and electronic payment devices.
Coil resonance frequency optimization ensures compatibility with target frequency bands (e.g., 13.56MHz).

(3) Sensors and Measurement

Applied in magnetic induction sensors and current sensors.
Differential PCB coil designs enhance signal sensitivity.

(4) Power and Electromagnetic Shielding

Used in PCB transformers and EMI suppression.
Coil turns and shape adjustments optimize electromagnetic compatibility (EMC).

Conclusion

As an essential magnetic component in modern electronic systems, PCB coil design requires careful consideration of materials, manufacturing processes, electromagnetic performance, and thermal management. With increasing demands for high-frequency, integrated, and flexible designs, PCB coil technology will continue to evolve for higher performance and broader applications. Through optimized design and process innovation, PCB coils will play a crucial role in emerging fields such as 5G communication, IoT, and electric vehicles.