Prediction of bending reliability of BGA solder joints on Flexible Printed Circuit (FPC)

In this paper, the mechanical performance of solder joints of BGA mounted on flexible printed circuit (FPC) was studied using finite element method (FEM). To optimize the electric components layout during the design, it is necessary to predict the bending reliability of components on FPC. Traditionally, 3-point or 4-point bending are common used to study if the substrate is rigid. In order to simulate the free bending condition with soft substrate, a new bending method is taken. That´s to fix the FPC at one side and move the other side to bend the FPC. Based on this novel model, the length of the FPC, the height and diameter of the solder joints are respectively taken as the optimized variables. With the variable change, there are many different cases. For one general case, the bending process is divided into two steps. At first, to rotate the FPC about 180deg at one side and make the two fixtures parallel; then move the fixture to control the bending. It is found that the stress of the solder joints in first step linearly proportioned increase with the bending. While in the second step, the stress reaches the peak and declines gradually with the two ends of the FPC get closer. Simulation results indicate that by using the longer FPC can improve the reliability significantly. Normally, reliability can be also improved by increasing the height and diameter of the solder joints. While in the translating-induced bending, increasing the diameter will worsen the solder reliability unless the diameter is larger than 0.3 mm.