Aging Response and Cryogenic Mechanical Properties of an In-Sn Eutectic Solder Alloy for Josephson Packaging
Josephson computing technology entails the use of superconducting Josephson junction devices which combine high speeds and low power dissipation compared to conventional semiconductor devices.1–3 A low melting point (390K) In (52 weight %) Sn (48 weight %) eutectic alloy which is superconducting at 4.2K is used as an interconnection solder in Josephson packaging to enable chip and package part replacements to be done at reasonably low temperatures. For a typical contact application, about 25µm thick film of the alloy is usually evaporated onto 200 µm diameter contact pads of 100 nm each of Pd and Au and 230 nm of Nb situated on a Si substrate and subjected to reflow prior to use4,5. In the anticipated service cycle, the alloy undergoes rapid solidification after the reflow operation and could experience aging at room temperature (298K) during storage and/or at 348K where a reflow operation is carried out for interconnections using a second lower melting solder in the package. Subsequently, the whole package is cooled down to near 4.2K at which the superconducting junction devices operate. In such a service cycle, stresses can arise due to thermal contraction mismatch between the substrate and the solder and due to joining of parts made of dissimilar materials. Further, since 298K and 348K are significant fractions of the alloy melting point, one would expect significant changes in the as quenched microstructure and, as a result, in the mechanical properties of the alloy. Accordingly, it is necessary to study the low temperature mechanical behavior of the alloy and its dependence on aging at 298K and 348K and correlate them with the attendant changes in the microstructure. Recently, Yeh 6 reported on the shear and tensile properties of this alloy in the overaged condition, tested between room temperature and 77K. In this paper we extend this data base to 4.2K and document the microstructual and mechanical property changes caused by 298K and 348K aging treatments.
KeywordsUltimate Tensile Strength Void Nucleation Interconnection Solder 298K Aging Eutectic Solder Alloy
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