Modeling, design and demonstration of ultra-short, fine-pitch solder-based interconnection systems with high-throughput assembly
Huang, Ting-Chia Nathan
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Emerging high-performance computing systems have been driving the need for advanced packaging solutions such as high-density 2.5D interposer packages with escalating pitch, performance, and reliability requirements for off-chip interconnections. The objectives of this research are to design, develop and demonstrate novel, manufacturable, solder-based interconnection and assembly technologies at 20µm pitch, addressing the scalability limitations of conventional Cu pillars in terms of thermomechanical reliability, thermal stability and power-handling capability. Pitch scaling with solders is accompanied by a necessary reduction in solder volume and subsequent increase in the volumetric contribution of intermetallics, raising serious concerns for stress management and reliability. Fine control of interfacial reactions is, therefore, key in extending solder-based interconnections to finer pitches and constitutes the first challenge addressed in this thesis. Intermetallic formation is primarily governed by the assembly process and the surface metallurgy that reacts with the solder, with the specific challenges and associated research tasks defined below. From assembly perspective, pitch scaling brings a shift from conventional reflow to thermocompression bonding. Leading-edge TC-NCP (thermocompression with non-conductive paste) processes, first established by Amkor, have shown high promises for fine-pitch assembly at 40µm pitch and below. Through dynamic control of the temperature gradient in the assembled package, TC-NCP enables shorter reaction times and improved solder collapse and assembly yield. However, thermocompression processes inherently have to be customized for any given package design, with no existing guidelines for process design. A fundamental understanding of TC-NCP is, therefore, required to design thermal and force profiles that provide accurate control over the reaction. This is the objective of the first research task. Finite element modeling of the heat transfer in TC assembly considering tool – materials – package interactions was first established and validated experimentally to provide guidelines for optimization of assembly profiles. The developed methodology was applied to design the bonding profiles used to build all specimens in this thesis. This research also enabled the first demonstration of thermocompression assembly on high-density, ultra-thin glass substrates. Intermetallic growth in solder interconnection systems has also been traditionally controlled through the reacting surface metallurgy applied on the substrate pads. Over the last decade, standard ENEPIG (electroless Ni – electroless Pd – immersion Au) finish has been the metallurgy of choice in high-end applications for its outstanding reliability. However, the typically high thicknesses of Ni in ENEPIG limits its pitch scalability and degrades its high-frequency performance. Novel, Ni-free, metallic surface finishes are, therefore, required to meet the needs of emerging high-performance systems and form reliable interconnections at 20µm pitch with less than 10µm solder height. With such limited solder volume, risks of Au embrittlement and subsequent joints failure are also increased. The second research task addresses these challenges with the design of a new metallurgical system for reliable, ultra-short Cu pillar interconnections, based on the novel electroless Pd autocatalytic Au (EPAG) finish supplied by Atotech GmbH. The EPAG composition was optimized based on a comprehensive study including wettability testing, analysis of interfacial reactions, shear testing and thermal cycling. The optimal finish composition yielded a unique reaction with formation of a single intermetallic, resistant to Au embrittlement. Cu pillar assemblies with the optimized EPAG composition exhibited a 3x improvement in fatigue life compared to assemblies with standard ENEPIG with less than 10µm solder height, demonstrating potential scalability of the Cu pillar technology to 20µm pitch. With further reduction in solder volume to achieve finer pitches, the solder is expected to fully react into intermetallics, if not during chip-level assembly itself, then during subsequent process steps. Solid-liquid interdiffusion (SLID) bonding has been proposed and extensively researched as an alternative technology to form all-intermetallic joints with improved pitch scalability, power handling capability and thermal stability. However, the adoption of existing SLID technologies has been limited to date due to reliability concerns, notably related to voiding, and manufacturability and cost challenges due to low assembly throughput. This last technical challenge was addressed in the third research task with the design and demonstration of a void-free, manufacturable SLID technology. In the proposed metastable SLID technology, the Cu6Sn5 metastable phase was isolated using Ni diffusion barrier layers, enabling full conversion into void-free intermetallics with highest interdiffusion rates. Metastable SLID was demonstrated, for the first time, at pitches down to 20µm on Si and glass substrates, with superior shear strength of 90MPa, outstanding electromigration resistance at 105A/cm2, good thermal stability after 1000h high temperature storage at 200℃ and excellent thermomechanical reliability.