Evolution of Ag3Sn at Sn–3.0Ag–0.3Cu–0.05Cr/Cu joint interfaces during thermal aging

The intermetallic compounds (IMC) in the solder and at the interface of Sn–3.0Ag–0.5Cu (SAC)/Cu and Sn–3.0Ag–0.3Cu–0.05Cr (SACC)/Cu joints were investigated after isothermal aging at 150 °C for 0, 168 and 500 h. Different shaped Ag3Sn phases were found near the IMC layer of the latter joint. Interestingly, fine rod-shaped and branch-like Ag3Sn were detected near the interface after soldering and long Ag3Sn changed into shorter rods and small particles during aging. It is investigated that the Cr addition and thermal aging have effect on the evolution of Ag3Sn morphologies and it is controlled by interfacial diffusion. Energy minimization theory and the redistribution of elements are used to explain the morphological evolution of Ag3Sn. Small Ag3Sn particles were also found on the IMC layer after aging, unlike the large Ag3Sn at that of SAC/Cu joints. In conclusion, a favorable morphology of the joint interface leads to better bonding properties for SACC/Cu joints against thermal aging than that for SAC/Cu.

► Different shaped Ag3Sn particles were found at interfaces of Pb-free solder joints.
► Fine rod-shaped and branch-like Ag3Sn existed near the IMC layer after soldering.
► Long Ag3Sn changed into shorter rods and small particles during aging.
► The evolution of Ag3Sn morphologies is controlled by interfacial diffusion.
► Cr along with Ag and Cu in Sn matrix was detected through TEM and SEM.