How to solve the problem of Rigid-Flex PCB expansion and shrinkage?

The root cause of the expansion and shrinkage is determined by the characteristics of the material. The expansion and shrinkage of the flexible printed circuit board material is mainly related to the material polyimide and the glue, that is, it has a great relationship with the imidization of polyimide. The higher the degree of imidization, the stronger the controllability of the expansion and shrinkage.

Strictly speaking, the internal stress of each roll of material is different, and the process control of each batch of produce the boards will not be exactly the same. Therefore, the grasp of the material expansion and shrinkage coefficient is based on a large number of experimental, then the process management and statistical analysis of data is particularly important. Specific to the actual operation, the expansion and shrinkage of the flexible board is phased:

First is from board cut to the baking, the expansion and shrinkage at this stage are mainly caused by temperature.

To ensure the stability of the expansion and shrinkage caused by the board baking, the consistency of the process control must first be achieved. Under the premise of uniform material, the operation of heating and cooling the board baking must be consistent, don't let the baked board put in the air for heat dissipation because of the pursuit of efficiency. Only in this way can removed the expansion and shrinkage caused by the internal stress of the material.

The second phase occurs during the transfer of the photo image, and the expansion and shrinkage at this stage are mainly caused by changes in the internal stress orientation of the material.

To ensure the stability of the line transfer process, all the baked boards can not be subjected to the grinding operation, the surface is pretreated directly by the chemical cleaning line, the surface after lamination should be flat and the resting time before and after exposure must be sufficient. After the completion of the line transfer, the flexible board will exhibit different degrees of curling and shrinking due to the change of the stress orientation. Therefore, the control of the line film compensation is related to the control of the rigid and flexible combination precision, at the same time, the determination of the range of the expansion and shrinkage of the flexible board is the basis for the production of the matching rigid board.

The third stage of expansion and shrinkage occurs in the process of rigid and flexible board lamination, and the expansion and shrinkage at this stage is mainly determined by the lamination parameters and material properties.

The factors affecting the expansion and shrinkage at this stage include the heating rate of lamination, the setting of the pressure parameters, and the residual copper rate and thickness of the core. In general, the smaller the residual copper ratio, the larger the expansion and shrinkage value, the thinner the core, the larger the expansion and shrinkage value. However, from large to small, it is a process of gradual change. Therefore, film compensation is particularly important. In addition, due to the nature of the flexible and rigid board materials, compensation is a factor that requires additional consideration.

rigid-flex PCB