First-Principles Calculation of Phase Stability and Cohesive Properties of Ni-Sn Intermetallics
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THE factors controlling the reliability of modern solder interconnects in electronic packages constitute a very active area of current research. This is primarily driven by the increasingly dense arrays of interconnects, the miniaturization of electronic packages, the requirement of new packaging technology, and the use of leadfree solders. The critical issues in microelectronic solder interconnects that affect reliability are the dynamics of the wetting behavior, the interfacial reaction with the underlying base metal governing the microstructure evolution during processing and in the service of devices, and electromigration. Due to the decreasing size of modern solder joints, the intermetallic layer continues to be proportionately thicker. In addition to the intrinsic properties of solder, the strength and reliability of solder joints are determined by the interfacial intermetallic(s), the interfacial properties (between solder and intermetallic), and the microstructural coarsening of solder. The phases that form in electronic and optoelectronic interconnects during fabrication and under normal service conditions G. GHOSH, Research Assistant Professor, is with the Department of Materials Science and Engineering, Robert R. McCormick School of Engineering and Applied Science, Northwestern University, Evanston, IL 60208-3108. Contact e-mail: [email protected] Manuscript submitted April 19, 2008. Article published online November 5, 2008 4—VOLUME 40A, JANUARY 2009
may affect the reliability of the devices through the following intrinsic properties: the coefficient of thermal expansion, the ductile/brittle transition (due to a change in composition and temperature), the elastic constants, the inelastic behavior, the heat capacity, and the thermal conductivity. While the formation of a thin intermetallic layer is desirable for achieving a good metallurgical bond, excessive intermetallic growth may have a deleterious effect on the mechanical reliability of the solder joints. The importance of some of these issues has been discussed in several recent reviews.[1–5] The intermetallics in solder interconnects may become the ‘‘weakest link,’’ if they are inherently brittle. Under-bump metallurgy (UBM) ranging from pure Ni to Ni-Cu, Ni-Pd, Ni-P, Ni-Ti, and Ni-V alloys is either used or is currently being developed for use in microelectronics packaging.[6–12] Even though Cu is the most commonly used solderable surface finish in a printed circuit board, Ni or a suitable Ni-base alloy is considered a better candidate, because the kinetics of the interfacial reaction between Ni and solder is much slower than that between Cu and solder. When used as UBM, the Ni layer should be thick enough to prevent its complete consumption to form Ni-Sn and other Ni-containing intermetallic(s) during reflow and during the lifetime of a device that may experience a range of temperatures. The evolving microelectronic soldering technology requires a quantitative understanding of processing-microstructure-property-performance links; METALLUR
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