The root cause of the expansion and contraction is determined by the characteristics of the material. To solve the problem of the expansion and contraction of the rigid flex joint plate, we must first introduce the flexible plate material polyimide;

Analysis and Improvement of PCB Expansion and Shrinkage

 (1) Polyimide has excellent heat dissipation performance and can withstand Thermal shock during lead-free welding high-temperature treatment;

 (2) For small devices that require greater emphasis on signal integrity, most equipment manufacturers tend to use flexible circuits;

 (3) Polyimide has the characteristics of high glass transfer temperature and high melting point. Generally, it needs to be processed above 350 ℃;

 (4) In terms of organic dissolution, polyimide is insoluble in general organic solvents.

The expansion and contraction of flexible board materials are mainly related to the matrix material PI and adhesive, which is closely related to the imidization of PI. The higher the degree of imidization, the stronger the controllability of expansion and contraction. According to normal production rules, flexible boards will experience varying degrees of shrinkage during the formation of graphic circuits and the process of rigid bending combined pressing after cutting. After etching the graphic circuits, the density and direction of the circuits will lead to a reorientation of stress on the entire board surface, ultimately leading to a general pattern of shrinkage changes on the board surface; During the process of rigid bending combined with compression, due to the inconsistent coefficient of expansion and contraction between the surface coating film and the matrix material PI, a certain degree of expansion and contraction can also occur within a certain range;

Essentially, the expansion and contraction of any material are caused by the influence of temperature. In the lengthy production process of PCB, the material undergoes many thermal and wet processes, and the expansion and contraction value will have varying degrees of subtle changes. However, based on long-term practical production experience, the changes are still regular.

How to control and improve:

The internal stress of each roll of material is different, and the process control of each batch of production boards will not be completely the same. Therefore, the grasp of material shrinkage coefficient is based on a large number of experiments, and process control and data statistical analysis are particularly important. In practical operation, the expansion and contraction of flexible plates are divided into stages;

Firstly, from the opening of the material to the baking board, the expansion and contraction at this stage are mainly caused by the influence of temperature;

To ensure the stability of the rise and fall caused by the baking board, it is first necessary to ensure consistency in process control. On the premise of uniform materials, the heating and cooling operations of each baking board must be consistent. It is not allowed to blindly pursue efficiency and place the baking board in the air for heat dissipation. Only in this way can the internal stress of the material be eliminated to the greatest extent.

The second stage occurs during the process of pattern transfer, and the expansion and contraction in this stage are mainly caused by changes in the internal stress orientation of the material.

To ensure the stability of the expansion and contraction during the line transfer process, all baked boards cannot be ground. They are directly treated with a chemical cleaning line for surface preparation. The surface after lamination must be flat, and the standing time of the board surface before and after exposure must be sufficient. After completing the line transfer, the flexible board will show varying degrees of curling and shrinkage due to changes in stress orientation, Therefore, the control of line film compensation is related to the control of stiffness and deflection combination accuracy. At the same time, the determination of the range of expansion and contraction values of the flexible plate is the data basis for producing its supporting rigid plate.

The third stage of expansion and contraction occurs during the compression process of the rigid and flexible plate, and the expansion and contraction in this stage are mainly determined by the compression parameters and material properties.

The factors affecting the expansion and contraction at this stage include the heating rate of pressing, the setting of pressure parameters, and the residual copper rate and thickness of the core plate. Overall, the smaller the residual copper rate, the greater the fluctuation value; The thinner the core board, the greater the expansion and contraction value. However, from large to small, it is a gradual process of change, so film compensation is particularly important. In addition, due to the inherent differences in the materials of flexible and rigid plates, their compensation is an additional factor that needs to be considered.