Characterization of In-Based Eutectic Alloys Used in Josephson Packaging
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INTRODUCTION
JOSEPHSON large scale integrated (LSI) digital circuits have the potential of offering very fast and low powerdensity computing systems. In the present Josephson computer technology, the experimental chips consist of junction intefferometers, Nb ground planes, and superconducting transmission lines) Low melting-point In alloy solders are used to connect circuit chips and package parts. The solder joints serve not only as the electrical connections for signal and power supply but also as the mechanical link between parts. Therefore, the solders must exhibit low electrical resistivity (preferably superconducting), reasonable mechanical strength, room temperature to low temperature cyclability, and low melting temperature for a process compatible with tunnel junctions with Pb alloy electrodes. In addition, an interface material has to be used between the Nb pad and solder for proper wetting, adhesion, and low contact resistance. To date, two kinds of In alloys are used as solders and a Pd (100 nm)/Au (100 nm) layer is chosen as the interface material (IF~). 2'3 The first solder, S1, is a eutectic alloy of InBiSn with composition In (51 pct wt) Bi (32.5 pct) Sn (16.5 pct) and melting point of 59 ~ The other solder, $2, is the eutectic alloy of InSn with composition In (52 pct wt) Sn (48 pet) and melting point of 117 ~ $1 is used for controlled col/apse chip joining and $2 is used for fillet vertical joining and pin joining, s The solders, in general, are vapor deposited from an induction heated crucible through molybdenum masks onto the substrates. After dicing, testing, and selection, parts to be joined are fluxed, butted to the counter part, and heated above the melting point (not necessarily in the above order). The joints are then cooled down and flux rinsed off. When all the parts are assembled, the assembly is cooled down for testing and operation. Inevitably, parts will be cycled between room temperature and liquid helium temperature for testing, changing of parts, or repairing. A rough estimate indicates that some parts may experience up to 100 cycles through the lifetime. If rework is necessary, parts may even go through the heating and fluxing cycles. It is obvious that these environmental exposures will have some effect on the properties of the solder joints, tn order to assess the solder joint performance, characterization of these solders and their relation to the interface under various conditions are needed. JAMES T.C. YEll is Research Staff Member with IBM Thomas J. Watson Research Center, Yorktown Heights, NY 10598. Manuscript submitted October 27, 1981. METALLURGICAL TRANSACTIONS A
The feasibility of In-based solders and interface material for the Josephson package application was studied by Grebe 6 and by Ting et al. 2 In this paper, we describe the physical and mechanical properties and structures of solders and interfaces. The structure of solders and their interaction with IFM under various conditions was studied in detail. Mechanical strengths of bulk solders were measured at v
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