Rigid-flex PCBs can be laminated using a one-time laminating method in which all the inner layers are pressed together, or a step-by-step lamination method in which the flexible inner layer is pressed first and then the rigid outer layer is pressed. The laminating method has a short processing cycle and low cost, but it is difficult to position the overlay during lamination. Lamination defects such as air bubbles, delamination and inner layer deformation can only be found after the outer layer is etched; step lamination can be Reducing the positioning difficulty of the overlay during lamination, it is also possible to find the pattern offset and lamination defects of the inner layer in time, and the step-by-step lamination can also take care of the characteristics of the flexible and rigid materials to optimize the process parameters, but step-by-step lamination Laminating laborious, time-consuming, and cost-assisted materials at a time.
(2) Selection of adhesive sheets:
The use of different types of bonding sheets has a direct influence on the structure of rigid-flex printed boards. FIGS. 3a-b are schematic views of a flex-rigid eight-layer printed board bonded with different types of bonding sheets. Among them, category a is an adhesive sheet in which an acrylic adhesive film is used as an inner layer. In this structure, the percentage of acrylic thickness is quite large, and the coefficient of thermal expansion of the entire rigid-flexible printed board is also large. Metallized holes tend to fail in thermal stress tests. In this structure, reducing the Z-axis expansion by reducing the thickness of the acrylic adhesive sheet is not practical. On the one hand, this is not conducive to bubble-free lamination, and on the other hand, the lack of increased thickness to compensate for its thickness often results in The offset of the flexible inner graphics is poor. The structure b is an acrylic adhesive sheet in which a glass cloth is used as a reinforcing material instead of a non-reinforced material. This acrylic material with a reinforcing material not only satisfies the requirement for bubble-free lamination but also increases the hardness of the structure. Its disadvantage is that it handles the protruding glass fiber head before it is pored. In structure c, epoxy glass cloth prepreg is used to bond the flexible inner layer of the cover layer.
Due to the poor adhesion of epoxy resin and polyimide film, during the installation and use, it is easy to produce the phenomenon of inner layer delamination, which can be achieved by adding a layer between epoxy glass cloth and polyimide. Acrylic adhesives increase the binding force, but as a result, acrylic acid is introduced again and the production complexity is also increased. In the structure d, the cover layer is removed, and the inner layer is bonded with epoxy glass cloth prepreg or epoxy glass cloth as the reinforcing material. The flexible copper clad substrate is exposed after the surface copper is etched away. Layer acrylic adhesive, so it has very good bonding with epoxy. At the same time, the large number of epoxy materials greatly reduces the coefficient of thermal expansion of the entire rigid-flex PCB, which greatly improves the reliability of the metallized holes. However, due to the removal of a large number of cover layers, the PCB is operated at high temperatures. The environment will become soft, especially in the flexible section, so add a reinforcement plate. Structure e uses polyimide laminates instead of epoxy laminates to improve the high temperature resistance of rigid-flex PCBs. In the structure ae, in addition to c should not be used, manufacturers can determine the rigid flex PCB structure based on their own equipment and technology and rigid flex PCB application requirements.
Recently, some manufacturers are trying a bold overburden partial laminating method (the schematic structure of this laminating method is shown in Fig. 4). This method retains the advantages of good binding force in the structure a, and also overcomes The disadvantage of large thermal expansion. In this configuration, the outermost cover layer of the flexible multilayer printed board extends only about 1/10 of the rigid area, and the rigid outer layer is bonded to the flexible inner layer using a non-flowable epoxy prepreg. Due to the absence of a cover layer, the epoxy prepreg is mainly bonded to the acrylic adhesive (copper foil and flexible substrate) bonded to the copper foil on the flexible substrate, so that the bonding force is good. At the same time, the coefficient of thermal expansion of the entire flex-finished printed circuit board is greatly reduced due to the removal of the two acrylic adhesive sheets that bond the rigid outer layer and the flexible inner layer and the acrylic adhesive sheets on the two cover layers, thereby increasing the metallized hole. The thermal shock resistance. Therefore, although the process of this structure is complex and costly, it increases the reliability of the rigid-flex PCB.