How to control and improve the problem of the expansion and contraction of soft and hard board?
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 soft and hard joint board, it is necessary to first introduce the material of the flexible board polyimide (Polyimide):
(1) Polyimide has excellent heat dissipation performance and can withstand the thermal shock of lead-free soldering at high temperature;
(2) For small devices that need more emphasis on signal integrity, most equipment manufacturers tend to use flexible circuits;
(3) Polyimide has the characteristics of high glass transition temperature and high melting point, and it is generally processed above 350 ℃;
(4) In terms of organic dissolution, polyimide is insoluble in common organic solvents.
The expansion and contraction of the flexible board material is mainly related to the base material PI and the glue, that is, it has a great relationship with the imidization of PI. The higher the degree of imidization, the stronger the controllability of the expansion and contraction.
According to the normal production rules, the flexible board will have different degrees of expansion and contraction during the formation of the graphic circuit after the material is opened, and the combination of soft and hard bonding. After the graphic circuit is etched, the density and direction of the circuit, It will lead to the reorientation of the stress on the entire surface of the board, which will eventually lead to a general regular change in expansion and contraction. During the process of soft and hard bonding, due to the inconsistency of the expansion and contraction coefficient of the surface coating film and the base material PI, it will There is a certain degree of ups and downs within the scope.
Essentially speaking, the expansion and contraction of any material is caused by the influence of temperature. In the lengthy PCB manufacturing process, the material will undergo slight changes in expansion and contraction values ​​after many hot and wet processes, but Judging from the long-term actual production experience, the changes are still regular.
How to control and improve?
Strictly speaking, the internal stress of each roll of material is different, and the process control of each batch of production board will not be the same. Therefore, the grasping of the expansion and contraction coefficient of the material is based on a large number of experiments. On the other hand, process control and data statistical analysis are particularly important. Specific to the actual operation, the expansion and contraction of the flexible board is in stages:
The first is from the opening to the baking board. The rise and contraction at this stage is mainly caused by the temperature:
To ensure the stability of the expansion and contraction caused by the baking board, first of all, the consistency of the process control is required. Under the premise of uniform materials, the operation of heating and cooling of the baking board must be consistent every time, not because of the blind pursuit of efficiency. The baked board is placed in the air for heat dissipation. Only in this way can the expansion and contraction caused by the internal stress of the material be removed to a considerable extent.
The second stage occurs in the process of pattern transfer. The expansion and contraction of this stage is mainly caused by the change of stress orientation in the material.
To ensure the stable expansion and contraction of the circuit transfer process, all the baked boards cannot be subjected to the grinding operation, and the surface pretreatment is directly carried out through the chemical cleaning line. The surface after the film must be flat, and the board time before and after exposure must be sufficient. After the line transfer is completed, due to the change in stress orientation, the flexible board will show different degrees of curling and shrinking, so the control of the line film compensation is related to the control of the combination of soft and hard precision. At the same time, the expansion and contraction value range of the flexible board The determination of is the data basis for producing its supporting rigid board.
The third stage of expansion and contraction occurs in the process of soft and hard board pressing, which is determined by the main pressing parameters and material characteristics.
The factors affecting the expansion and contraction at this stage include the temperature rise rate of pressing, the setting of pressure parameters, and the copper residual rate and thickness of the core board. In general, the smaller the residual copper rate, the greater the shrinkage value; the thinner the core plate, the greater the shrinkage value. However, from large to small, is a gradual change process, therefore, film compensation is particularly important. In addition, due to the different nature of the materials of the flexible board and the rigid board, its compensation is an additional factor to be considered.
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