1. Flexible Printed Circuit (FPC) is an electrical component fabricated on a flexible substrate, which may or may not include a cover layer (commonly utilized for powering the FPC). Given its ability to bend, fold, or move repetitively in various directions, FPCs offer advantages over standard rigid boards (PCBs), including lightweight, thinness, and flexibility, leading to their increasing applications.
2. The base layer materials for FPCs typically consist of polyimide (commonly known as PI), though polyester (referred to as PET) is also used. The material thickness generally comes in 12.5/25/50/75/125 micrometers, with 12.5 and 25 micrometers being the most frequently employed. For high-temperature soldering applications, PI is usually selected, while FR4 is commonly used as the base material for PCBs.
3. The cover layer (CoverLayer) of an FPC is a laminated structure made from a dielectric film and adhesive, or a coating of a flexible medium, serving to protect against contamination, moisture, scratches, and other environmental factors. The primary materials used are consistent with those of the base layer, namely polyimide and polyester, with a typical thickness of 12.5 micrometers.
4. FPC design requires bonding each layer together, necessitating the use of FPC adhesives (Adhesive). Common adhesives for flexible boards include acrylics, modified epoxies, phenolic butyrals, reinforced adhesives, and pressure-sensitive adhesives, although single-layer FPCs do not require adhesive bonding.
5. In various applications, such as device soldering, flexible boards often require stiffeners for external support. The primary materials employed include PI or polyester film, glass fiber, polymers, steel sheets, and aluminum sheets. PI or polyester films are standard reinforcement materials for flexible boards, typically with a thickness of 125 micrometers. Glass fiber (FR4) reinforcement offers greater hardness than PI or polyester, but it can be more challenging to process in applications where increased rigidity is necessary.
Compared to PCB pad processing methods, FPC pad processing offers various techniques, the most common of which include:
1. **Chemical Nickel Gold**: Also known as chemical immersion gold, this method typically uses an electroless nickel layer thickness of 2.5um-5.0um on the PCB copper surface, with immersion gold (99.9% pure) at 0.05um-0.1um. This replaces the gold in the PCB pool. **Technical advantages**: provides a flat surface, longer storage life, and easy solderability; ideal for fine-pitch components and thinner PCBs, making it especially suitable for FPC. **Disadvantages**: environmentally unfriendly.
2. **Tin-Lead Plating**: This allows for direct application of flat lead and tin to the pad, ensuring good solderability and uniformity. It is essential for certain processes like HOTBAR on FPC. **Disadvantages**: lead oxidation occurs easily, resulting in short storage life; requires pulling the electroplated wire; also not environmentally friendly.
3. **Selective Gold Electroplating (SEG)**: This involves using electroplated gold in specific areas of the PCB, with other areas undergoing different surface treatments. Initially, a nickel layer (2.5um-5.0um) is applied before electroplating gold (0.05um-0.1um). **Advantages**: thicker gold plating, excellent oxidation and wear resistance; often used for “Golden Fingers.” **Disadvantages**: environmentally unfriendly due to cyanide pollution.
4. **Organic Solderability Layer (OSP)**: This process involves covering the exposed copper surface of the PCB with specific organic materials. **Advantages**: provides a very flat surface and meets environmental standards; suitable for PCBs with fine-pitch components. **Disadvantages**: requires conventional wave soldering and selective wave soldering processes, which do not permit OSP treatments.
5. **Heat Air Leveling (HASL)**: This method covers the exposed metal surface of the PCB with a 63/37 lead-tin alloy, with coating thickness ranging from 1um-25um. Controlling the coating thickness and land pattern is challenging, making it unsuitable for fine-pitch components. It significantly impacts thin FPCs, so this treatment is not recommended.
In design, FPC is often used alongside PCB. Connections between the two typically utilize board-to-board connectors, gold finger connectors, HOTBAR, soft and hard boards, and manual soldering, allowing designers to choose appropriate methods for different applications.
In practical use, it’s essential to assess whether ESD shielding is necessary. If flexibility demands are low, solid copper and thicker media can be employed. For high flexibility requirements, a copper skin grid and conductive silver paste can be utilized.
Due to the inherent flexibility of FPC, stress can lead to breakage, necessitating specific methods:
1. Maintain a minimum inner corner radius of 1.6mm on flexible contours; larger radii enhance reliability and tear resistance. Adding a trace near the board edge can help prevent tearing.
2. Any crack or slot on the FPC must end in a round hole with a diameter of at least 1.5mm, especially when adjacent parts need independent movement.
3. To optimize flexibility, the bending area should have uniform width, avoiding variations in FPC width and uneven wiring density.
4. Stiffeners, or reinforcement, provide external support using materials like PI, polyester, fiberglass, polymer materials, aluminum, or steel. Thoughtful design of the reinforcement board, area, and material significantly reduces the risk of FPC tearing.
