Solder metalization interdiffusion in microelectronic interconnects

We investigated Intermetallic compound formation mechanisms and their effect on the integrity of ball grid array Cu/Ni/Au/solder joints integrity were investigated. Substrates with three types of Au plating, and thus three different thicknesses [Electrolytic (2.6 and 0.75μm), Immersion (0.25μm), and Selective (0.02μm)] were used. After solder reflow, the solder joints were annealed for up to 1000 hrs at 150°C. Optical and electronic metallography together with Energy Dispersive Spectroscopy were used to locate and identify phases present in the joint for different annealing times. Brittle failure of solder joints was ascribed to the formation of a ternary intermetallic (Au_0.5Ni_0.5)Sn₄ at the interface solder/substrate. In the absence of post-reflow thermal aging, only Ni₃Sn₄ was observed at the interface and it did not decrease the mechanical reliability of the joint. Tensile-shear stress tests were performed on unaged samples as well as samples aged for 1 hr, 4 hrs and 450 hrs.