Effect of boundary conditions on fatigue life of board-level BGA solder joints under random vibration

We investigated the effect of fastening area of a board with mounted packages on the fatigue life of solder balls under random vibration. First, we conducted a finite element analysis (FEA) and a lifetime test to develop the S–N curve of solder balls through which the packages and printed circuit board (PCB) are connected. In the FEA, the global–local modeling technique was applied, and the stress of solder balls was determined with the sine-sweep excitation near the first natural frequency. Also, the finite element (FE) model was verified by comparing the simulated natural frequencies and mode shapes with measured ones. An experimental setup to apply the sine-sweep excitation was developed to determine the number of cycles to failure of solder balls. Utilizing the developed S–N curve of the solder ball and the Steinberg three-band method, the lifetime of solder balls under random vibration was predicted in two different boundary conditions of the PCB, i.e., screw joints only and screw joints with plain washers. Results indicate that the first natural frequency increases with increased fastening area due to the addition of washers between the PCB and screws. In addition, the increased fastening area due to the washers increases the lifetime of solder balls under random vibration. Finally, the simulated lifetime of the solder balls under random vibration was verified experimentally.