What Is a Flexible Printed Circuit Board?
Flexible Printed Circuit Board, abbreviated as “flexible board” and commonly known as FPC in the industry, is a printed circuit board made of flexible insulating substrates, mainly polyimide or polyester films. It has many advantages that rigid printed circuit boards do not possess. For example, it can be freely bent, coiled, and folded. Under normal operating conditions, it can withstand more than 100,000 dynamic bending and folding operations and still ensure the stable operation of the circuit. The use of FPC can greatly reduce the volume of electronic products, meeting the needs of the development of electronic products towards high density, miniaturization, and high reliability.
By Layers: Single-sided, Double-sided, and Multi-layer flex PCBs
Based on the number of layers present in a flexible printed circuit board, they can be classified into single-sided, double-sided, and multi-layered PCBs. Each type has its advantages and features. While a higher number of layers allows a connection of more components, the complications, and overall costs also increase along with. Therefore, engineers at QFPCB study the project requirements carefully to find the best-suited type of flexible printed circuit board for an application.
Single-sided flex PCBs: A single-sided PI (Polyimide) flexible copper-clad board material is used, with conductive traces laid on one side. The desired circuit pattern is formed on the surface through specific processes, such as etching. The circuit connections and signal transmission are all completed on this side. Cover with a protective film, resulting in a flexible printed circuit board with only one layer of conductor.
Double-sided flex PCBs: A double-sided PI (Polyimide) copper-clad board material is used, with the circuits on both sides connected through plated-through holes (PTH), enabling more complex circuit functions. The vias are small holes drilled through the substrate, and through processes like electroplating, the inner walls are made conductive, connecting the circuits on the top and bottom layers to form a complete electrical path，and both sides are covered with a protective film.
Multilayer flex PCBs:Multilayer FPC (Flexible Printed Circuit Board) is made by stacking multiple layers of base material, copper foil, and adhesive layers, along with a cover film. Three or more single-sided or double-sided flexible circuit layers are laminated together, and metalized holes are formed by drilling and electroplating to create conductive pathways between different layers. Multilayer FPCs allow for more complex routing and signal isolation, but the flexibility is relatively reduced, and the cost is higher. They are commonly used in devices that require high-density connections in limited spaces,such as robotics, industrial equipment, and medical equipment.
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Basic Layer Structure of FPC (Flexible Printed Circuit)
The structure of an FPC (Flexible Printed Circuit) is typically made up of multiple layers of materials to meet the needs of different circuit designs. Below are the common layers of an FPC:
1. Substrate Layer:
The substrate is the most fundamental layer in an FPC, usually made from materials such as Polyimide (PI) or Polyester (PET), which provide excellent flexibility, heat resistance, and mechanical strength. The thickness of the substrate typically ranges from 0.0125mm to 0.1mm, and it can be selected according to the specific requirements.
Polyimide (PI): Known for its high heat resistance and insulation properties, it is often used in applications where high heat resistance is required.
Polyester (PET): A cost-effective material with good flexibility and processability but relatively lower heat resistance, typically used in general consumer electronics.
2. Copper Layer:
The copper layer serves as the conductive layer of the FPC, typically made from electrolytic copper foil or rolled copper foil, with thickness ranging from 12μm to 70μm. The thickness of the copper layer directly affects the conductivity and durability, and should be chosen based on the actual current requirements and flexibility needs.
Electrolytic Copper Foil: Electrolytic copper foil（ED copper foil） is a copper foil produced by an electrodeposition process, with one side having a glossy surface and the other side having a special treatment to give it a dull and matte appearance. This copper foil has excellent flexibility and can be made into different thicknesses and widths according to requirements. In addition, the specially treated dull side surface can significantly improve its bonding ability. On the other hand, forged copper foil not only maintains flexibility, but also has hard and smooth characteristics, which is particularly suitable for occasions that require dynamic deflection.
