The FPC Flexible Printed Circuit is a circuit form made on a flexible cut-off surface that can be either covered or not (usually used to protect FPC circuits). Since FPC can be bent, folded or repetitively moved in various ways, it is more and more widely used,
The base film of FPC is generally made of polyimide (Polyimide, PI for short) and polyester.
(Polyester, referred to as PET), the material thickness is 12.5/25/50/75/125um, commonly used 12.5 and 25um. If the FPC needs to be soldered at a high temperature, the material is usually made of PI, and the substrate of the PCB is usually FR4.
The cover layer of the FPC is made of a film of a dielectric film and a glue, or a coating of a flexible medium, which is protected from contamination, moisture, scratches, etc., and the main material is the same as the base material, that is, the polyimide. amine (Polyimide) and Polyester (Polyester), the commonly used material thickness is 12.5um.
FPC design needs to bond the layers together, this time you need to use FPC glue (Adhesive). Flexible sheets are commonly used in Acrylic, Modified Epoxy, Phenolic Butyrals, Reinforced Plastics, Pressure Sensitive Adhesives, etc., while single layer FPCs are bonded without the use of glue.
In many applications, such as soldering a device, the flexible board requires a stiffener to obtain external support. The main materials are PI or Polyester film, glass fiber, polymer material, steel sheet, aluminum sheet and so on. PI or Polyester film is a commonly used material for flexible board reinforcement, and the thickness is generally 125um. The hardness of the glass fiber (FR4) reinforcing plate is higher than that of PI or Polyester, and is used for a harder place.
There are many ways to handle the pad of the FPC relative to the way the PCB pad is processed. The following are common:
1. Chemical nickel gold is also known as chemical immersion gold or immersion gold. Generally, the thickness of the electroless nickel layer used on the copper metal 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, a PCB factory worker used the replacement method to Replace the gold in the pcb pool). The technical advantages: smooth surface, long storage time, easy to solder; suitable for fine pitch components and thinner PCB. For FPC, it is more suitable because it is thinner. Disadvantages: not environmentally friendly.
2. Tin-Lead Plating Advantage: It can directly add flat lead tin to the pad, which has good solderability and uniformity. For certain processing processes such as HOTBAR, this method must be used on FPC. Disadvantages: lead is easy to oxidize, storage time is short; need to pull plating wire; not environmentally friendly.
3. Selective electroplating gold (SEG) selective electroplating gold means that the local area of the PCB is plated with gold, and other areas are treated with another surface treatment. Electroplating gold refers to the application of a nickel layer on the copper surface of the PCB, followed by electroplating the gold layer. The thickness of the nickel layer is 2.5 um to 5.0 um, and the thickness of the gold layer is generally 0.05 um to 0.1 um. Advantages: The gold plating is thicker and has strong oxidation resistance and wear resistance. "Golden Finger" generally adopts this type of treatment. Disadvantages: not environmentally friendly, cyanide pollution.
4. Organic Solderability Protective Layer (OSP) This process refers to the surface covering of a bare PCB copper surface with specific organic matter. Advantages: Provides a very flat PCB surface that meets environmental requirements. A PCB suitable for fine pitch components.
Disadvantages: PCBA with conventional wave soldering and selective wave soldering processes is required, and OSP surface treatment is not allowed.
5. Hot Air Leveling (HASL) This process refers to the 63/37 lead-tin alloy covering the exposed metal surface of the PCB. The thickness of the 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 land pattern. It is not recommended for PCBs with fine pitch components because the fine pitch components require high flatness of the pads; the hot air leveling process is for thin FPCs.
In design, FPC often needs to be used with PCB. In the connection between the two, board-to-board connectors, connectors plus gold fingers, HOTBAR, soft and hard bonding boards, and manual soldering are usually used for different applications. Environment, the designer can use the corresponding connection method.
In practical applications, it is determined whether ESD shielding is required according to the application requirements. When the FPC flexibility requirement is not high, it can be realized by solid copper skin and thick medium. When the flexibility requirement is high.
Due to the softness of FPC, it is easy to break when subjected to stress, so some special means are needed for FPC protection.
Common methods are:
1. The minimum radius of the internal angle on the flexible profile is 1.6 mm. The greater the radius, the higher the reliability and the stronger the tear resistance. In the corner of the shape, a line near the edge of the board can be added to prevent the FPC from being torn.
2. The crack or slot on the FPC must end in a circular hole of not less than 1.5 mm in diameter, which is also required in the case where the adjacent two parts of the FPC need to be moved separately.
3. In order to achieve better flexibility, the curved area needs to be selected in a uniform width area, and the FPC width variation and the trace density are not uniform in the bending area as much as possible.
4. Stiffener (Stiffener), also known as stiffener, is mainly used to obtain external support, the use of materials are PI, Polyester, fiberglass, polymer materials, aluminum, steel and so on. Reasonable design of the position, area and material of the reinforcing plate has a great effect on avoiding FPC tearing.
5. In the multi-layer FPC design, the air gap layering design is needed for the area that needs to be bent frequently during the use of the product. Try to use the thin material PI material to increase the FPC softness and prevent the FPC from breaking during the repeated bending process.
6. When the space permits, the double-sided adhesive fixing area should be designed at the joint 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 junction between the FPC and the connector to prevent the FPC from being skewed during the assembly process.
Due to the special nature of FPC, the following points should be noted when routing:
Routing rules: priority is to ensure that the signal routing is smooth, short, straight, and less perforated as the principle, try to avoid long, thin and rounded lines, mainly horizontal, vertical and 45-degree lines, avoiding arbitrary The angle line, the bending 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 spacing: According to the current production capacity of most manufacturers, the design line pitch (Pitch) is 0.10mm.
3. Line margin: The distance between the outermost line and the FPC contour is designed to be 0.30mm. The larger the space is, the better.
4. Fillet: The minimum value of the fillet on the FPC contour is designed as the radius R=1.5mm
5. The wire is perpendicular to the bending direction
6. The wire should pass through the curved area evenly
7. The wire should be as thick as possible to bend the area
8. There must be no additional plating metal in the curved area (the curved area wire is not plated)
9. Line width is consistent
10. Double-panel traces cannot overlap to form an "I" shape
11. Minimize the number of layers in the curved area
12. There must be no through holes and metallized holes in the curved area.
13. The bending center axis should be placed at the center of the wire. The material coefficients and thickness on both sides of the wire are as uniform as possible. This is very important in dynamic bending applications.
14. Horizontal plane torsion follows the principle of reducing the curved section to increase flexibility or partially increasing the area of the copper foil to increase toughness.
15. Vertical surface bending is to increase the bending radius, reduce the number of layers in the center area, etc.
16. For products with EMI requirements, if high-frequency radiation signal lines such as USB and MIPI are on the FPC, the conductive silver foil layer should be added to the FPC according to the EMI measurement condition and the conductive silver foil should be grounded to prevent EMI.
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 more and summarize, I believe that designing FPC is not difficult, and it can be easily used.