Flexible Printed Circuit (FPC)

Overview

FPC, also known as Flexible Printed Circuit, is favored for its lightweight, thin profile, and ability to bend and fold freely. With the rapid development of the electronics industry, circuit board design is increasingly moving towards high precision and high density. Traditional manual inspection methods can no longer meet production needs, leading to the inevitable trend of automatic defect detection in FPC manufacturing.

FPC is made from flexible substrates, which enable it to bend and fold without compromising performance. This makes FPC ideal for compact spaces and complex shapes in electronic devices such as wearables, foldable phones, and camera modules.

Key Features and Applications of FPC

Flexibility and Thin Design: FPC can bend and fold, making it suitable for compact and complex electronic devices.

Lightweight: Its flexible substrate makes FPC lighter than traditional rigid boards, advantageous for applications requiring lightweight components, like aerospace and portable electronics.

High-Density Connections: FPC supports high-density electrical connections and wiring, suitable for complex circuit designs, particularly in devices like flat panel displays and digital cameras.

Vibration and Shock Resistance: The flexible nature of FPC offers good resistance to vibrations and shocks, making it suitable for high-reliability applications such as automotive electronics and medical devices.

Automated Manufacturing: FPC production utilizes similar processes to printed circuit boards (PCBs), allowing for automated manufacturing and increased production efficiency.

Applications

FPC is widely used across various fields, including:

Mobile Devices: Foldable phones, tablets, etc.

Medical Equipment: Medical sensors, imaging devices, etc.

Automotive Electronics: In-vehicle displays, cameras, etc.

Aerospace: Avionics, internal connections in spacecraft, etc.

Consumer Electronics: Digital cameras, headphones, smartwatches, etc.

Composition of FPC Materials

Insulating Film

The insulating film forms the base layer of the circuit, with adhesives bonding copper foil to the insulating layer. In multilayer designs, it is bonded to inner layers, protecting the circuit from dust and moisture, and reducing stress during bending. Copper foil forms the conductive layer.

In some flexible circuits, rigid elements made from aluminum or stainless steel provide dimensional stability, physical support for component placement, and eliminate stress. Adhesives bond these rigid elements to the flexible circuit. Additionally, bonding layers can be applied on both sides of the insulating film for environmental protection and electrical insulation.

Common insulating materials include polyimide and polyester. Approximately 80% of flexible circuit manufacturers in the U.S. use polyimide, while about 20% use polyester. Polyimide is non-flammable, geometrically stable, has high tear strength, and withstands soldering temperatures. Polyester, also known as PET, is similar in physical properties but has a lower dielectric constant and is less moisture-absorbing, with a melting point of 250°C and a glass transition temperature of 80°C.

Conductors

Copper foil is suitable for flexible circuits and is available in electrode (ED) or plated forms. One side of ED copper foil is shiny, while the other is dull. It is a flexible material available in various thicknesses and widths, with the matte side often treated to enhance adhesive bonding.

Adhesives

Adhesives bond the insulating film to conductive materials and can also serve as protective coatings. The main difference between adhesive and cover layers lies in their application methods. Not all laminated structures contain adhesives; adhesive-free laminates form thinner circuits with greater flexibility and improved thermal conductivity.

FPC Manufacturing Processes

FPC types include single-sided, double-sided, and multilayer boards.

Double-sided or multilayer board process

Cutting → Drilling → PTH → Plating → Pretreatment → Dry Film Lamination → Alignment → Exposure → Development → Graphic Plating → Stripping → Pretreatment → Dry Film Lamination → Alignment Exposure → Development → Etching → Stripping → Surface Treatment → Adhesive Cover Film → Pressing → Curing → Nickel-Gold Plating → Printing → Cutting → Electrical Testing → Punching → Final Inspection → Packaging → Shipping.

Single-sided board process

Cutting → Drilling → Dry Film Lamination → Alignment → Exposure → Development → Etching → Stripping → Surface Treatment → Cover Film → Pressing → Curing → Surface Treatment → Nickel-Gold Plating → Printing → Cutting → Electrical Testing → Punching → Final Inspection → Packaging → Shipping.

These processes can be adjusted based on actual conditions.

Advantages and Disadvantages of FPC

Advantages of Flexible Printed Circuits

Bendable and Foldable: FPC can be freely bent and arranged according to spatial layout requirements, allowing for integrated assembly and electrical connections in three-dimensional space.

