The growth behavior of intermetallic compounds (IMCs) at the liquid-solid interfaces in Cu/Sn/Cu interconnects during reflow at 250 °C and 280 °C on a hot plate was investigated. Being different from the symmetrical growth during isothermal aging, the interfacial IMCs showed clearly asymmetrical growth during reflow, i.e., the growth of Cu₆Sn₅ IMC at the cold end was significantly enhanced while that of Cu₃Sn IMC was hindered especially at the hot end. It was found that the temperature gradient had caused the mass migration of Cu atoms from the hot end toward the cold end, resulting in sufficient Cu atomic flux for interfacial reaction at the cold end while inadequate Cu atomic flux at the hot end. The growth mechanism was considered as reaction/thermomigration-controlled at the cold end and grain boundary diffusion/thermomigration-controlled at the hot end. A growth model was established to explain the growth kinetics of the Cu₆Sn₅ IMC at both cold and hot ends. The molar heat of transport of Cu atoms in molten Sn was calculated as + 11.12 kJ/mol at 250 °C and + 14.65 kJ/mol at 280 °C. The corresponding driving force of thermomigration in molten Sn was estimated as 4.82 × 10⁻¹⁹ N and 6.80 × 10⁻¹⁹ N.

As the diameter of solder interconnects in three-dimensional integrated circuits (3D ICs) downsizes to several microns, how to achieve a uniform microstructure with thousands of interconnects on stacking chips becomes a critical issue in 3D IC manufacturing. We report a promising way for fabricating fully intermetallic interconnects with a regular grain morphology and a strong texture feature by soldering single crystal (111) Cu/Sn/polycrystalline Cu interconnects under the temperature gradient. Continuous epitaxial growth of η-Cu₆Sn₅ at cold end liquid-Sn/(111)Cu interfaces has been demonstrated. The resultant η-Cu₆Sn₅ grains show faceted prism textures with an intersecting angle of 60° and highly preferred orientation with their $⟨11\overline{2}0⟩$ directions nearly paralleling to the direction of the temperature gradient. These desirable textures are maintained even after soldering for 120 min. The results pave the way for controlling the morphology and orientation of interfacial intermetallics in 3D packaging technologies.

β-Sn grain orientation and configuration are becoming crucial factors to dominate the lifetime of solder interconnects in three-dimensional integrated circuit packaging. In this paper, we found that a temperature gradient during solidification significantly dominated the orientation and configuration of the final β-Sn grains in Cu/SnAgCu/Cu micro interconnects. Being different from the random orientations and growth fronts meeting or cyclic twin boundary forming near the center after homogeneous temperature bonding, the β-Sn grains solidified under a certain temperature gradient were observed to follow a highly preferred orientation with their c-axis departing from the direction of temperature gradient by about 45°–88°. Meanwhile, these preferred oriented β-Sn grains consisted of low angle grain boundary structures with misorientation in the range of 0°–15°. The mechanism was explained in terms of the anisotropy and directional growth of β-Sn grains. The results pave the way for grain orientation control in 3D packaging technology.

A novel solid–liquid interdiffusion (SLID) bonding method with the assistance of temperature gradient (TG) was carried out to bonding Cu and Ni substrates with Sn as interlayer. The element distribution and grain morphology of interfacial intermetallic compound (IMC) in Cu/Sn/Ni micro-joints during both SLID and TG-SLID bonding and in the final Cu/(Cu,Ni)₆Sn₅/Ni full IMC micro-joints were analyzed. Under the effect of Cu-Ni cross-interaction, interfacial (Cu,Ni)₆Sn₅ dominated the IMC growth at all the interfaces. The morphology of the (Cu,Ni)₆Sn₅ grains was closely related to Ni content with three levels of low, medium and high. The full IMC micro-joints consisted of L-(Cu,Ni)₆Sn₅, M-(Cu,Ni)₆Sn₅ and H-(Cu,Ni)₆Sn₅ grains after SLID bonding or TG-SLID bonding with Ni as hot end, while only L-(Cu,Ni)₆Sn₅ grains after TG-SLID bonding with Cu as hot end, showing that the direction of TG had a remarkably effect on the growth and morphology of the interfacial (Cu,Ni)₆Sn₅ during TG-SLID bonding. Thermodynamic analysis revealed the key molar latent heat and critical Ni content between fine-rounded-like (Cu,Ni)₆Sn₅ and block-like (Cu,Ni)₆Sn₅ were 17,725.4 J and 11.0 at.% at 260 °C, respectively. Moreover, the growth kinetic of the interfacial IMC was analyzed in detail during bonding with and without TG. Under the combination of TG and Cu-Ni cross-interaction, void-free full IMC micro-joints were fast formed by TG-SLID bonding with Cu as hot end. This bonding method may present a feasible solution to solve the problems of low formation efficiency and inevitable Cu₃Sn growth of full IMC joints for 3D packaging applications.