5. In multi-layer FPC design, frequent bending areas should include air gap layers. Thin PI materials should be used to enhance softness and prevent breakage during repeated bending.
6. Where space allows, incorporate a double-sided tape area at the junction of the gold finger and connector to prevent detachment during bending.
7. A positioning silk screen line should be established at the FPC-connector junction to prevent misalignment during assembly, facilitating production inspections.
Given the unique characteristics of FPC, consider the following wiring guidelines:
**Wiring Rules**: Prioritize smooth signal routing, adhering to the principles of short, straight paths with minimal perforation. Avoid long, thin, or circular wiring, favoring horizontal, vertical, and 45-degree lines, and steering clear of sharp angles. Detailed conditions include:
1. **Line Width**: Due to differing requirements for data and power lines, an average reserved wiring space of 0.15mm is recommended.
2. **Line Pitch**: Based on current manufacturing capabilities, a design line pitch of 0.10mm is advised.
3. **Line Margin**: Design a distance of 0.30mm between the outermost line and the FPC contour; larger spaces are preferable.
4. **Internal Fillet**: The minimum internal fillet on the FPC profile should be a radius of R=1.5mm.
5. **Wire Orientation**: Wires should be perpendicular to the bending direction.
6. **Wire Distribution**: Wires must pass through the bending area uniformly.
7. **Bending Area Filling**: Maximize wire presence in the bending region.
8. **Plating Restrictions**: No additional plating metal should be present in the bending area (unplated wires only).
9. **Consistent Width**: Maintain uniform line width.
10. **Trace Configuration**: Traces on double panels should not overlap to form an “I” shape.
11. **Layer Minimization**: Reduce the number of layers within the bending area.
12. **Via Restrictions**: The bending area should avoid via holes and metallized holes.
13. **Center Axis Alignment**: Set the bending center axis at the wire’s center, ensuring consistent material coefficients and thickness on both sides; crucial for dynamic bending.
14. **Horizontal Torsion**: Minimize the bending section to enhance flexibility, or selectively increase copper foil area for improved toughness.
15. **Vertical Bending**: Increase bending radius and reduce layer count in the bending’s central area.
16. **EMI Considerations**: For products requiring EMI shielding, if high-frequency signal lines (like USB, MIPI) are present, incorporate a conductive silver foil layer on the FPC, grounded to mitigate EMI.
As FPC applications expand, this information may evolve, but careful design and thoughtful consideration will make FPC design a manageable task.
2. The base layer materials for FPCs typically consist of polyimide (commonly known as PI), though polyester (referred to as PET) is also used. The material thickness generally comes in 12.5/25/50/75/125 micrometers, with 12.5 and 25 micrometers being the most frequently employed. For high-temperature soldering applications, PI is usually selected, while FR4 is commonly used as the base material for PCBs.
3. The cover layer (CoverLayer) of an FPC is a laminated structure made from a dielectric film and adhesive, or a coating of a flexible medium, serving to protect against contamination, moisture, scratches, and other environmental factors. The primary materials used are consistent with those of the base layer, namely polyimide and polyester, with a typical thickness of 12.5 micrometers.
4. FPC design requires bonding each layer together, necessitating the use of FPC adhesives (Adhesive). Common adhesives for flexible boards include acrylics, modified epoxies, phenolic butyrals, reinforced adhesives, and pressure-sensitive adhesives, although single-layer FPCs do not require adhesive bonding.
5. In various applications, such as device soldering, flexible boards often require stiffeners for external support. The primary materials employed include PI or polyester film, glass fiber, polymers, steel sheets, and aluminum sheets. PI or polyester films are standard reinforcement materials for flexible boards, typically with a thickness of 125 micrometers. Glass fiber (FR4) reinforcement offers greater hardness than PI or polyester, but it can be more challenging to process in applications where increased rigidity is necessary.
Compared to PCB pad processing methods, FPC pad processing offers various techniques, the most common of which include:
1. **Chemical Nickel Gold**: Also known as chemical immersion gold, this method typically uses an electroless nickel layer thickness of 2.5um-5.0um on the PCB copper surface, with immersion gold (99.9% pure) at 0.05um-0.1um. This replaces the gold in the PCB pool. **Technical advantages**: provides a flat surface, longer storage life, and easy solderability; ideal for fine-pitch components and thinner PCBs, making it especially suitable for FPC. **Disadvantages**: environmentally unfriendly.
2. **Tin-Lead Plating**: This allows for direct application of flat lead and tin to the pad, ensuring good solderability and uniformity. It is essential for certain processes like HOTBAR on FPC. **Disadvantages**: lead oxidation occurs easily, resulting in short storage life; requires pulling the electroplated wire; also not environmentally friendly.
