Rigid flex PCB Manufacturing and Material Selection
With the birth and development of FPC and PCB, a new product Rigid Flex PCB was born. The rigid flex board is a circuit board with FPC characteristics and PCB characteristics, which is formed by combining the flexible circuit board and the rigid circuit board according to the relevant process requirements, and through processes such as pressing.
Initially, the basic design concept and manufacturing process of the multi-layer rigid-flex board was developed from aerospace equipment because of the need for reliable wiring in a limited space. In some complex products, even rigid-flex boards with more than 30 conductor layers are used. On the other hand, in consumer electronic products such as mobile phones and digital cameras, high-density and low-cost wiring technology has always been required, so new design concepts and manufacturing processes have emerged.
The combination of the flexible board and the rigid board can be called Rigid-Flex PCB or RFPCB. When their multi-layer flexible medium uses flexible materials instead of glass-epoxy resin, also known as multilayer flexible board.
Multi-layer rigid-flex board is basically a combination of rigid board and flexible board. However, if circuit board manufacturers want to successfully combine the two, they need to have a good level of rigid PCB and flexible PCB manufacturing processes. Therefore, before designing this kind of PCB, it is necessary to clearly understand the capabilities and limitations of the PCB manufacturer.
Basic structure
There are many different structure types in the design of the rigid-flex board. Figure 1, Figure 2 shows the basic structure of the multilayer flexible and rigid board, including plan view and cross-sectional view (laminated structure).
See Figure 3–1, a non-adhesive layered structure is adopted between multiple flexible layers, which can provide better bending performance. In extreme cases, this structure can be used to make PCBs with more than 30 layers for use in aerospace products. Because of the demand for high reliability, fine circuit patterns and micro-hole technology cannot be used. Moreover, using leaded components instead of SMT components, this structure usually requires a larger line width and line space. And large-diameter through holes with thick copper walls. This kind of rigid-flex board can be designed into a variety of structures according to needs, such as folding type, flying swallow type, book type, and so on.
The above table lists several materials necessary for the production of the rigid-flex board. It should be pointed out that with the advancement of technology, the performance of these materials has been significantly improved.
The material must have high heat resistance and good dimensional stability during heating. High-reliability fields (such as military industry, aerospace, etc.) recommend the use of thick polyamide amine film (greater than 50um) because the substrate needs to have good stability and durability during processing. However, the consumer electronics field is based on the development trend of light, thin and short, generally using thinner media (less than 50um) materials. In the adhesive copper clad laminate, cover film and adhesive sheet, the adhesion of acrylic adhesive is better, but the heat resistance is slightly worse, and the shrinkage rate is higher. The use of epoxy adhesive materials has better heat resistance, but the fixing time is longer and the bonding force is slightly worse. The non-adhesive copper clad laminate substrate manufactured by the casting or pressing industry usually has a higher heat resistance and lower thermal expansion coefficient, and can reduce the thickness of the final product, so it is very advantageous in the production of rigid and flexible laminates. It can also significantly reduce drilling slag. However, this material must be processed at a temperature exceeding 300°C, so special equipment and process conditions are required.
Manufacturing process
Since the rigid-flex board has a variety of complex structural forms, the manufacturing process is also different. Figure 4 shows a typical rigid-flex board laminate structure manufactured according to the standard process shown in Figure 5. In Figure 5, the process starts with the manufacture of double-sided flexible copper clad laminates.
Except for through holes, other conductor patterns are formed using traditional etching processes. Therefore, on the flexible area conductor cover, there is no opening covering film, and the multilayer flexible boards are bonded with the adhesive sheet that has been opened in the flexible area in advance so that the flexible bending area will not be affected. The rigid outer cover (refers to the rigid area of the outermost layer) uses a double-sided rigid copper clad laminate.
The first step is to process the rigid flex outer cover layer. It is necessary to press the electroplating pattern of the layer on the outer layer, and then use a CNC milling machine, punching, or laser method to cut the part of the rigid layer in the flexible zone to the hollow or half the depth of the gong. The flexible board and the processed rigid outer cover are bonded by an adhesive sheet. The adhesive sheet has been pre-opened in the flexible part.
During the lamination process, if the rigid outer layer adopts a hollow structure, a suitable pressure-matched filling board should be prepared for the flexible part. A vacuum press can be used to obtain better pressing quality, and at the same time with some auxiliary conformal materials (PE film, etc.), so that the pressing process can provide uniform pressure to the entire board, so that the low-flow adhesive sheet can fully flow and fill the gaps, especially for complex structures. Before bonding or laminating, proper baking should be carried out as needed to remove water vapor.
The laminated rigid-flex board can adopt a through-hole treatment process similar to that of the multilayer rigid board, but the difference lies in the removal of drilling dirt. The method of de-drilling depends on the material used. Like the rigid board, the hole is drilled after it is fully baked, and then the resin residue in the hole wall is removed by the plasma etching process. The plasma treatment also needs to be baked before the plasma treatment to remove water vapor. It is generally recommended that the etching degree not exceed 13um, and the through-hole plating process of conventional rigid boards can be used, but the specific process parameters of plating should be determined according to the reliability test data of the through-hole.
The following process is similar to that of the multilayer rigid board. The outer layer etching, covering film (solder mask), surface treatment, etc. can all use similar processes. When the shape is made, the pressure filling board or the control of the flexible area can be used. After the deep rigid outer cover layer is removed from the corresponding part of the flexible area, it can be formed into a rigid-flex board.
