Sintering Fundamentals of Nano-Metallic Particle Interconnects

Abstract

This thesis systematically investigated themicrostructure evolution and mechanical behavior of sintered copper (Cu) nanoparticles (NPs) using an integrated approach combining multiscale simulations with micromechanical tests. The research addressed critical challenges in the design and optimization of sintered Cu for advanced electronic applications, with a focus on anisotropic fracture behavior, porosity-dependent properties, thermal effects, interfacial strength, particle morphology, and the role of ALD coatings.

Chapter 1 established the foundational context for this study by underscoring the significance of sintered Cu NPs in electronics packaging and identifying critical gaps in the existing literature. It defined the research objectives and outlined the thesis structure, emphasizing the necessity of an integrated experimental and computational approach to link microstructure evolution with mechanical performance.

Chapter 2 systematically investigated the influence of sintering pressure on the evolution of anisotropic microstructures in sintered Cu NPs. By integrating molecular dynamics (MD) simulations with micro-cantilever bending tests, the study revealed that uniaxial sintering pressure promoted preferential neck alignment along the loading direction, resulting in pronounced mechanical anisotropy. These findings underscored the critical role of processing parameters in governing microstructural orientation and, consequently, in tailoring the mechanical performance of sintered Cu NPs. This chapter established a foundational understanding of the process–structure–property relationships central to nanoparticle-based sintering.

Chapter 3 examined the influence of sintering pressure on densification and mechanical performance. Sintered Cu NPs exhibited a favorable combination of high strength and low elastic modulus, highlighting their suitability for electronic packaging. A two-stage deformation mechanism was identified, governed by the material’s porous architecture and heterogeneous microstructure. Initial compressive loading compacted voids and formed dense interparticle contacts, followed by plastic deformation primarily accommodated through intragranular slip.

Chapter 4 investigated the strain rate- and temperature-dependent viscoplastic behavior of sintered Cu NPs. Micro-pillar compression tests, coupled with the Anand viscoplastic model, revealed the competing effects of strain-rate hardening and thermal softening. Phase-field fracture simulations further elucidated the influence of porosity on crack initiation and propagation, providing critical insight into fracture mechanisms under varying loading conditions.

Chapter 5 focused on the interfacial strength and fracture behavior of sintered Cu NPs bonded to Cu substrates. Micro-cantilever bending tests and cohesive zone model (CZM) demonstrated that interface-notched specimens exhibited superior fracture resistance, with a stress intensity factor (KQ) of 2.88 ± 0.10 MPa·m1/2, compared to 2.12 ± 0.11 MPa·m1/2 for Cu NP-notched specimens. Simulations aligned with ex perimental results, showing that reduced fracture resistance in Cu NP-notched samples stemmed from porosity and stress concentration, while enhanced strength at the interface was attributed to strong bonding andminimized void formation.

Chapter 6 examined the effect of Cu particle morphology on densification and fracture toughness. Fracture resistance was found to be highly morphology-dependent: bimodal sintered Cu exhibited the greatest toughness, attributed to effective crack deflection and bridging. Monomodal Cu showed moderate resistance, while flake-shaped Cu displayed the lowest toughness, primarily due to weak interlamellar bonding and anisotropic porosity.

Chapter 7 revealed that the fracture toughness of sintered Cu NPs was strongly dependent on loading mode. Mode II (in-plane shear) exhibited the highest toughness due to shear-induced particle interlocking, followed by mode I (tensile) characterized by ductile tearing, and mode III (out-of-plane shear), which showed the lowest toughness due to triaxial stress and unstable crack propagation. The application of an Al2O3 coating enhanced fracture toughness across all modes, with the greatest improvement observed under shear-dominated and mixed-mode loading. The coating constrained surface deformation, delayed crack initiation, and strengthened interparticle boundaries, while preserving the intrinsic ductile fracture mechanism.

Chapter 8 concluded the thesis by synthesizing the key findings and outlining future research directions, including the mitigation of oxidation, investigation of sizedependent mechanical behavior, understanding of stiffness mismatch, and exploration of metamaterial-inspired design strategies.