3. **Selective Gold Electroplating (SEG)**: This involves using electroplated gold in specific areas of the PCB, with other areas undergoing different surface treatments. Initially, a nickel layer (2.5um-5.0um) is applied before electroplating gold (0.05um-0.1um). **Advantages**: thicker gold plating, excellent oxidation and wear resistance; often used for “Golden Fingers.” **Disadvantages**: environmentally unfriendly due to cyanide pollution.
4. **Organic Solderability Layer (OSP)**: This process involves covering the exposed copper surface of the PCB with specific organic materials. **Advantages**: provides a very flat surface and meets environmental standards; suitable for PCBs with fine-pitch components. **Disadvantages**: requires conventional wave soldering and selective wave soldering processes, which do not permit OSP treatments.
5. **Heat Air Leveling (HASL)**: This method covers the exposed metal surface of the PCB with a 63/37 lead-tin alloy, with coating thickness ranging from 1um-25um. Controlling the coating thickness and land pattern is challenging, making it unsuitable for fine-pitch components. It significantly impacts thin FPCs, so this treatment is not recommended.
In design, FPC is often used alongside PCB. Connections between the two typically utilize board-to-board connectors, gold finger connectors, HOTBAR, soft and hard boards, and manual soldering, allowing designers to choose appropriate methods for different applications.
In practical use, it’s essential to assess whether ESD shielding is necessary. If flexibility demands are low, solid copper and thicker media can be employed. For high flexibility requirements, a copper skin grid and conductive silver paste can be utilized.
Due to the inherent flexibility of FPC, stress can lead to breakage, necessitating specific methods:
1. Maintain a minimum inner corner radius of 1.6mm on flexible contours; larger radii enhance reliability and tear resistance. Adding a trace near the board edge can help prevent tearing.
2. Any crack or slot on the FPC must end in a round hole with a diameter of at least 1.5mm, especially when adjacent parts need independent movement.
3. To optimize flexibility, the bending area should have uniform width, avoiding variations in FPC width and uneven wiring density.
4. Stiffeners, or reinforcement, provide external support using materials like PI, polyester, fiberglass, polymer materials, aluminum, or steel. Thoughtful design of the reinforcement board, area, and material significantly reduces the risk of FPC tearing.
5. In multi-layer FPC design, frequent bending areas should include air gap layers. Thin PI materials should be used to enhance softness and prevent breakage during repeated bending.
6. Where space allows, incorporate a double-sided tape area at the junction of the gold finger and connector to prevent detachment during bending.
7. A positioning silk screen line should be established at the FPC-connector junction to prevent misalignment during assembly, facilitating production inspections.
Given the unique characteristics of FPC, consider the following wiring guidelines:
**Wiring Rules**: Prioritize smooth signal routing, adhering to the principles of short, straight paths with minimal perforation. Avoid long, thin, or circular wiring, favoring horizontal, vertical, and 45-degree lines, and steering clear of sharp angles. Detailed conditions include:
1. **Line Width**: Due to differing requirements for data and power lines, an average reserved wiring space of 0.15mm is recommended.
2. **Line Pitch**: Based on current manufacturing capabilities, a design line pitch of 0.10mm is advised.
3. **Line Margin**: Design a distance of 0.30mm between the outermost line and the FPC contour; larger spaces are preferable.
4. **Internal Fillet**: The minimum internal fillet on the FPC profile should be a radius of R=1.5mm.
5. **Wire Orientation**: Wires should be perpendicular to the bending direction.
6. **Wire Distribution**: Wires must pass through the bending area uniformly.
7. **Bending Area Filling**: Maximize wire presence in the bending region.
8. **Plating Restrictions**: No additional plating metal should be present in the bending area (unplated wires only).
9. **Consistent Width**: Maintain uniform line width.
10. **Trace Configuration**: Traces on double panels should not overlap to form an “I” shape.
11. **Layer Minimization**: Reduce the number of layers within the bending area.
12. **Via Restrictions**: The bending area should avoid via holes and metallized holes.
13. **Center Axis Alignment**: Set the bending center axis at the wire’s center, ensuring consistent material coefficients and thickness on both sides; crucial for dynamic bending.
14. **Horizontal Torsion**: Minimize the bending section to enhance flexibility, or selectively increase copper foil area for improved toughness.
15. **Vertical Bending**: Increase bending radius and reduce layer count in the bending’s central area.
16. **EMI Considerations**: For products requiring EMI shielding, if high-frequency signal lines (like USB, MIPI) are present, incorporate a conductive silver foil layer on the FPC, grounded to mitigate EMI.
As FPC applications expand, this information may evolve, but careful design and thoughtful consideration will make FPC design a manageable task.
