Effect of thermal ageing on the shear strength of lead-free solder joints

In recent years, as a response to the proposed legislation and regulations restricting the use of lead-based solders in the electronics industry, efforts to develop alternative solders have increased dramatically. In this work lead-free solder alloys with compositions of Sn96.5-Ag3.5, Sn96.3-Ag3.2-Cu0.5, Sn93.5-Ag3.5-Bi3 and Sn90-Ag2-Bi7.5-Cu0.5 (wt%) have been used to solder copper "components" to Ni/Au and Sn/Pb metallisations in order to simulate the solder joint in electronics. The shear strength of the soldered joints after thermal ageing at 25 degrees C, 100 degrees C and 150 degrees C for 200 hours, 500 hours and 1000 hours have been evaluated. The fracture surfaces have been studied using scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS). In general the shear strength decreases as the ageing time increases, however the Sn-Ag systems are shown to be more stable with ageing temperature and time. Bismuth containing alloys are shown to have the highest strength initially but after ageing the strength of the joints become very weak.