As solder joints are being scaled down, intermetallic compounds (IMCs) are playing an increasingly critical role in the reliability of solder joints, and thereby an in-depth understanding of IMCs microstructure evolutions in micro-joints is of great significance. This study focused on coarsening behaviors of Ag₃Sn compounds in Sn-3.0Ag-0.5Cu (SAC305) micro-joints of flip chip assemblies using thermal shock (TS) tests. The results showed that the Ag₃Sn compounds grew and rapidly coarsened into larger ones as TS cycles increased. Compared with such coarsening behaviors during thermal aging, TS exhibited a significantly accelerating influence. This predominant contribution is quantitatively determined to be induced by strain-enhanced aging. Moreover, based on observations for Ag₃Sn microstructure evolutions during TS cycling, one particular finding showed that there are two types of coarsening modes (i.e., Ostwald ripening and Necking coalescence) co-existing in the Ag₃Sn coarsening process. The corresponding evolutions mechanism was elucidated in a combination of simulative analysis and experimental validation. Furthermore, a kinetic model of the Ag₃Sn coarsening was established incorporating static aging and strain-enhanced aging constant, the growth exponent (n) was calculated to be 1.70, and the predominant coarsening mode was confirmed to be the necking coalescence.

In this work, SAC305 lead-free solder reinforced with 0.1 wt. % fullerene nanoparticles was prepared using a powder metallurgy method. A lab-made setup and a corresponding Cu/solder/Cu sample for thermo-migration (TM) test were designed and implemented. The feasibility of this setup for TM stressing was further verified with experimental and simulation methods; a temperature gradient in a solder seam was calculated as 1070 K/cm. Microstructural evolution and mechanical properties of both plain and composite solder alloys were then studied under the condition of TM stressing. It was shown that compared to unreinforced SAC305 solder, the process of diffusion of Cu atoms in the composite solder seam was remarkably suppressed. After the TM test for 600 h, Cu/solder interfaces in the composite solder seam were more stable and the inner structure remained more intact. Moreover, the addition of fullerene reinforcement can considerably affect a distribution of Cu₆Sn₅ formed as a result of dissolution of Cu atoms during the TM test. Hardness data across the solder seam were also found notably different because of the elemental redistribution caused by TM.

In this study, 96.5Sn–3Ag–0.5Cu (SAC305) lead-free composite solder containing graphene nanosheets (GNS) decorated with Ni nanoparticles (Ni-GNS) was prepared using a powder metallurgy method. A lab-made set-up and a corresponding Cu/solder/Cu sample design for assessing thermo-migration (TM) was established. The feasibility of this setup for TM stressing using an infrared thermal imaging method was verified; a temperature gradient in a solder joint was observed at 1240 K/cm. Microstructural evolution and diffusion of Cu in both plain and composite solder joints were then studied under TM stressing conditions. Compared to unreinforced SAC305 solder, the process of diffusion of Cu atoms in the composite solder joint was significantly reduced. The interfacial intermetallic compounds (IMCs) present in the composite solder joint also provide a more stable morphology after the TM test for 600 h. Furthermore, during the TM test, the Ni-GNS reinforcement affects the formation, migration and distribution of Ni–Cu–Sn and Cu–Sn IMCs by influencing the dissolution rate of Cu atoms.

This study focuses on the microstructural evolution process of Cu–Sn–Ni intermetallic compounds (IMCs) interlayer in the micro-joints, formed from the initial Ni/Sn (1.5 µm)/Cu structure through transient liquid phase (TLP) soldering. Under the bonding temperature of 240 °C, the micro-joints evolve into Ni/(Cu, Ni)₆Sn₅/(Cu, Ni)₃Sn/Cu structure, where the interfacial reactions on Cu/Sn and Sn/Ni are suppressed by the atoms diffusing from the opposite side. The thickness of (Cu, Ni)₃Sn layer on plated Cu layer still increases with the prolonged dwell time. When the bonding temperature was elevated to 290 °C, the phase transformation of (Cu, Ni)₆Sn₅ into (Cu, Ni)₃Sn has been accelerated, thus the majority of IMCs interlayer is constituted with (Cu, Ni)₃Sn. However, a small amount of Ni-rich (Cu, Ni)₆Sn₅ phases still remain near the Ni substrate and some of them close to the center-line of IMCs interlayer. The state between (Cu, Ni)₆Sn₅ and the adjacent (Cu, Ni)₃Sn tends to reach equilibrium in Ni content based on the observation from Transmission Electron Microscope (TEM). In addition, the Cu–Sn–Ni IMCs micro-cantilevers were fabricated from these micro-joints using Focus Ion Beam (FIB) for the in situ micro-bending test, the results indicate a high ultimate tensile strength as well as the brittle fracture in the inter- and trans-granular modes.