Studies of Solder Joint Reliability under Mechanical Bending Test on FR-4 PCBs with Sn-4.0Ag-0.5Cu Solder Paste

The mechanical bending test results, as well as the modeling and calculation data were presented in this study to characterize the solder joint reliability. Bending test was completed simply by loading a series of displacements on the FR-4 printed circuit board (PCB) with devices and solder joints in single direction. A special bending tester that can control displacement exactly by a cam system was designed and used for reliability study. The three group samples need to endure bend distortion with different deflections respectively, namely 3 mm, 4 mm, and 5 mm. The electrical resistance was probed every 3600 cycles by multimeter. When the resistance of solder joint got 10% increase, the failure can be defined. The experimental results indicate that the fatigue life decreased obviously with the displacement increased. By using optical microscope and SEM photographs, two kinds of failure model were found in solder joints. The majority failure model, which took place at the bottom corner of solder joint under the termination of resistor initially, and propagated into the solder matrix. The delamination appeared at the interface between the termination of resistor and its ceramic body. The finite element analysis was presented to support this experiment. The shear strain ranges under the different deflections were obtained from simulation. A Coffin-Mason equation of Sn-4.0Ag-0.5Cu from literature was used to compare with this experimental result and prove the feasibility of two points bending test in fatigue life prediction.