The origin of the expansion and Contraction of Rigid-Flex PCB is determined by the characteristics of the materials. To solve the problem of the expansion and contraction of the soft and hard binding plate, the polyimide material of the flexible plate must be introduced first:
(1) Polyimide has excellent heat dissipation performance and can withstand the heat-shock of lead-free welding 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 transfer temperature and high melting point. Generally, it needs to be processed above 350 ℃;
(4) In terms of organic dissolution, polyimide is not soluble in general organic solvents.
The expansion and contraction of the flexible board material are mainly related to the matrix material PI and glue, that is to say, it has a great relationship with PI imidization. The higher the degree of imidization, the stronger the controllability of the expansion and contraction.
According to the normal fabrication experience, the flexible board will produce different degrees of expansion and contraction during the process of the formation of the graphic circuit and the rigid-flex PCB after the cutting of the material.
After the etching of the graphic circuit, the density and trend of the circuit will lead to the reorientation of the stress on the whole board surface and eventually lead to the general regular expansion and contraction of the board surface.
In the process of the rigid-flex PCB fabrication, the coefficient of expansion and contraction of the surface coating film is different from that of the matrix material PI, which will produce a certain degree of expansion and contraction in a certain range.
In essence, the expansion and contraction of any material are caused by the influence of temperature. During the lengthy PCB manufacturing processes, the expansion and contraction value of materials will have different degrees of subtle changes after many hot and wet processes, but in the long-term practical production experience, the changes are still regular.
How to control and improve the Expansion and Contraction of Rigid-Flex PCB?
In a strict sense, the internal stress of each roll of material is different, and the process control of each batch of production plate is not exactly the same. Therefore, the control of material expansion and contraction coefficient is based on a large number of experiments.
And the process control and data statistical analysis are particularly important. In practical operation, the expansion and contraction of flexible plate are divided into stages:
1. First of all, from opening to baking, the rise and fall of this stage are mainly caused by temperature:
In order to ensure the stability of the expansion and contraction caused by the baking board, the consistency of process control is the first thing. On the premise of unified materials, the operation of heating and cooling of the baking board must be consistent each time.
It is not allowed to put the baking board in the air for heat dissipation because of the pursuit of efficiency. Only in this way can we get rid of the rise and fall caused by the internal stress of the material.
2. The second stage occurs in the process of pattern transfer, which is mainly caused by the change of internal stress orientation.
In order to ensure the stability of the expansion and contraction of the circuit transfer process, all baked boards cannot be ground.
The surface shall be treated directly through the chemical cleaning line. The surface shall be flat after the film is pressed, and the standing time of the board surface before and after the exposure shall be sufficient.
After the circuit transfer is completed, due to the change of the stress orientation, the flexible boards will show different degrees of curl and contraction, so the circuit phenanthrene The control of forest compensation is related to the control of the accuracy of the combination of hard and soft.
At the same time, the determination of the range of the expansion and contraction value of the flexible plate is the data basis for the production of its supporting rigid plate.
3. In the third stage, the expansion and contraction occur in the process of soft and hard plate compression, which is determined by the main compression parameters and material characteristics.
In this stage, the influencing factors include heating rate, pressure parameter setting, residual copper rate, and thickness of core plate.
In general, the smaller the residual copper rate is, the greater the rise and fall value is; the thinner the core plate is, the greater the rise and fall value is. However, it is a gradually changing process from large to small. Therefore, film compensation is particularly important.
In addition, due to the different nature of the flexible plate and the rigid plate, compensation is an additional factor to be considered.