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MATERIALS ENGINEERING
SEMINAR
"Transient Liquid Phase Bonding for High Temperature Interfaces"
By
John R. Holaday
Purdue MSE Ph.D. Final Exam
Advisor: Professor Carol A. Handwerker
ABSTRACT
Transient liquid phase bonding (TLPB) is a type of interdiffusion bonding between metals that has been proposed for a variety of electronic interconnect applications. TLPB takes advantage of the formation of solid intermetallic compounds (IMC) formed by interdiffusion between a liquid phase, low-melting temperature component, such as Sn or a Sn alloy, and a solid, high-melting temperature component, such as Cu, Ni, or Ag. In conventional soldering, relatively thin layers of IMC form at interfaces and are dispersed in the bulk of the solder which has been heated above the liquidus temperature of the solder alloy and solidified by cooling. In TLPB, isothermal solidification occurs by the complete consumption of the low-melting temperature phase in the formation of IMC. Under the correct conditions, the resulting IMCs will exhibit a melting temperature greater than the initial processing temperature. The elevated melting temperature of the IMC is intended to facilitate high temperature operation and hierarchical device fabrication.
A framework for interpreting equilibrium binary and ternary phase diagrams to predict non-equilibrium TLPB behavior was implemented. Detailed thermodynamic data in the form of calculated phase diagram (CALPHAD) databases exist for a broad set of Sn alloys. This framework was applied to screen potential TLPB formulations from a larger design space. Ternary diagrams were generated at regular temperature intervals using Thermo-Calc to compare outcomes over possible processing temperature ranges. Evaluations of binary and ternary phase diagrams are presented for several Sn alloys with Ag, Cu, and Ni as the high-melting temperature phase. Known interfacial reaction products were also discussed to demonstrate circumstances in which a different phase is known to form than that expected based on the proposed thermodynamic interpretation method. This analysis was performed to provide insights into potential TLPB formulations regarding effective composition ranges, resulting phases, processing temperatures, and operating temperature ranges.
Application of the thermodynamic framework lead to the identification of a novel Bi-rich, Sn-Bi-Cu TLPB formulation. The relatively low eutectic temperature (139°C) of Sn-Bi is an attractive characteristic for use in TLPB, specifically where low temperature processing is desirable. Ideally, the melting temperature and thus the maximum operating temperature is elevated to that of the precipitated Bi phase(~271°C) that forms after Sn is consumed by the formation of Cu6Sn5 and Cu3Sn. Previous investigations of Sn-Bi TLPB focused on characterizing the reaction of eutectic Sn-Bi and Sn-rich, Sn-Bi with Cu. Unfortunately, eutectic Sn-Bi reacted with Cu results in a persistent melting event at 200°C due to the presence of Cu6Sn5. Analysis of the ternary phase diagrams and invariant reactions in the Cu-Sn-Bi ternary reveal that a shift in the IMC phase in equilibrium with Bi causes a liquid forming reaction at 200°C. This reaction can be avoided by using a Bi-rich, Sn-Bi composition such that Cu3Sn forms at the Cu-SnBi interface as opposed to a layered Cu6Sn5 and Cu3Sn structure. The concept of processing regimes and processing regime maps was introduced as a method to clarify description of the temperature and composition ranges resulting in the same processing outcomes.
Experimental assessment of Bi-rich, Sn-Bi was performed with Cu and Ni. Interfacial reactions between Sn-80Bi (wt. %) and Cu substrates confirmed the direct formation Cu3Sn. Planar TLPB assemblies were fabricated by quickly soldering together substrates in a planar configuration with a bondline thickness of 10 to 20µm. These assemblies were then thermally processed using the DSC, cross-sectioned, polished, and examined via SEM. DSC and and EDS confirmed complete isothermal solidification in less than 60 minutes at 300°C. Solid-liquid interfacial reaction couples were also processed in a tube furnace. Interfacial reactions were tested to confirm the viability of Bi-rich Sn-Bi where Ni interfaces will be bonded. Experimental results demonstrated that Sn-Bi-Cu is a practical TLPB system when a Bi-rich low-melting temperature phase is processed above 200°C.
Transient liquid phase bonding is a compelling method for forming high temperature interconnects. The thermodynamic framework developed for TLPB design establishes the opportunity for additional novel formulations to be developed. Bi-rich, Sn-Bi with Cu has been demonstrated to be a novel, thermodynamically viable TLPB system. Although further development will be needed to characterize and refine mechanical performance.
Date: Thursday, July 19, 2018
Time: 9:00 A.M.
Place: HAMP 2123
PURDUE MSE
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