Mechanical properties of fine pitch devices soldered joints based on creep model

Abstract

Nonlinear analyses of quad flat package (QFP) on printed circuit board (PCB) assemblies subjected to thermal cycling conditions are presented.Two different solders are considered,namely, Sn37Pb and Sn3.5Ag.The stress and strain response of fine pitch devices soldered joints was investigated by using finite element method based on Garofalo-Arrheninus model.The simulated results indicate creep distribution of soldered joints is not uniform,the heel and toe of soldered joints, the area between soldered joints and leads are the creep concentrated sites.The similar phenomena of stress curves simulated based on Garofalo-Arrhenlnus model and Anand equations is confirmed,and the creep strain value of Sn3.5Ag soldered joints is lower than that of Sn37Pb soldered joints. Thermal cycling results show that Sn3.5Ag strongly outperforms Sn37Pb for QFP devices under the studied test condition.This is well matched with the experimental outcome analyzed.In addition,the soldered devices were tested by micro-joints tester,the tensile strength of Sn3.5Ag soldered joints is found to be higher than that of Sn37Pb soldered joints.By analyzing the fracture microstructure of soldered joints,it is found that fracture mechanism of Sn3.5Ag soldered joints is toughness fracture, while fracture mechanism of Sn37Pb soldered joints includes brittle fracture and toughness fracture. The results of this study provide an important basis of understanding the mechanical properties of fine pitch devices with traditional Sn37Pb and Sn3.5Ag lead-free soldered joints.