Development of Cu particle sintering as die attach solution for high temperature electronics applications

This PhD thesis, “Development of Cu Particle Sintering as Die Attach Solution for High Temperature Electronics Applications”, submitted to TU Berlin, investigates innovative copper (Cu) sintering techniques for die-attach applications in high-temperature electronics, particularly for wide bandgap semiconductors like silicon carbide (SiC). The increasing demand for reliable interconnects in power electronics, driven by rapid electrification, necessitates materials that operate above 200°C. Conventional lead-free solders, with melting points of 220–230°C, are limited to below 150°C, prompting exploration of alternatives such as high-temperature solders, transient liquid phase (TLP) bonding, and silver (Ag) sintering. While Ag sintering is widely adopted, its high cost, susceptibility to electromigration, and environmental impact highlight the need for sustainable alternatives. Cu emerges as a promising candidate due to its comparable thermal, mechanical, and electrical properties, though challenges like oxidation and high sintering temperatures must be addressed. The research develops a novel Cu sintering approach utilizing micro-scale brass flakes engineered for nano-scale sintering behaviour. This is achieved through selective wet chemical etching of zinc (Zn) from α-brass flakes using hydrochloric acid (HCl), a process known as dezincification, which creates surface modifications that enhance material transport during sintering. Polyethylene glycol 600 (PEG600) is incorporated as an organic binder to reduce Cu and Ag oxides in-situ, enabling effective sintering in an open bond chamber under nitrogen flow. This method yields a Cu sinter paste that achieves a shear strength exceeding 50 MPa under industrially viable conditions: 275°C, 10 MPa bonding pressure, and 5 minutes of sintering time. The study provides a comprehensive analysis of sintering theory, covering mechanisms such as surface diffusion, grain boundary diffusion, and densification, supported by models like Herring’s scaling law and the Mackenzie-Shuttleworth equation. Material characterization, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and die shear strength testing, confirms the enhanced sinterability of the etched brass flakes. A review of current Cu sintering methods underscores the advantages of this approach over alternatives like Cu nanoparticles or reducing atmospheres. Results demonstrate that the etched brass flakes, with residual Zn, produce a sintered microstructure and mechanical performance comparable to commercial Ag sinter pastes, but at significantly lower cost. The PEG600 binder ensures process stability by mitigating oxidation, while the low-temperature, low-pressure sintering process aligns with existing Ag sintering equipment, facilitating industrial integration. The thesis concludes by outlining prospects for process scalability and paste optimization for broader applications in power electronics packaging. This work establishes a cost-effective, high-performance Cu-based die-attach solution that addresses the economic and environmental limitations of Ag sintering, contributing to the advancement of sustainable high-temperature electronics packaging.