Dissertation Defense - Ting-Chia (Nathan) Huang
Prof. Rao Tummala, Advisor, MSE
Prof. Naresh Thadhani, MSE
Prof. Eric Vogel, MSE
Prof. Suresh Sitaraman, ME
Prof. Raj Pulugurtha, ECE
Modeling, Design and Demonstration of Ultra-Short, Fine-Pitch Solder-Based Interconnection Systems with High-Throughput Assembly
Emerging high-performance systems are driving the needs for advanced packaging solutions such as 3D ICs and 2.5D system integration with increasing performance and reliability requirements for off-chip interconnections. The research objectives are to design, develop and demonstrate novel, manufacturable, solder-based interconnections addressing the limitations of conventional Cu microbump technology in terms of pitch scalability, thermomechanical reliability, thermal stability and power-handling capability to extend the applicability of solders to next-generation high-performance packaging, at 20µm pitch and below. Pitch scaling with solders is hindered by three major technical challenges: a) shift from reflow to thermocompression bonding for improved control of solder collapse and intermetallic growth, but with high-customized processes; b) reliability of ultra-short interconnections with reduced solder volume; c) non-controlled intermetallics formation leading to stress management issues and voiding. To address the aforementioned challenges, three research tasks were proposed and executed, beginning from the assembly process, then the metallization on the substrate, and finally to the redesign of entire metallurgical bonding system.
At first, the study focused on the fine pitch assembly process to enable the high as-bonded yield on substrates with high routing density, and optimize the process for intermetallic growth. Amkor’s TC-NCP (thermocompression bonding with non-conductive paste) process has been demonstrated as one of the most promising process to achieve fine-pitch assembly at 40um pitch and below. Tool – material – process interactions have to be taken into account when designing thermocompression bonding heating and force profiles. Although Cu microbump TC bonding is an established technology on organic substrates and silicon interposers, there are no standard process guidelines and all process recipes are highly-customized given a test vehicle design, material set and bonder. This research constitutes the first demonstration of thermocompression assembly on ultra-thin glass substrates. Finite element modeling of the heat transfer in assembly, considering tool-package interactions, was first established and experimentally validated to provide guidelines for setup of assembly profiles. This methodology was also applied to design the assembly process for all the specimens built in this thesis.
Secondly, a novel metallic surface finishes without Ni are required in replacement of standard ENEPIG and ENIG finishes to achieve high reliability with reduced solder height. This transition was also driven by the needs of sub-10um interconnect pitch on high-density substrates without extraneous plating, and with improved high-frequency performance. A comprehensive study of the new electroless palladium autocatalytic gold (EPAG) finish supplied by Atotech GmbH was executed. The EPAG composition was optimized based on wettability testing, analysis of interfacial reactions, thermal cycling. Assemblies with the optimized EPAG composition exhibited a 3x improvement in fatigue life compared to assemblies with standard ENEPIG.
Further reduction of the solder volume to achieve finer pitches may give rise to severe interfacial stresses at the residual solder-to-intermetallics interfaces, drawing the limit of pitch scalability. Solid-liquid interdiffusion (SLID) bonding has been proposed and extensively researched as an alternative technology to form all-intermetallic joints with improved pitch scalability and power handling capability. However, the adoption of existing SLID technologies is limited due to reliability concerns, notably related to voiding, manufacturability and cost with relatively low-throughput processes. The proposed metastable SLID technology addresses these challenges by isolating the Cu6Sn5 metastable phase using Ni diffusion barrier layers, enabling full conversion into void-free intermetallics with highest interdiffusion rates. Metastable SLID was for the first time demonstrated at pitch down to 20µm on Si and glass substrates, with superior shear strength of 90MPa, outstanding electromigration resistance at 105A/cm2, good thermal stability after 10x reflow cycle and excellent thermomechanical reliability, even with ultra-low-K devices.