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FPC Pad Processing And Material

The FPC flexible printed circuit is a form of circuit fabricated on a flexible cut surface that may or may not be covered (usually used to protect FPC circuits). Since FPC can be bent, folded or repeatedly moved in various ways, it is used more and more widely.

The base film of FPC is usually made of polyimide (polyimide, PI) 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 high temperatures, the material is typically made of PI and the substrate of the PCB is typically FR4.

The cover layer of the FPC is made of a dielectric film and a glue film or a flexible medium coating to prevent contamination, moisture, scratches, etc., and the main material is the same as the substrate, that is, polyimide. Amine (polyimide) and polyester (polyester), commonly used materials with a thickness of 12.5um.

FPC design requires bonding the layers together, in which case FPC glue (adhesive) is required. Flexible sheets are commonly used in acrylics, modified epoxy resins, phenolic butyral, reinforced plastics, pressure sensitive adhesives, etc., while single layer FPCs are not bonded using adhesives.

In many applications, such as welding devices, flexible sheets require reinforcement to obtain external support. The main materials are PI or polyester film, glass fiber, polymer material, steel plate, aluminum plate and so on. PI or polyester film is a commonly used material for flexible board reinforcement, and the thickness is generally 125um. Glass fiber (FR4) reinforced sheets have a higher hardness than PI or polyester and are used in harder places.

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There are several ways to handle the handling of the FPC's pads relative to the PCB pads. The following are common:

1. Chemical nickel gold is also known as chemical immersion gold or immersion gold. Typically, 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 (formerly PCB). Replace the gold coins in the pcb pool. 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 advantages: can directly add flat lead tin on the pad, with good solderability and uniformity. For some processes (such as HOTBAR), this method must be used on the FPC. Disadvantages: Lead is easy to oxidize, storage time is short; need to pull plating line; not environmentally friendly.

3. Selective Electroplating Gold (SEG) Selective plating gold means that part of the PCB is plated with gold, while 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 and then electroplating the gold layer. The thickness of the nickel layer is from 2.5 μm to 5.0 μm, and the thickness of the gold layer is usually from 0.05 μm to 0.1 μm. Advantages: The gold plating layer is thick and has strong oxidation resistance and wear resistance. "Golden Finger" generally uses this treatment. Disadvantages: not environmentally friendly, cyanide pollution.

4. Organic Solderability Protective Layer (OSP) This process refers to the surface coating of the bare PCB copper surface with specific organic matter. Advantages: Provides a very flat PCB surface to meet environmental requirements. Suitable for PCBs with fine pitch components.

Disadvantages: PCBA with traditional 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 63/37 lead-tin alloy covering the bare metal surface of the PCB. The hot air leveling lead-tin alloy coating has a thickness of 1 um to 25 um. The hot air leveling process makes it difficult to control the thickness of the plating and the land pattern. Not recommended for PCBs with fine pitch components because fine pitch components require high flatness of the pads; hot air leveling is suitable for thin FPCs.

 

 

In design, FPC often needs to be used with PCBs. Among the connections between the two, board-to-board connectors, connectors and gold fingers, HOTBAR, soft and hard bond plates, and manual soldering are often used for different applications. On the environmental side, designers can use the appropriate connection method.

In practical applications, it is determined according to the application requirements whether ESD shielding is required. When FPC flexibility is not high, it can be achieved with solid copper and thick media. When flexibility is required.

FPC PAD processing

Due to the softness of the FPC, it is prone to breakage when subjected to pressure, so FPC protection requires some special means.

 

Common methods are:

1. The minimum radius of the internal angle on the flexible profile is 1.6 mm. The larger the radius, the higher the reliability and the stronger the tear resistance. At the corners 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 groove on the FPC must end with a circular hole having a diameter of not less than 1.5 mm, which is also necessary in the case where the adjacent two parts of the FPC need to be moved separately.

3. In order to obtain better flexibility, it is necessary to select a curved region in a uniform width region, and as far as possible, the FPC width variation and the track density are not uniform in the curved region.

4, reinforced ribs (Stiffener), also known as reinforced ribs, mainly used to obtain external support, the use of materials are PI, polyester, glass fiber, 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 multi-layer FPC design, air gap stratification design is required in areas that are often bent during product use. Try to use a thin material PI material to increase the FPC softness and prevent the FPC from breaking during repeated bending.

6. When 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 screen should be designed at the junction between the FPC and the connector to prevent the FPC from skewing during assembly.

FPC Routing

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

Routing rules: The priority is to ensure that the signal routing is smooth, short, straight, and less perforated. Try to avoid long and thin round lines, mainly horizontal, vertical and 45-degree lines, avoiding any angle lines, curved parts of the arc, 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 remaining line spacing is 0.15 mm on average.

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

3. Line margin: The distance between the outermost line and the FPC contour is designed to be 0.30 mm. The bigger the space, 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 bending 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 bend area

12. There must be no through holes and metallized holes in the curved area.

13. The bending center axis should be 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 increase 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, and the like.

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 to the FPC according to the EMI measurement conditions, and the conductive silver foil should be grounded to prevent EMI.

 

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