FPC soft board is a circuit form made on the flexible cut-off surface, which can have a cover layer or not (usually used to protect the FPC circuit). Because FPC soft board can bend, fold or repeat in many ways, it has the advantages of light, thin and flexible compared with ordinary hard board (pcb), so it is more and more widely used. So what should we pay attention to when designing.

In the design, FPC often needs to be used in conjunction with PCB. In the connection between the two, the methods of board-to-board connector, connector plus gold finger, HOTBAR, soft and hard combination board, and manual welding are usually used for connection. For different application environments, designers can adopt corresponding connection methods.

In practical applications, determine whether ESD shielding is required according to the application needs. When FPC flexibility requirements are not high, solid copper sheet and thick medium can be used to realize. When the flexibility requirement is high, copper skin grid and conductive silver paste can be used.

Due to the flexibility of FPC soft board, it is easy to break when bearing stress, so some special means are needed for FPC protection,

Common methods include:

1. The minimum radius of the internal corner on the flexible contour is 1.6mm. The greater the radius, the higher the reliability and the stronger the tear resistance. A line near the edge of the board can be added at the corner of the shape to prevent the FPC from being torn.

2. Cracks or grooves on the FPC must terminate in a circular hole with a diameter of not less than 1.5mm, and this requirement is also applicable when the FPC of two adjacent parts needs to be moved separately.

3. In order to achieve better flexibility, the bending area needs to be selected in the area with uniform width, and try to avoid the FPC width change and uneven routing density in the bending area.

4. STIfener, also known as stiffening plate, is mainly used to obtain external support. The materials used include PI, Polyester, glass fiber, polymer material, aluminum sheet, steel sheet, etc. Reasonable design of the location, area and material of the stiffening plate plays a great role in avoiding FPC tearing.

5. In the design of multi-layer FPC, the air gap delamination design is required for the areas that need to be frequently bent during the use of the product. Thin material PI material should be used as far as possible to increase the flexibility of the FPC and prevent the FPC from breaking during repeated bending.

6. If space allows, a double-sided adhesive fixation area should be designed at the connection between the golden finger and the connector to prevent the golden finger and the connector from falling off during bending.

7. The FPC positioning screen line shall be designed at the connection between the FPC and the connector to prevent the FPC from deflecting and not inserting in place during the assembly process. It is conducive to production inspection.

Due to the particularity of FPC, the following points should be noted when routing:

Wiring rules: give priority to ensure smooth signal wiring, take short, straight and less through holes as the principle, try to avoid long, thin and round wiring, take horizontal lines, vertical lines and 45-degree lines as the main line, avoid any angle line, and bend the part of the arc line, the above details are as follows:

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

2. Line distance: According to the current production capacity of most manufacturers, the design line distance (Pitch) is 0.10mm

3. Line edge distance: the distance between the outermost line and the FPC contour is designed to be 0.30mm, and the larger the space is, the better

4. Inner fillet: the minimum value of inner fillet on FPC profile is designed as radius R=1.5mm

5. The wire is perpendicular to the bending direction

6. The conductor shall pass through the bending area evenly

7. The wire shall cover the area of bending area as much as possible

8. There shall be no additional plating metal in the bending area (the wire in the bending area shall not be plated)

9. Consistent line width

10. The wiring of double-sided boards cannot overlap to form "I" shape

11. Minimize the number of layers in the bending area

12. There shall be no through hole and metallized hole in the bending area

13. The bending center axis shall be set at the center of the conductor. The material coefficient and thickness on both sides of the conductor shall be consistent as far as possible. This is very important in the application of dynamic bending.

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

15. The bending of the vertical plane shall be carried out by increasing the bending radius and reducing 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, the conductive silver foil layer should be added on the FPC according to the EMI measurement and the conductive silver foil should be grounded to prevent EMI.

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