Harmonic vibration analysis and S-N curve estimate of PBGA mixed solder joints

With the development of lead-free in electronic products, mixed soldering of lead-free component with lead solder is inevitable in high reliability electronic assembly. For the mixed assemblies, the primary task is to ensure the reliability of mixed solder joints. This study is aimed at finding out the harmonic vibration fatigue properties of plastic ball grid array (PBGA) mixed solder joints under 1st natural frequency and getting the relation curve between equivalent stress and number of fatigue cycles of the mixed solder joints. A specified PBGA assembly was designed with a piece of daisy-chained printed circuit board (PCB). Modal test and harmonic vibration test were implemented. The PCB strain was measured by strain gages, and the time to failure of solder joints was monitored by an event detector. After comparing natural frequencies, modal shapes and PCB strain with the simulation corresponding results, finite element model (FEM) was validated. Then, the volume averaged equivalent stress of mixed solder joints was discovered with modified model. Finally, S-N curve of mixed solder joints was fitted with Basquin power law relation. In the hopeful future, combining the fitted S-N curve with other methodologies, the vibration fatigue life of mixed solder joints could be estimated.