FPC flexible board is a form of circuit fabricated on a flexible finish surface, with or without a cover layer (usually used to protect FPC circuits). Because FPC soft board can be bent, folded or repeated movement in a variety of ways, compared with ordinary hard board (PCB), has the advantages of light, thin, flexible, so its application is more and more widely, so we need to pay attention to what we design, the following small make up to say in detail.
In the design, FPC often needs to be used with PCB, in the connection between the two usually adopt the board-to-board connector, connector and gold finger, HOTBAR, soft and hard combination board, manual welding mode for connection, according to different application environment, the designer can adopt the corresponding connection mode.
In practical applications, it is determined whether ESD shielding is needed according to application requirements. When FPC flexibility is not high, solid copper skin and thick medium can be used to achieve it. When the requirement of flexibility is high, copper mesh and conductive silver paste can be used
Due to the softness of FPC soft plate, it is easy to break under stress, so some special means are needed for FPC protection.
Common methods are:
1. The minimum radius of the inner Angle of the flexible contour is 1.6mm. The larger the radius, the higher the reliability and the stronger the tear resistance. A line can be added near the edge of the plate at the corner of the shape to prevent the FPC from being torn.
2. Cracks or grooves in the FPC must end in a circular hole not less than 1.5mm in diameter, even if two adjacent FPCS need 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 FPC width variation and uneven line density in the bending area.
STIffener board is used for external support. Materials STIffener board includes PI, Polyester, glass fiber, polymer, aluminum sheet, steel sheet, etc. Reasonable design of the position, area and material of the reinforcement plate plays a great role in avoiding FPC tearing.
5. In multi-layer FPC design, air gap stratification design should be carried out for areas that need frequent bending during the use of the product. Thin PI material should be used as far as possible to increase the softness of FPC and prevent FPC from breaking in the process of repeated bending.
6. If space permits, double-sided adhesive fixing area should be designed at the connection of gold finger and connector to prevent gold finger and connector from falling off during bending.
7. FPC positioning silk screen line should be designed at the connection between FPC and connector to prevent deviation and improper insertion of FPC during assembly. Conducive to production inspection.
Due to the particularity of the FPC, pay attention to the following points during cabling:
Routing rules: Give priority to ensuring smooth signal routing, follow the principle of short, straight and few holes, avoid long, thin and circular routing as far as possible, take horizontal, vertical and 45 degree lines as the main, avoid arbitrary Angle line, bend part of the radian line, the above details are as follows:
1. Line width: Considering that the line width requirements of data cable and power cable are inconsistent, the average space reserved for wiring is 0.15mm
2. Line spacing: According to the production capacity of most manufacturers, the design line spacing (Pitch) is 0.10mm
3. Line margin: the distance between the outermost line and FPC contour is designed to be 0.30mm. The larger the space allows, the better
4. Interior fillet: The minimum interior fillet on the FPC contour is designed as a radius R=1.5mm
5. The conductor is perpendicular to the bending direction
6. The wire should pass evenly through the bending area
7. The conductor should cover the bending area as much as possible
8. No extra plating metal in the bending area (the wires in the bending area are not plating)
9. Keep the line width the same
10. The cabling of the two panels cannot overlap to form an "I" shape
11. Minimize the number of layers in the curved area
12. There shall be no through holes and metallized holes in the bending area
13. The bending center axis shall be set at the center of the wire. The material coefficient and thickness on both sides of the conductor should be the same 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. The bending radius of vertical plane should be increased and the number of layers in the bending center should be reduced
16. For products with EMI requirements, if high frequency radiation signal lines such as USB and MIPI are on FPC, conductive silver foil layer should be added and grounded on FPC according to EMI measurement to prevent EMI.