Introduction:

Rigid-flex PCB technology has rapidly proven its ability to simplify the interconnection and assembly of complex structures. Thanks to its superior mechanical and electrical reliability, rigid-flex PCBs are widely used in military, aerospace, automotive, computer peripherals, and portable electronics industries.

Characteristics and Applications of Rigid-Flex PCB:

The demand for rigid-flex PCBs has been driven by four key factors:

• Weight reduction.
• Improved reliability.
• Need for compact packaging.
• Simplified automatic assembly without special fixtures.

Rigid-flex PCBs contribute to weight reduction by using thin substrates. Standard rigid boards often use a 0.1mm core or thicker, while flexible circuits use cores as thin as 0.05-0.06mm. Naturally, thinner cores and circuits result in lighter products. The elimination of jumper cables and connectors also boosts reliability, and the rigid-flex PCB design allows for a compact and easily assembled package.

Research and Development of New Products:

The manufacturing process for rigid-flex PCBs is highly complex and varies with different designs. The initial product is a double-sided rigid-flex PCB, which combines a single-sided flex layer with a single-sided rigid layer and modified epoxy prepreg, as shown in Figure 1.

Figure 2 illustrates a construction featuring a double-sided flexible printed circuit with a single-sided rigid laminate on each side. The adhesive used is also a modified epoxy prepreg.

Several key processes are critical during the research and development of rigid-flex PCBs:

• In the photochemical patterning stage, the quality of the image is primarily determined by the thickness of the resist and the light source. A highly collimated, intense light source is essential for generating clean and precise images. The thinner the resist, the easier it is to achieve a fine image.
• Microvia formation is a crucial process. Rigid PCB manufacturers have made significant progress in the size and quality of drilled holes, while flexible PCBs offer various methods for via generation. This operation is particularly challenging due to the differences in material hardness—rigid-PCBs are relatively hard, whereas polyimide and adhesives are much softer.
• Acrylic adhesive materials with a high Thermal Coefficient of Expansion (TCE) are used. Due to the differing TCE values, the flexible portion can form grooves. A new family of low-flow and no-flow prepreg epoxies can reduce the tension caused by the adhesive.
• After drilling, polyimide and adhesives may generate smearing that cannot be removed with conventional chemical processes, potentially compromising the reliability of rigid-flex PCBs. Standard desmearing methods, like using potassium permanganate, are not feasible due to the adhesive’s incompatibility with strong alkaline chemicals.

Conclusion:

In recent years, rigid-flex PCB technology has steadily advanced. However, in China, research in this area is still in the development stage, and several challenges remain to be addressed.

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