Pressing Process Of Flexible Printed Circuit Board Warping Effect Of The Finite Element Research

With the development of assembly technology of electronic components, the improvement of the assembly technology increases the requirements of the flatness of flexible printed circuit board (FPC). Thermal strain and residual thermal stresses of FPC are the main reasons causing the warping in repression process, which will lead the component placement and solder joint to failure, and gold fingers to degradation. The ANSYS software based on the finite element principle can be used to simulate the three-dimensional model of FPC in laminating process, analyze the factors of warpage, and find out the reasons causing warping, which can avoid or reduce the FPC warping problems and improve the quality of FPC. Therefore, it has significant applications in theory and practical manufacture.In the thesis, the finite element method was employed to build the mathematical model of the temperature field and thermal stress in the laminating process of FPC. The3D physical models of FPC were built using the ANSYS software based on the finite element principle to simulate three-dimensionally the laminating process of FPC. In summary, through the simulation and calculation, the thermal strain and stress distribution map of FPC were obtained. The effect of material thickness of FPC on the thermal strain and stress distribution in the laminating process were investigated, and the results indicated that the FPC with small thickness is beneficial to the low thermal stress, and the thermal stress is mainly involved in the laminating process under the large thermal strain. The PFC is a multilayer composition, including copper foil, polyimide (PI) layer and bonding layer. We analyzed the thermal strain and stress distribution of every layer in FPC. Obviously, the material located at different layer suffers from the discriminatory thermal stress and corresponding strain. Through the comparison of thermal stress and strain in different materials, we found that the main reasons causing the warping are the modulus of elasticity and the coefficient of thermal expansion of the epoxies in bonding layer with a great difference at～48℃. Meanwhile, the coefficient of thermal expansion of the epoxies is larger than those of copper foil and polyimide, and thermal expansion process is limited by copper foil and polyimide layers. The thermal stress derived from the limited thermal expansion causes the thermal strain. The laminating processes of FPC have two methods:hot repression and fast hot repression. According to the simulative results, the fast hot repression can reduce effectively the thermal strain and stress in the laminating process, resulting the decrease of the warpage. Hence, we regard that the valid approaches to reduce the warpage in the laminating process are decreasing the thickness of banding layer, increasing the thicknesses of copper foil and PI layers and using the fast hot repression method.