Rolled Copper Foil: Rolled copper foil(Calendered copper foil) is created by pressing copper. This type of copper foil boasts have excellent electrical conductivity, making it ideal, for transmitting high frequency signals. Compared to electrolytic copper Rolled copper foils feature compact copper molecules rendering them more resistant to bending. However, the calendering process is more intricate and expensive with a range than electrolytic copper foil.
3. Coverlay:
The coverlay layer is used to protect the copper traces from external influences such as moisture, dust, and mechanical damage. The coverlay is typically made from the same material as the substrate, such as Polyimide or Polyester, with thickness ranging from 0.0125mm to 0.05mm. The coverlay forms an insulating protective layer on the surface of the flexible circuit and can also have openings for soldering and electrical connections.
4. Adhesive Layer:
The adhesive layer is primarily used to bond different layers of materials together. Common adhesive materials include acrylic and epoxy resin, both of which perform well in terms of heat resistance, chemical resistance, and flexibility. The thickness of the adhesive layer is generally between 0.01mm and 0.05mm, and the strength and flexibility of the bond should be considered during design.
5. Solder Mask Layer:
The solder mask layer protects the soldering areas of the circuit, preventing solder from flowing and shorting the traces during the soldering process. Solder mask layers are typically applied using green, blue, or transparent solder mask ink.
6. Surface Treatment Layer:
The surface treatment layer is applied at the contact points or endpoints of the FPC to improve the conductivity and corrosion resistance of these points. Common surface treatments include gold plating, tin plating, and nickel plating. Specifically, the gold fingers (metal contact areas) often undergo gold plating to ensure wear resistance and conductivity.
Flex PCB and Rigid-flex: advantages and disadvantages
Flexible printed circuit board (FPC) and rigid-flex board and HDI flexible PCBs are both types of flexible circuit boards. The main difference between them is the physical strength. In FPC or HDI flexible PCBs, a flexible substrate is mainly used, and the characteristics of a flexible circuit board are realized through a combination of different reinforcement layers and adhesive layers. In a rigid-flex board, a hard FR-4 or ceramic substrate is combined with a flexible FPC circuit board to achieve the combination of the advantages of flexibility and hardness.
Flexible FPC Circuit Boards
Flexible FPC boards(or HDI flexible PCBs) are a type of circuit board typically made from flexible materials such as Polyimide (PI). FPC circuit boards are known for their high flexibility and reliability, with the ability to achieve three-dimensional free bending. They are widely used in electronic devices where flexible circuit boards and space-saving designs are required.
Advantages:
Compared to rigid circuit boards, flexible FPCs(or HDI flex PCB) offer several advantages. First, FPCs provide excellent flexibility and bendability, allowing them to adapt to complex three-dimensional structures, significantly enhancing the design freedom of electronic devices. Additionally, FPCs offer higher density and shorter wiring lengths, improving their ability to resist interference and enabling faster signal transmission speeds. FPCs also enable lightweight designs, saving both device costs and space. Furthermore, FPCs typically use pin connectors, which are easy to implement and facilitate easier maintenance.
Disadvantages:
FPCs(or HDI FPC PCBs) also have some drawbacks. For instance, compared to rigid circuit boards, the manufacturing process for FPCs is more complicated, making them more expensive. Additionally, since FPCs are often required to bend during use, this bending could lead to fatigue and potential performance degradation over time.
Applications:
Flexible FPC circuit boards(or HDI FPC circuit boards) are widely used in various fields such as mobile phones, tablets, cameras, handheld computers, automotive displays, LED lighting, medical devices, aerospace, and industrial automation. Specific applications include keyboards, screens, sensors, audio and video devices, and even implantable medical devices.