Compact Size: FPC significantly reduces the size and weight of electronic products, suitable for high-density, miniaturized, and highly reliable applications, widely used in aerospace, military, mobile communication, laptops, peripherals, PDAs, digital cameras, etc.

Good Thermal Conductivity and Solderability: FPC is easy to assemble and has lower overall costs. The combination of soft and rigid designs partially compensates for the slight limitations of flexible substrates in component load capacity.

Disadvantages of Flexible Printed Circuits

High Initial Costs: Due to the custom design and manufacturing for specific applications, initial circuit design, wiring, and photographic masters incur higher costs. It is generally not advisable to use flexible PCBs for low-volume applications unless there is a specific need.

Difficult Replacement and Repair: Once made, flexible PCBs require changes from the original layout or pre-set programming, making modifications challenging. The protective film covering must be removed for repairs, adding complexity.

Size Limitations: Flexible printed circuits are typically produced using batch methods, limiting their length and width due to manufacturing equipment constraints.

Fragility: Improper handling during installation and connection can damage the circuits, and soldering and reworking require trained personnel.

FPC Soldering Procedures

Apply flux to the pads before soldering and use a soldering iron to treat them to prevent poor tinning or oxidation.

Carefully place the PQFP chip on the PCB with tweezers, aligning it with the pads. Adjust the soldering iron temperature to over 300°C, applying a small amount of solder to the tip before pressing down on the aligned chip and soldering two diagonal pins to secure it.

Begin soldering all pins by applying solder to the tip of the soldering iron and wetting all pins with flux. Touch the tip of the soldering iron to the end of each pin until the solder flows into place.

After soldering, clean the pins with flux, removing excess solder to eliminate shorts and overlaps. Use tweezers to check for soldering defects, and clean the circuit board with alcohol and a stiff brush to remove residual solder.

SMD resistors and capacitors are relatively easy to solder, requiring proper alignment and soldering one end before checking placement.

FPC Gold Finger Technology

Zero Insertion Force (ZIF) connectors are used to connect flexible circuits (FPC) to mainboards or other electronic components. This technology allows insertion or removal of FPC without applying extra force, hence the name “zero insertion force.”

Key Aspects of ZIF Connections

Mechanism: ZIF connectors feature a clamping socket where the gold fingers of the FPC insert without additional force, making it user-friendly and reducing wear.

Gold Fingers: The end of the FPC typically features gold fingers formed from metal conductors that correspond with the pins of the socket.

Alignment: Correct alignment of the gold fingers with the socket pins is crucial to ensure reliable connections.

Applications: ZIF connectors are widely used in portable devices, digital cameras, and medical equipment, providing reliable connections and reducing wear on components.

Considerations

Care must be taken during ZIF connections to avoid bending the FPC, ensure the cleanliness of gold fingers and pins, and limit the number of insertions to maintain connection reliability.

FPC Simulation

As signal rates increase, especially in high-speed consumer products like phones and laptops, FPC simulation becomes vital. To mitigate signal interference, solid copper is often used as a reference beneath signal lines. Proper design is essential for effective simulation.

Protection Methods for FPC

Minimum Bend Radius: The minimum inner corner radius on flexible profiles should be 1.6 mm; larger radii enhance reliability and tear resistance.

Cracks or Slots: Must end in round holes with a diameter of at least 1.5 mm, necessary for adjacent flexible circuit sections that need to move independently.

Flexibility: Bend areas should have uniform widths, with variations in width and trace density minimized for better flexibility.

Stiffeners: Used for external support, materials include PI, polyester, fiberglass, polymers, aluminum, and steel. Proper design of the stiffener’s position, area, and material greatly aids in preventing tearing.

Layering: In multilayer Flexible Printed Circuit designs, gap layering in areas subject to frequent bending is advisable. Thin PI materials can enhance flexibility and prevent cracking during repeated bends.

Double-sided adhesive: Should be designed at the junction between gold fingers and connectors to prevent detachment during bending.

Positioning: Positioning features should be incorporated between the FPC and connectors to prevent misalignment during assembly.

conclusion

Flexible Printed Circuit (FPC) are highly reliable, lightweight, and flexible circuit boards made from materials like polyimide and polyester. They offer advantages over traditional rigid boards, including high wiring density, excellent thermal conductivity, and adaptability to compact spaces, making them ideal for applications in mobile devices, medical equipment, automotive electronics, and more. The production process involves sophisticated techniques, and while FPCs reduce size and weight, they can be more costly to produce and difficult to repair. With technological advancements, Flexible Printed Circuit continue to play a crucial role in the evolving electronics landscape.