Solder metallization interdiffusion in microelectronic interconnects

We investigated the growth of intermetallic compounds in Cu/Ni/Au/PbSn solder joints. The substrates that we investigated had been Au plated by one of two different techniques. The Au finish thicknesses ranged from 0.25 to 2.6 /spl mu/m. After solder renew, structural examinations using optical and electron microscopy of cross-sectioned solder joints revealed the growth of Ni/sub 3/Sn/sub 4/ at the solder/Ni interface after reflow. Solder joints with thicker layers of Au annealed in Ar gas at a temperature of 150/spl deg/C for up to 450 h, displayed an appreciable growth of Au/sub 0.5/Ni/sub 0.5/Sn/sub 4/ at the Ni/sub 3/Sn/sub 4//solder interface. Previous investigators correlated growth of a Au-Sn alloy with the degradation of the mechanical properties of the solder joint. The determination of the stoichiometry of the Au/sub 0.5/Ni/sub 0.5/Sn/sub 4/ phase provides some understanding of why this phase grew at the Ni/sub 3/Sn/sub 4//solder interface, as Sn, Au and Ni are all readily available at this interface. The growth of this ternary alloy is also consistent with trends observed in the kinetics of formation of solder alloys.