Introduction:
The rigid-flex PCB technology almost immediately delivered on its promise of simplifying interconnection and assembly of complex interconnection structures. Because of the superiorly mechanical and electrical reliability, the rigid-flex PCBs have been used in the fields of military, aerospace, automotive, computer peripherals and portable electronics.
Characteristics and Applications of Rigid- Flex PCB:
There have always been 4 drivers that have created the need for rigid-flex:
1) Weight reduction.
2) Reliability improvement.
3) The need for a compact package.
4) Easier automatically assemble without special fixtures.
Rigid-flex PCB fits weight reduction needs by using a very thin substrate. Typical rigid boards will be run on 0.1mm core or thicker. Typical flexible circuits are run on 0.05-0.06mm core material. Obviously, the thinner the core and the circuit are, the less it weighs. Because eliminating jumper cables and connectors or flat cable would be used to connect different rigid boards together, it gets a reliability improvement. Finally, the rigid-flex PCB allows manufacturers to assemble the product and fold it into a nice compact package.
Research and Development of New Product:
The manufacturing process of rigid-flex PCB are very complex. Developing on different designs, there is distinctly dissimilar process. First product is shown in figure 1. It’s a double-side rigid-flex PCB. The construction consists of a single-side flex and a layer of single-side rigid together with a modified epoxy prepreg.
The figure 2 shows that construction which is comprised of a double-side flexible printed circuit with a single-side rigid laminated on each side. The adhesive is also modified epoxy prepreg.
During the research, there are several processes should be noticed.
1) At the stage of the photochemical patterning of circuit, the quality of the image depend primarily the thickness of resist and the light source. A highly collimated, intense lights source. A highly collimated, intense lights source is critical to generating a straight, clean image. The thinner the resist is, the easier it is to generate a fine image.
2) The formation of microvia. In this stage, rigid manufacturers have made tremendous strides in the size and quality of drilled holes that they can generate with today’s equipment. Flex-PCB, however, offers a variety of methods for via generation. This operation is very critical, since the rigid-flex PCB is built-up with materials at very different hardness. Whereas the materials of rigid-PCB is relatively hard, the polyimide and in particular the adhesive are very soft.
3) It uses acrylic adhesive materials that have high Thermal Coefficient of Expansion (TCE). Because of the different of TCE, the flexible portion can form groove. A new family of low-flow and no-flow prepreg epoxies can decrease the tensility, which is induced by adhesive.
4) After drilling, the polyimide and in particular the adhesive generate some smear which can’t be removed by conventional chemical disposal and can induce low-reliability of rigid-flex PCBs. Ordinary desmearing with potassium permanganate, as used in conventional rigid board production, is not feasible, since the adhesive is not compatible with strong alkaline chemicals.
Conclusion:
In recent years, this technology has been steadily developed. But in China, the research about rigid-flex PCB is in the developing stage, and there are still many challenges needed to be solved.
