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PCB Tech

PCB Tech

PCB Tech

PCB Tech

The performance and selection method of flexible circuit board materials

Flexible printed circuit (Flexible Printed Circuit) can be freely bent, wound, and folded. The flexible circuit board is processed by using polyimide film as the base material. It is also called soft board or FPC in the industry. Flexible circuit board is based on The number of layers is different, the process flow is divided into double-sided flexible circuit board process flow, multi-layer flexible circuit board process flow. The FPC soft board can withstand millions of dynamic bending without damaging the wires. It can be arranged arbitrarily according to the space layout requirements, and can be moved and stretched arbitrarily in three-dimensional space, so as to achieve the integration of component assembly and wire connection; flexible circuit boards can be The volume and weight of electronic products are greatly reduced, and it is suitable for the development of electronic products in the direction of high density, miniaturization and high reliability.

Material performance and selection method

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1. Substrate: The material is polyimide (POLYMIDE), which is a high-temperature resistant, high-strength polymer material. It can withstand a temperature of 400 degrees Celsius for 10 seconds, and its tensile strength is 15,000-30,000 PSI. The 25μm thick substrate is the cheapest and the most common application. If the circuit board needs to be harder, a 50μm substrate should be used. Conversely, if the circuit board needs to be softer, a 13μm substrate is used.

Second, the transparent glue of the base material: It is divided into two types: epoxy resin and polyethylene, both of which are thermosetting glue. The strength of polyethylene is relatively low. If you want the circuit board to be softer, choose polyethylene. The thicker the substrate and the transparent glue on it, the harder the circuit board. If the circuit board has a relatively large bending area, a thinner substrate and transparent glue should be used as much as possible to reduce the stress on the surface of the copper foil, so that the chance of micro-cracks in the copper foil is relatively small. Of course, for such areas, single-layer boards should be used as much as possible.

3. Copper foil: divided into two types: rolled copper and electrolytic copper. Rolled copper has high strength and bending resistance, but the price is more expensive. The price of electrolytic copper is much cheaper, but its strength is poor and it is easy to break. It is generally used in occasions where it is rarely bent. The thickness of the copper foil should be selected according to the minimum lead width and minimum spacing. The thinner the copper foil, the smaller the minimum achievable width and spacing. When choosing rolled copper, pay attention to the rolling direction of the copper foil. The rolling direction of the copper foil should be the same as the main bending direction of the circuit board.

Fourth, the protective film and its transparent glue: 25μm protective film will make the circuit board harder, but the price is cheaper. For circuit boards with relatively large bends, it is best to choose a 13μm protective film. Transparent glue is also divided into epoxy resin and polyethylene, and the circuit board using epoxy resin is relatively hard. After the hot pressing is completed, some transparent glue will be extruded from the edge of the protective film. If the size of the pad is larger than the opening size of the protective film, the extruded glue will reduce the size of the pad and cause its edges to be irregular. At this time, transparent glue with a thickness of 13μm should be used as much as possible.

V. Pad plating: For circuit boards with large bending and exposed pads, electroplated nickel + electroless gold layer should be used. The nickel layer should be as thin as possible: 0.5-2μm, chemical gold layer 0.05-0.1μm .