A Study of Electromigration from Superconducting to Normal Conducting Metals

Abstract

A study of the potential for electromigration to occur in indium solder balls at liquid helium temperature was conducted.  The solder balls were interconnected by means of niobium traces, which transitioned from niobium to indium throughout a flip-chip assembly.  The niobium traces were superconducting in the liquid helium.  By observation of Black’s equation, it was reasonable to assume that electromigration does not occur in normal conducting metals at cryogenic temperatures.  However because of reliability concerns with the use of indium in superconductivity applications, the potential for electromigration to occur at the superconducting niobium to normal conducting indium transitions was conducted.  Because electromigration is a function of current density, temperature, and geometry, flip-chips were manufactured with indium solder balls of varying pitches.  The flip-chips were stressed at a current level that was 90% of the niobium’s critical current, and two tests were conducted.  Each stress test lasted a period of 21 days, and the solder ball pitches for the first test were 15 micrometers; for the second test, the solder ball pitches were 25 micrometers and 35 micrometers.  Resistance of all samples was monitored constantly by automating the test equipment with Labview, and the resistance did not change.  With electromigration, an increase in the samples’ resistance was expected to occur, and no test samples exhibited any increases in their resistance, validating the notion that it is very unlikely for electromigration to occur at liquid helium temperature.