Microstructure and shear strength of Au-20wt%Sn solder joints fabricated by thermo-compression bonding for LED packages

Solder joints of Au-20wt%Sn (Au-20Sn) between real light-emitting diode Si chips and AlN substrates were fabricated using thermo-compression (TC) bonding. We investigated the microstructure of TC-bonded solder joints using scanning electron microscopy, transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM) and its effect on shear strength. The metallization system for the Au-20Sn solder consisted of Pt/Ti thin films on Si chips and an electroless Ni/Pd/Au layer on AlN substrates. The TC bonding was carried out at 280, 290, 300, and 310 °C for 0.2, 0.6, and 1.2 s. Samples were aged at 200 °C for up to 1000 h in a conventional oven. The performance of the solder joints was sensitive to joining temperature. When the solder joint was formed at a relatively low temperature, shear strength was low due to the unbonded regions in the solder joints. The shear strength increased with joining temperature and time. When the solder joints were formed at a relatively high temperature, shear strength was high. The solder joints were composed of δ-phase at both interfaces and ζ’-phase at the center. After aging at 200 °C for up to 1000 h, shear strength slightly decreased. The main fracture modes before and after aging were Si cohesive fracture and interfacial fracture between Si and Ti, respectively. TEM and STEM clearly indicated that the Pt layer remained at the solder interface after aging. The high shear strength was attributed to the strong interface between Pt and the Au-20Sn solder.