The shear strength and fracture surfaces of Sn-Pb bump and Au stud bump for photodiode packages after isothermal aging testing were studied experimentally. Al/Au stud bumps and Cu/Sn-37 mass%Pb solders were adopted, and aged at 373 K and 423 K for up to 900 hours to analyze the effect of intermetallic compound (IMC). The shear strength decreased with aging time. The diffraction patterns of Cu₆Sn₅, scallop-shaped IMCs, and planar-shape Cu₃Sn were observed by transmission electron microscopy (TEM). The formation of Kirkendall voids with the growth of IMCs at the solder was found to be a possible mechanism for shear strength reduction. IMCs between Au stud bumps and Al pads were identified as AlAu₂.