Effect of nickel microalloying on the microstructure and properties of In-35 wt%Sn low temperature solder

As the microelectronics industry faces challenges in sustaining Moore's Law due to physical limitations in component miniaturization, the reliability and performance of solder joints in electronic assemblies becomes crucial. The adoption of lead-free solder materials has raised concerns related to processing temperatures inducing dynamic warpage, reduced yield, and reliability concerns. To address these challenges, this thesis investigates the use of low temperature solder (LTS), specifically In-35Sn (wt%) solder with 0.05 wt% nickel (Ni) microalloying. Various techniques, including synchrotron Xray radiography imaging (SXRI), scanning electron microscope (SEM), transmission electron microscope (TEM), differential scanning calorimeter (DSC), and high-speed shear (HSS) testing are employed to comprehensively analyze the prepared solder, microstructure and soldering behavior. In-situ real-time SXRI observations reveal complex interactions during the solidification of the alloy, providing insights into how Ni suppresses primary arm growth by a factor of 0.652 and reduces secondary arm spacing by a factor of 0.502, impacting microstructure evolution. These interactions are challenging to observe using conventional methods that provide micrographs of solidified microstructures. Furthermore, the thesis explores microstructure evolution during solidification and the formation of interfacial Cu3(Sn,In) intermetallic compounds (IMCs) in In-35Sn solder alloys on copper (Cu) substrates. The SXRI observations reveal Ni microalloying providing nucleation sites for the β-In3Sn phase and reducing the degree of undercooling to near zero. Additionally, the TEM observations reveal that Ni microalloying results in refining microstructures of interfacial IMCs formation in In- 35Sn solder on Cu substrates, and improving mechanical properties by increasing shear strength by 5.62% and 3.45%, respectively, and increasing fracture initiation energy by 4.35% and 18.55%, compared to the reference solder joint during shear at speeds of 100 mm/s and 2000 mm/s. Furthermore, the thesis investigates how multiple reflow cycles during assembly processes impact primary Cu2(In,Sn) particles, the interfacial Cu3(Sn,In) layer, and solder joint shear strength. The findings validate nucleation and solidification kinetics and demonstrate a 60 % refinement of Cu2(In,Sn) particles and increased energy absorption of the solder joint by 20.77%, 35.40%, and 6.85% during the first, third, and sixth reflows. Collectively, the results show that Ni microalloying profoundly impacts alloy solidification, interfacial reactions, and phase formation. This approach effectively modifies microstructure and enhances the properties of In-35Sn solder joints on Cu substrates. The findings not only deepen the understanding of alloy behavior but also offer practical implications for the design of novel LTS compositions. This work contributes to the advancement of microelectronics packaging, bridging the gap between scientific understanding and industrial applicability, paving the way for improved and reliable LTS joint solutions in the development of Pb-free electronics assembly.