Rigid-Flex FPC Circuit Boards
Rigid-Flex Printed Circuit Boards (or HDI Rigid-Flex circuit boards) are hybrid circuit boards that combine both rigid and flexible PCB technologies. The core part consists of a rigid board made from FR-4, ceramic, or other materials, seamlessly integrated with a flexible FPC circuit board. RFPCBs leverage the advantages of both rigid and flexible boards to meet the requirements of specialized applications. For example, in high-speed and high-density data transmission applications, RFPCBs offer high reliability and reduced noise interference, making them an ideal solution.
Advantages:
Compared to standalone rigid PCBs or flexible FPCs, HDI RFPCBs provide several advantages. First, RFPCBs can be customized in size and shape to accommodate application-specific requirements, making them particularly suitable for designs with unique shapes and space constraints. Additionally, RFPCBs offer greater structural rigidity, allowing them to withstand higher mechanical stress and weight. Moreover, the circuit layout of RFPCBs is more compact, enabling higher circuit density and better signal transmission performance, ultimately enhancing system reliability and performance.
Disadvantages:
Despite their benefits, RFPCBs(or HDI RFPC PCBs) also have some drawbacks. These include higher manufacturing costs, increased design complexity, and more complicated repair processes. Additionally, while the flexible portions of RFPCBs can bend, their bend radius is limited compared to standalone FPCs, imposing some restrictions on their maximum bending angle.
Applications:
RFPCBs (or HDI Rigid-Flexible PCBs) are widely used in high-end electronics and specialized applications. Key industries include medical devices, aerospace, industrial automation, automotive electronics, humanoid robotics, consumer electronics, and telecommunications equipment. For example, in aviation-related devices, RFPCBs enable designs that accommodate angled, curved, or bent surfaces while maintaining compact assembly spaces. In medical devices, RFPCBs integrate both macro-level structural requirements and micro-level electronic design needs, ensuring safety, stability, and portability, while enabling innovation and functional upgrades.
Design Considerations for Flexible Printed Circuit (FPC) Boards
Selection of Appropriate Materials and Thickness:
Based on the bending requirements, conductivity needs, and operating environment of the product, it is crucial to select the appropriate base material, copper foil thickness, and adhesive layer materials and thickness. For applications involving high-frequency bending, it is recommended to use rolled copper foil and thin substrates to ensure the flexibility and durability of the FPC board.
FPC Proper Via Design:
In multilayer FPC structures, vias are used for interlayer connections, and their design must ensure reliability and stability. It is important to avoid placing vias within a minimum distance of 0.8mm from the board outline to prevent breakage due to bending. Additionally, via-in-pad design is not recommended for FPC, as resin-filled vias are not feasible, which may result in solder leakage.
FPC Circuit Design
(1) Large Copper Area Oxidation: When designing large copper areas, air is difficult to expel during coverlay lamination. The moisture in the trapped air reacts with the copper surface under high temperature and pressure, leading to oxidation and affecting the appearance, though functionality remains intact. To mitigate this issue, it is recommended to use a mesh copper pattern with a 45-degree orientation for better signal transmission. The suggested line width/spacing is 0.2/0.2mm, or alternatively, a solder mask opening can be added to the large copper area.
(2) Avoiding Independent Pads: As shown in the diagram, the pads are independent and overlap on both sides. Since the FPC core material is only 25µm thick, the pads can easily detach. It is recommended to add copper pour around the pads and connect them with traces at the corners to enhance adhesion. Additionally, the top and bottom pads should be offset to improve bonding strength.
(3) Edge Gold Finger Design: During laser cutting, high temperatures can cause carbonization at the board edges, leading to micro-shorts between gold fingers. To prevent this, gold fingers should be designed at least 0.2mm away from the board outline. Furthermore, vias within the soldering area of the gold fingers should not be arranged in a straight line to avoid stress concentration, which may cause via breakage.
4. FPC Panelization Design
(1) Panelizing Heavy Boards with Steel Stiffeners: When an entire panel consists of steel stiffeners, the weight may cause the FPC to stretch and deform, making SMT assembly difficult. It is recommended to maintain a minimum 3mm spacing between boards with steel stiffeners, with slot widths of 0.5mm and connection points of 1mm, adding a connection point approximately every 15mm.
