Hardware design in action first FPC design Raiders

- Dec 05, 2019-

Hardware design in action first FPC design Raiders


FPC Flexible Printed Circuit (FPC Flexible Printed Circuit) is a circuit form made on a flexible cut-off surface, with or without a cover layer (usually used to protect FPC circuits). Because FPC can perform bending, folding or repetitive motion in a variety of ways, compared to ordinary rigid boards (PCBs), it has the advantages of lightness, thinness, flexibility, etc., so its applications are becoming more and more widespread.


PCThe base film of FPC is generally made of polyimide (PI), and polyester is also useful.

(Polyester, PET for short), the thickness of the material is 12.5 / 25/50/75 / 125um, usually 12.5 and 25um. If the FPC needs to be soldered at high temperature, the material is usually PI, and the PCB substrate is usually FR4.

The cover layer of FPC is a laminated body of dielectric film and glue, or a coating of flexible medium, which has the protective effect of avoiding contamination, humidity, scratches, etc. The main material is the same as the base material, that is, polyimide amine

Polyimide and Polyester, the common material thickness is 12.5um.


FPC design needs to bond the layers together, and FPC adhesive (Adhesive) is needed at this time. Common adhesives for flexible boards are acrylic, modified epoxy, modified butyrals, reinforced adhesives, pressure-sensitive adhesives, etc., and single-layer FPC does not use adhesives for bonding.


In many applications such as device welding, flexible boards need to be reinforced (Stiffener) to obtain external support. The main materials are PI or Polyester film, glass fiber, polymer material, steel sheet, aluminum sheet, etc. PI or Polyester film is a commonly used material for flexible board reinforcement, and its thickness is generally 125um. Glass fiber (FR4) reinforced board has higher hardness than PI or Polyester. It is used in places that require harder, and it is relatively difficult to process.


Compared to the PCB pad processing method, there are many methods for FPC pad processing. The following are common:

Chemical nickel gold is also called chemical immersion gold or immersion gold. Generally, the thickness of the non-electrolytic nickel layer used on the copper surface of the PCB is 2.5um-5.0um, and the thickness of the immersion gold (99.9% pure gold) layer is 0.05um-0.1um (previously it was reported that a PCB factory worker uses the replacement method to Replace the gold in the pcb pool). Advantages of this technology: flat surface, long storage time, easy to solder; suitable for fine-pitch components and thinner PCBs. For FPC, it is more suitable because it is thinner. Cons: Not environmentally friendly.

Tin-Lead Plating Advantages: You can directly add flat lead-tin to the pad, with good solderability and uniformity. For some processing technologies such as HOTBAR, this method must be adopted on FPC. Disadvantages: Lead is susceptible to oxidation and has a short storage time; it needs to be pulled and plated; it is not environmentally friendly.

Selective plating gold (SEG) Selective plating gold refers to the use of electroplated gold in some areas of the PCB and other surface treatment methods. Electroplated gold refers to the first coating of nickel on the copper surface of PCB, and then electroplating of gold. The thickness of the nickel layer is 2.5um-5.0um, and the thickness of the gold layer is generally 0.05um-0.1um. Advantages: The gold plating layer is thicker and has strong oxidation resistance and wear resistance. "Golden Finger" generally adopts this processing method. Disadvantages: not environmentally friendly, cyanide pollution.

Organic Solderability Protective Layer (OSP) This process refers to covering the bare PCB copper surface with a specific organic substance. Advantages: Can provide a very flat PCB surface and meet environmental protection requirements. Suitable for PCBs with fine pitch components.

Disadvantages: PCBA that requires conventional wave soldering and selective wave soldering welding processes is not allowed to use the OSP surface treatment process.

Hot air leveling (HASL) This process refers to covering the exposed metal surface of the PCB with 63/37 lead-tin alloy. The thickness of hot-air leveling lead-tin alloy coating is 1um-25um. The hot air leveling process is difficult to control the thickness of the plating layer and the pad pattern. It is not recommended for PCBs with fine-pitch components because the fine-pitch components have high pad flatness requirements; the hot air leveling process is for thin FPC The impact is large, this surface treatment is not recommended.

