Microstructural evolution and mechanical properties of chip-stacking micro-solder joints under TG bonding process

As chip-stacking micro-joints are downsized to less than 20 μm, the microstructural evolution induced by multiple TG bonding has emerged as a critical concern for mechanical properties. The evolution of microstructure and mechanical properties in miniaturized micro-solder joints was systematically investigated across various TG bonding durations. The interfacial IMCs exhibit a rapid and asymmetric microstructural evolution at both ends. This characteristic intensified until the continuously growing columnar Cu 6 Sn 5 grains at the hot end span the Sn interlayer to form single grains. Moreover, the Sn grains and predominant columnar Cu 6 Sn 5 grains display highly preferred orientations, while the small Cu 3 Sn grains exhibit random orientation. These evolutions differ markedly from those observed under isothermal bonding. The kinetic mechanisms driving this unique evolution were comprehensively elucidated through multiple factors. Subsequent tensile testing revealed correlations between microstructural evolution, tensile strength, and fracture behavior. Specifically, the thickness ratio of interfacial IMCs dominates the fracture mode, while transformed morphology primarily affects fracture paths and mechanical strength. The fracture mechanisms were thoroughly analyzed to explain the anomalous changes in tensile strength with IMCs microstructural evolutions. Interestingly, network-like diffusion channels were unexpectedly found within the Cu 3 Sn grains on the hot end after tensile testing, offering an additional explanation for this asymmetric evolution. These results provide valuable insights that can guide the design and implementation of next-generation reliable interconnections in 3D chip stacking. • TG bonding induced an asymmetric microstructural evolution, including IMCs' thickness, morphology, and orientation. • This evolution results from Cu unidirectional diffusion, IMCs dynamic equilibrium, and Cu directional diffusion channels. • Such a unique evolution correlates strongly with the tensile strength and fracture mechanisms of the micro-joints. • IMCs thickness dominates the fracture mode, whereas morphology evolution affects the fracture path and tensile strength. • Highly preferred orientation of Sn grains and Cu 6 Sn 5 grains can ensure stable mechanical properties of micro-joints.