(2) Placement of Panel Connection Points: Connection points should not be placed on gold fingers, as this may result in uneven edges at the front end of the gold fingers.
(3) Number of Connection Points: If there are too few connection points, the boards may detach easily. Each PCB should have at least two connection points, with a minimum width of 0.8mm. The number of connection points should be adjusted based on the board size—the larger the board, the more connection points required.
(4) Handling Small Boards: If the board is too small and has too many connection points, depaneling can become difficult. If SMT is not required, each board should have only two 0.3mm-wide connection points to facilitate manual depaneling.
(5) Avoiding Board Loss During Laser Dust Extraction: Small-sized boards may be sucked away by the dust extraction system during laser cutting. For boards smaller than 20×20mm, it is recommended to deliver them in panelized form or depanel them only after production is complete.
5. FPC Stiffener Design
Stiffeners in flexible circuits reinforce specific areas to facilitate assembly. PI stiffeners are suitable for gold-finger insertion applications, FR4 is used for lower-end products, and steel stiffeners provide excellent flatness and are ideal for chip-mounting applications. For more details, refer to the stiffener specification diagram.
(1) Avoid Using Steel Stiffeners for Plug-in Holes: Steel stiffeners may cause short circuits. Additionally, since steel exhibits weak magnetism, it is unsuitable for Hall-effect components. Steel stiffeners should also be avoided in gold-finger insertion applications.
(2) Specifying Overall Thickness for Gold Finger Stiffeners: When designing FPCs with gold-finger insertion, the total thickness requirement must be clearly specified. The total thickness is typically defined in the connector specification. The PI stiffener thickness should not be calculated by simply subtracting the FPC thickness from the total thickness.
(3) Stiffener Cutout Design: Stiffeners should avoid component holes or pads underneath them. It is best for the customer to define the cutout design. By default, stiffeners are cut out with a 0.3mm clearance around pads. If the remaining stiffener width is less than 2mm after cutout, the entire region will be left without a stiffener. Any special requirements should be clearly specified.
(4) Gold Finger Stiffener Height: The stiffener height should extend at least 1.0mm beyond the gold finger pads to prevent fractures during use.
(5) Electromagnetic Film Considerations: Electromagnetic films may conduct electricity on both sides. If the bottom layer of the film is not connected to the same network, it is recommended to remove the electromagnetic film design.
(6) Grounding Resistance for Electromagnetic Films: If there are specific grounding resistance requirements for the electromagnetic film, the grounding solder mask opening should be designed accordingly. If no specific requirement is provided, a default 1.0mm or larger solder mask opening will be added randomly. Note: An ungrounded electromagnetic film may absorb excessive electromagnetic waves, potentially affecting signal integrity, so prototyping is recommended for verification.
(7) Steel Stiffener Placement on Pads: Placing steel stiffeners on pads may result in short circuits.
(8) Minimum Stiffener Width: FR4 stiffeners with very narrow widths may break or carbonize easily. It is recommended to use PI or steel stiffeners instead if the width is less than 5mm. Additionally, the minimum width for adhesive-backed stiffeners should be at least 3mm.
(9) Stiffener or Adhesive Near SMT Pads: Stiffeners or adhesives should not be placed around SMT pads, as they may interfere with solder paste printing. If necessary, the assembly process should be adjusted so that SMT assembly is completed before attaching stiffeners or adhesives.
6. Board Thickness Specification
The overall board thickness includes the coverlay, copper thickness, and PI substrate thickness. If there are non-copper areas or regions without a coverlay, the board thickness may be reduced accordingly. Designers should pay close attention to these variations.
7. Manufacturing Process Adaptability
The stack-up structure design should consider manufacturability, avoiding overly complex layer configurations to reduce production difficulty and costs. During fabrication, material distribution across layers should be uniform to ensure consistency and reliability of the FPC.