In design, FPC often needs to be used in conjunction with PCB. In the connection of the two, board-to-board connectors, connector gold fingers, HOTBAR, soft and hard board, and manual soldering are usually used for connection. For different applications Environment, designers can use the corresponding connection.

 In practical applications, determine whether ESD shielding is required according to the needs of the application. When FPC flexibility is not high, it can be realized with solid copper and thick dielectric. When high flexibility is required, it can be achieved by copper grid and conductive silver paste.

Because of the softness of FPC, it is easy to break when subjected to stress, so some special measures are needed for FPC protection.

The common methods are:


1. The minimum radius of the inner corner of the flexible contour is 1.6mm. The larger the radius, the higher the reliability and the stronger the tear resistance. The corner of the outline can be added with a trace near the edge of the board to prevent the FPC from being torn.

2. The crack or slot on the FPC must end in a circular hole with a diameter of not less than 1.5mm. This requirement is also required when the FPC of two adjacent parts needs to be moved separately.

3. In order to achieve better flexibility, the curved area needs to be selected in an area with a uniform width, and the FPC width variation and uneven routing density in the curved area should be avoided as much as possible.

4. Stiffener, also known as reinforcing plate, is mainly used to obtain external support. The materials used are PI, Polyester, glass fiber, polymer materials, aluminum sheet, steel sheet, etc. Reasonably designing the position, area and material of the reinforcing plate has a great effect on  avoiding FPC tearing.

5. When designing multi-layer FPC, air gap layered design is needed for the areas that need to be bent frequently during the use of the product. Try to use thin PI materials to increase the FPC softness and prevent the FPC from breaking during repeated bending.

6. Where space permits, a double-sided adhesive fixing area should be designed at the connection between the gold finger and the connector to prevent the gold finger and the connector from falling off during the bending process.

7. The FPC positioning silk screen should be designed at the connection between the FPC and the connector to prevent the FPC from being skewed or inserted in place during the assembly process. Conducive to production inspection.


Because of the special nature of FPC, the following points need to be paid attention to when routing:


Routing rules: Priority is provided to ensure smooth signal routing. Short, straight, and less punched holes are the principle. Try to avoid long, thin, and circular routing, mainly horizontal, vertical, and 45-degree lines. The angle line and the curved part follow the arc line. The above situation is explained in detail as follows:


1. Line width: Considering that the line width requirements of the data line and the power line are inconsistent, the reserved trace space is 0.15mm on average

2. Pitch: According to the current production capacity of most manufacturers, the design pitch is 0.10mm

3. Line margin: the distance between the outermost line and the FPC outline is designed as 0.30mm, the larger the space, the better

4. Fillet: The minimum value of the fillet on the FPC contour is designed as a radius R = 1.5mm

5. The wire is perpendicular to the bending direction

6. The wire should pass through the bending area evenly

7. Make the wire as large as possible

8. There must be no extra plated metal in the bend area (the wire in the bend area is not plated)

9. Keep line width consistent

10. The traces of the two panels cannot overlap to form an Ishape.

11. Minimize the number of layers in the curved area

12. Do not have vias or metallized holes in the curved area

13. The bending center axis should be set in the center of the wire. The material coefficient and thickness on both sides of the wire are as consistent as possible. This is very important in dynamic bending applications.

14. Horizontal torsion follows the following principles-reduce the bending section to increase flexibility, or partially increase the copper foil area to increase toughness.

15. For vertical plane bending, increase the bending radius and reduce the number of layers in the bending center area.

16. For products with EMI requirements, if there are high-frequency radiation signal lines such as USB and MIPI on the FPC, you should add a conductive silver foil layer on the FPC and ground the conductive silver foil to prevent EMI according to the EMI measurement.


With the expansion of the FPC application environment, the above content will continue to be enriched or not applicable, but as long as you design carefully in your work, think and summarize, I believe that it is not difficult to design FPC, and you can easily get started.

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