PCB Manufacturing: Rigid, Flexible, and HDI Types

The manufacturing process of PCB directly determines its form, performance, and applicable scenarios. Rigid PCB meets fixed installation needs, flexible PCB adapts to bendable devices, and special process PCB such as high-frequency and HDI supports high-end functions. Classified by manufacturing process, PCB can be divided into five major categories: rigid PCB, flexible PCB, rigid-flex PCB, high-frequency PCB, and HDI PCB. Each type differs significantly in process flow, key technologies, and form characteristics.

I. Rigid PCB (The Most Mainstream Process Type)

Rigid PCB has a hard substrate and a fixed form. It cannot be bent. It is the most common type in electronic equipment, with mature manufacturing processes and controllable costs.

Manufacturing flow: substrate cutting, drilling, copper deposition, electroplating, etching, solder mask, silkscreen, profiling, testing.

Key process points:

Etching: line width tolerance control, plus or minus 0.02 mm, to avoid side etching, less than or equal to 0.01 mm.

Lamination: uniformity of lamination pressure, 30 to 35 kg per square centimeter, and temperature, 175 plus or minus 5 degrees Celsius, for multilayer boards.

Surface treatment: immersion gold, OSP, silver plating, and so on, suitable for soldering and corrosion protection.

Form characteristics: high hardness, flexural modulus greater than or equal to 20 GPa, cannot be bent, dimensional stability, warpage less than or equal to 1.5 percent.

Application scenarios: consumer electronics, industrial control, automotive electronics, and medical equipment, such as mobile phone motherboards, PLC controllers, and automotive ECU. It accounts for more than 85 percent of total PCB output.

II. Flexible PCB (Exclusive for Bending and Miniaturization)

Flexible PCB, or FPC, uses flexible substrates such as PI and PET. It can be bent, folded, and wound, adapting to narrow spaces and wearable devices.

Manufacturing flow: flexible substrate cutting, drilling, copper deposition, electroplating, etching, coverlay lamination, stiffener, profiling, testing.

Key process points:

Substrate selection: PI substrate, temperature resistance from minus 200 to 260 degrees Celsius, or PET substrate, low cost, temperature resistance from minus 40 to 120 degrees Celsius.

Coverlay lamination: no bubbles, diameter less than or equal to 0.3 mm, adhesion greater than or equal to 0.5 N per mm.

Stiffener design: add rigid stiffener plates, FR-4 or stainless steel, at connectors and chip areas to enhance mechanical strength.

Form characteristics: bendable, bending radius greater than or equal to 10 times board thickness, foldable, folding times greater than or equal to 10,000, thin thickness, 0.1 to 0.5 mm.

Application scenarios: wearable devices, mobile phone cables, automotive sensors, and folding screen phones. For example, smartwatch PCB with PI substrate is 0.2 mm thick and can be bent to fit the watch shell. The FPC at the hinge of a folding screen phone can fold 100,000 times without breaking.

III. Rigid-Flex PCB (Integration of Rigid and Flexible Advantages)

Rigid-flex PCB consists of rigid areas and flexible areas. The rigid areas carry chips and connectors, and the flexible areas realize bending connections, balancing integration and flexibility.

Manufacturing flow: rigid substrate and flexible substrate processed separately, lamination bonding, drilling, copper deposition, etching, solder mask, stiffener, profiling.

Key process points:

Bonding area treatment: during lamination of rigid and flexible substrates, avoid bubbles and peeling, peel strength greater than or equal to 0.8 N per mm.

Stress release: design serpentine routing in flexible areas to absorb bending stress.

Positioning accuracy: alignment deviation between rigid and flexible areas less than or equal to plus or minus 0.03 mm.

Form characteristics: local rigidity and local flexibility, 360 degree bending in flexible areas, dimensional stability in rigid areas.

Application scenarios: high-end phones, drones, medical endoscopes, and automotive electronics. For example, drone gimbal PCB has a rigid area carrying the image sensor and a flexible area connecting the body, adapting to gimbal rotation. Medical endoscope PCB has a flexible area extending into the human body and a rigid area carrying control chips.

IV. High-Frequency PCB Process (Low-Loss Signal Transmission)

High-frequency PCB process targets signals above 1 GHz. Through special substrates, impedance control, and low roughness treatment, it reduces signal attenuation and reflection.

Manufacturing flow: low-loss substrate cutting, high-precision etching with line width tolerance plus or minus 0.01 mm, impedance calibration, high-frequency surface treatment such as silver plating and immersion gold, shielding design, testing.

Key process points:

Impedance control: microstrip line width calculation, error less than or equal to plus or minus 0.01 mm, substrate dielectric constant fluctuation less than or equal to plus or minus 0.05.

Surface roughness: copper foil Ra less than or equal to 0.1 micrometer, reducing skin effect loss.

Shielding process: metal shielding cavity and grounding via array to suppress electromagnetic interference.

Technical indicators: insertion loss less than or equal to 0.5 dB per 100 mm at 10 GHz, return loss less than or equal to minus 15 dB, crosstalk less than or equal to minus 30 dB.

Application scenarios: 5G base stations, millimeter-wave radar, satellite communications, and WiFi 6E devices. For example, 77 GHz automotive radar PCB with PTFE substrate has an insertion loss of 0.8 dB per 100 mm at 28 GHz. 5G base station Massive MIMO PCB with Rogers 4350B has an insertion loss of 0.3 dB per 100 mm at 3.5 GHz.

V. HDI PCB Process (High-Density Interconnect)

HDI PCB process achieves high integration and miniaturization through micro via holes, dense wiring, and laser drilling, adapting to high-end electronic equipment.

Manufacturing flow: substrate cutting, laser drilling with hole diameter less than or equal to 0.15 mm, copper deposition, electroplating, lamination, etching, solder mask, testing.

Key process points:

Laser drilling: hole diameter precision plus or minus 0.005 mm, hole wall roughness Ra less than or equal to 1.6 micrometers.

Micro via metallization: hole wall copper thickness greater than or equal to 20 micrometers, no voids, void rate less than or equal to 1 percent.

High-density wiring: line width and spacing less than or equal to 0.1 mm, wiring density greater than or equal to 100 points per square centimeter.

Technical indicators: interconnection density is 3 to 5 times that of ordinary PCB, and size is reduced by more than 40 percent.

Application scenarios: flagship phones, tablets, high-end servers, and FPGA packaging. For example, a flagship phone motherboard with an 8-layer HDI board has a laser drill hole diameter of 0.1 mm and line width and spacing of 0.08 mm. It integrates a 5G baseband, camera module, and fast charging circuit, with an area of only 8 square centimeters.

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