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PCB Tech
The processing of PCB pads and the use of FPC materials
PCB Tech
The processing of PCB pads and the use of FPC materials

The processing of PCB pads and the use of FPC materials


FPC flexible printed circuit is a form of circuit made on a flexible cutting surface, which can be covered or uncovered (usually used to protect FPC circuits). Since FPC can be bent, folded or moved repeatedly in various ways, it is becoming more and more widely used.

The base film of FPC is usually made of polyimide (polyimide, PI) (abbreviated) and polyester.

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

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

FPC design needs to bond the layers together, and FPC glue (adhesive) is needed at this time. Flexible boards are usually used for acrylic, modified epoxy, phenolic butyral, reinforced plastics, pressure-sensitive adhesives, etc., while single-layer FPC does not use adhesives for bonding.

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In many applications, such as welding devices, flexible boards need reinforcements for external support. The main materials are PI or polyester film, glass fiber, polymer material, steel plate, aluminum plate, etc. PI or polyester film is a commonly used material for flexible board reinforcement, and its thickness is generally 125um. The hardness of glass fiber (FR4) reinforced board is higher than PI or polyester, and it is used in harder places.

There are many ways to deal with the way the FPC pad is handled relative to the PCB pad. The following are common:

1. Chemical nickel gold is also called 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 PCB). Factory workers use alternative methods. Replace the gold coins in the pcb pool. Technical advantages: smooth surface, long storage time, easy soldering; suitable for fine-pitch components and thinner PCBs. For FPC, it is more suitable because it is thinner. Disadvantages: not environmentally friendly.

2. The advantages of tin-lead electroplating: flat lead-tin can be directly added to the pad, which has good solderability and uniformity. For some processes (such as HOTBAR), this method must be used on the FPC. Disadvantages: lead is easy to oxidize, and the storage time is short; it needs to pull the electroplating line; it is not environmentally friendly.

3. Selective gold electroplating (SEG) Selective gold electroplating means that some areas of the PCB are plated with gold, while other areas are treated with another surface treatment. Gold electroplating refers to applying a nickel layer on the copper surface of the PCB, and then electroplating the gold layer. The thickness of the nickel layer is 2.5 μm to 5.0 μm, and the thickness of the gold layer is usually 0.05 μm to 0.1 μm. Advantages: The gold-plated layer is thicker, with strong oxidation resistance and wear resistance. "Golden Fingers" generally use this type of treatment. Disadvantages: not environmentally friendly, cyanide pollution.

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

Disadvantages: PCBA using 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 the 63/37 lead-tin alloy covering the exposed metal surface of the PCB. The thickness of the hot air leveling pewter 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 fine-pitch components require high flatness of the pad; the hot air leveling process is suitable for thin FPC.

In design, FPC often needs to be used with PCB. Among the connections between the two, board-to-board connectors, connectors and golden fingers, HOTBAR, soft and hard bonding boards, and manual soldering are usually used for different applications. In terms of environment, designers can use the corresponding connection method.

In actual applications, determine whether ESD shielding is required according to application requirements. When FPC flexibility is not high, it can be achieved by solid copper and thick media. When flexibility is demanding.

Due to the softness of FPC, it is easy to break when under pressure, so FPC protection requires some special means.

Common methods are:

1. The minimum radius of the inner corner 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, you can add a line near the edge of the board to prevent the FPC from being torn.

2. The cracks or grooves on the FPC must end with a round hole with a diameter of not less than 1.5 mm, which is also necessary when two adjacent parts of the FPC need to be moved separately.

3. In order to obtain better flexibility, it is necessary to select the bending area in a uniform width area, and as far as possible the FPC width change and the uneven trace density in the bending area.

4. Stiffener, also known as stiffener, is mainly used to obtain external support. The materials used include PI, polyester, glass fiber, polymer materials, aluminum, steel, etc. Reasonable design of the position, area and material of the reinforcement board has a great effect on avoiding FPC tearing.

5. In the multi-layer FPC design, the air gap layered design needs to be carried out in the area that is often bent during the use of the product. Try to use thin PI material to increase the softness of the FPC and prevent the FPC from cracking during repeated bending.

6. When space permits, a double-sided tape fixing area should be designed at the joint between the golden finger and the connector to prevent the golden finger and the connector from falling off during the bending process.

7. The FPC positioning screen should be designed at the connection between the FPC and the connector to prevent the FPC from skewing during the assembly process.