Size effects on IMC growth of Cu/Ni/Sn-3.5Ag microbump joints during isothermal aging and prediction of shear strength using ANN

Significant downsizing trend is currently directing the advanced microelectronic packaging industry, which poses new challenges to the manufacturing process and reliability of high-density chip interconnections. In this paper, to investigate the size effect of microbump joints on the interfacial structure and bonding strength, the chip-to-chip bonding joints fabricated by microbumps of same Cu/Ni/Sn-3.5Ag construction but of various sizes were experimentally examined. After thermocompression bonding process to fabricate the chip-to-chip microbump interconnections, the intermetallic compound (IMC) growth on the Ni/Sn-3.5Ag interface during 150 °C isothermal aging was compared for joints of 100, 40 and 25 μm in diameter as well as 35, 25 and 15 μm in solder layer height, using field emission electron microscopy and electron probe microanalysis. A clear size effect on the thickness and morphology of Ni3Sn4 IMC was observed, showing thicker IMC in joints of smaller diameter and lower height. The Ni concentration distribution in the solder layer was verified to be dependent on the joint size, with possible reasons being enhanced sidewall diffusion and grain boundary diffusion through IMC grains in small-diametered joints, as well as shorter diffusion path in lower joints. The shear strength of chip-to-chip bonding vehicles was also found to be size-dependent. In addition, artificial neural network (ANN) approach was applied to establish the numerical correlation of structural factors and bonding strength. It was found that a properly constructed ANN model can successfully predict the influence of the complex combinations of geometric and interfacial factors with highly accurate output, providing an effective tool for design and optimization of microbump interconnection structures and processes.