Mechanical properties of Sn-Pb based solder joints and fatigue life prediction of PBGA package structure

Solder joints are generally the weakest part in electronic packaging structure whose fatigue life depends to a large extent on the durability of solder joints. In pursuit of a balance between environmental protection and soldering performance, commonly used Sn-Pb solder should be modified by adding other chemical elements to reduce Pb content. This work aims to investigate the performance of different solder materials and explore the impact of different electronic package structures. Firstly, tensile experiments are performed on three kinds of solders (Sn-Pb, Sn-Pb-In, Sn-Pb-Bi) to obtain their mechanical behavior, and then the constitutive laws of them are calibrated by Anand model. Additionally, finite element method (FEM) is used simulate the response of plastic ball grid array (PBGA) package structure under thermal cycles, and mechanical variables including deformation, equivalent stress, and equivalent strain are analyzed. Finally, Coffin-Manson and Engelmaier equations are adopted to predict the fatigue life of package structures and the effects of various factors such as solder materials, dimensions of components, and welding quality are further investigated. Results reveal that adding Bi into Sn-Pb solder can make the solder joints have better performance. From the perspective of optimum fatigue life, the plastic package thickness is recommended in the range of 0.8 mm-0.9 mm, the chip thickness is recommended as 0.35 mm, the substrate thickness is recommended to be less than 0.34 mm, and the ratio of stand-off height to solder ball diameter should be controlled between 0.6 and 0.7. According to the prediction, the solder ball diameter is suggested to be 0.54 mm so that fatigue life could reach 194 cycles, and the stand-off height for this package structure is suggested to be 0.36 mm so that fatigue life could reach 123 cycles.