Date of Award
Doctor of Philosophy (PhD)
Carol A. Handwerker
Committee Member 1
Committee Member 2
Eric P. Kvam
Committee Member 3
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 has been studied as an alternative to other high temperature interconnect materials such as high-Pb solders. The goal of eliminating Pb and other toxic materials from electronics manufacturing processes, and most importantly, waste streams has been part of an ongoing effort internationally to improve the sustainability of electronics manufacturing. Pb containing solders have largely been eliminated from most consumer applications, but continuing exemptions for high-Pb solders have been provided by regulators due to the lack of suitable replacement for high temperature interconnects. 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.
Research studies involving TLPB can be categorized into two types of work. Proof-of-concept studies have demonstrated the feasibility of producing TLP bonds using materials and methods either common to current commercial electronics manufacturing or with some novel processing or design improvement. Complete isothermal solidification in which the bonds have been shown to remain solid up to an increased melting temperature by characterization of shear strength above the initial processing temperature are key results that have been achieved using a
variety of materials and processes. Characterization of the resulting phases and microstructures via cross-sectioning and inspection using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX or EDS) have been used to show complete or adequate conversion of the low-melting temperature component to IMC as well. Powder compacts in a reducing atmosphere or paste formulations comprising mixtures of the low-melting temperature component, the high-melting temperature component, a flux, and or binding agents are most commonly used. Thin layers of these components in a planar configuration can also be used. An important result in any of these approaches is to form a dense enough bond such that adequate mechanical, thermal, and electrical performance of the interconnect can be achieved with each component in the scale of tens of microns such that interdiffusion can occur rapidly enough. Most studies have employed materials already widely adopted in electronic interconnects such Sn, Ag, Cu, Ni, Bi, or In in pure and or alloyed forms.
A second type of TLPB research has focused instead on characterizing underlying processing mechanisms such as the kinetics of IMC formation in specific TLPB candidate systems such as Cu-Sn or Ag-Sn. Solid-liquid interdiffusion couples were processed at several temperatures for various durations. Cross-sections inspected using SEM and EDS were used to inspect the rate of formation of interfacial IMC layers. In some cases, differential scanning calorimetry (DSC) was used to measure heat flows during thermal processing. By analyzing the melting and solidification events in conjunction with the known sample dimensions and mass, the amount of IMC formed for each condition could be estimated as opposed to direct observation of cross-sections. An analytical model of the intermetallic growth kinetics would then be presented based on the observed rates. Other researchers prioritized investigating the thermodynamic implications of alloying either the low-temperature component or the high-temperature component. Once again, SEM and EDS were used to identify the phases formed and DSC was used to identify significant phase transitions relative to temperature. Typically, some discussion of key thermodynamic characteristics affected by the alloying type employed with the goal of identifying processing or applications limits for a specific material set.
In the work presented in this thesis, a comprehensive approach is taken to TLPB design encompassing both practical bond engineering concerns but also applying analysis of key materials science concepts. First, the key thermodynamic concepts required for successful TLPB are examined. Cu-Sn, Sn-Ag-Cu, and Sn-Bi-Cu example TLPB are used to introduce the concept of TLPB in further detail. Further introductions cover manufacturing and design constrains, wetting and microstructural inhomogeneities, and the technical demands for various applications.
Rather than making basic modifications to existing TLPB formulations, 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 in order 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.
Competing approaches to high temperature interfaces were also reviewed to better understand the requirements for commercially viable TLPB design. Various high temperature solder alloys have been investigated, and some are currently used in niche applications. Inclusion of precious metals or mechanical performance limits the practicality of the most promising alloys although Zn-Sn solders exhibits promising properties. Sintered Ag also exhibits excellent electrical, mechanical, and thermal properties. Examples of novel TLPB geometry, processing techniques, and optimization were also reviewed. These studies demonstrate that design and process optimization on an application basis will be needed to make TLPB a competitive technology.
Application of the thermodynamic framework lead to the identification of a novel Birich, 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 heated in a tube furnace. Interfacial reactions were also tested to confirm the viability of Bi-rich Sn Bi where Ag or 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. 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. The thermodynamic framework developed for TLPB also establishes the opportunity for additional novel formulations to be developed.
Holaday, John R., "Transient Liquid Phase Bonding for High Temperature Interconnects" (2018). Open Access Dissertations. 1954.