How to solve the problem of expansion and reduction of rigid flex PCB

- Dec 13, 2017-

The root of expansion is determined by the properties of the material to solve flex board expansion problems must first of Polyimide Flexible Plate (Polyimide) to be introduced:

(1) polyimide has excellent heat dissipation properties and can withstand the heat shock of high temperature treatment of lead-free welding.

(2) for small devices that require more emphasis on the integrity of the signal, most of the equipment manufacturers tend to use flexible circuits;

(3) polyimide has the characteristics of high glass transfer temperature and high melting point. In general, it should be processed above 350 centigrade.

(4) in organic dissolution, polyimides do not dissolve in the general organic solvents.

The flexible plate material. The main relationship with PI material and rubber, is also a great relationship with the PI imide, higher imidization degree, the stronger the controllability.

In accordance with the law of normal production, flexible plate in material, formed in the process of combination of hardware and software and graphics line, pressing in both will have varying degrees of expansion, in line graph after etching, intensive degree and trend line, will lead to stress re orientation should be the whole panel, leading to these changes in the general appearance of the board the regularity of the process; in the combination of soft and hard pressed, because the expansion coefficient of surface coating and the base material PI is inconsistent, will also have a degree in a certain range.

The essential reason that any material expansion is caused by the influence of the temperature, in the process of making PCB long, after a lot of material hot wet process, expanding value will have subtle changes in different degree, but the actual production experience in the long term, change or regular.

How to control and improve?

Strictly speaking, the stress of each volume of material is different, each batch of production process control board is not exactly the same, so the expansion coefficient of the material master is based on experiments, statistical analysis and data process control is particularly important. Specific to the actual operation, flexible plate is divided into several stages:

The first is from the expected baking board at this stage, expansion is mainly affected by temperature caused by:

To ensure the stable expansion caused by the baking board, first of all to the consistency of process control, in the premise of material under the unified, each must be consistent with the baking board heating cooling operation, not because of the pursuit of efficiency, and will bake end plate on the air for cooling. Only in this way, in order to maximize the elimination of materials caused by internal stress.

The second stage occurs in the process of pattern transfer in this stage of expansion is mainly affected by the internal stress caused by the change of orientation.

To ensure the stable expansion line of the transfer process, all good baking boards are not grinding board operation, direct surface pretreatment by chemical cleaning line, after lamination surface shall be flat, surface before and after exposure of standing time must be sufficient, after the completion of transfer line, due to the stress orientation changes, flexible plate will be presented different degrees of curling and contraction, so the line film compensation control in relation to the control accuracy of the combination of hardware and software, at the same time, the expanding range of flexible plate is determined, the production data supporting rigid plate.

In the process of pressing the soft hard occurred in third stages. In this stage, the expansion and contraction of the main pressing parameters and material properties are determined.

The factors influence this stage include the expansion pressure of the heating rate, copper residue rate and thickness of several pressure parameter settings and core board. In general, the residual copper rate is smaller, the greater the value of expansion; the core plate is thin, the greater the value. However, from the large to the small, is a gradual process of change, therefore, the compensation is particularly important. In addition, due to the different nature of the flexible plate and the rigid plate material, the compensation is an additional factor to be considered.

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