| 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 at the liquid helium temperature. 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 diameters. The flip-chips were stressed at a current level that was ~90% of the niobium's critical current, which ranged from 100 mA to 123 mA, and two tests were conducted. Each of the two such stress tests lasted a period of ~504 hours (3 weeks). The solder ball pitches for the first test were 35 micrometers with 17,200 solder balls within each flip-chip; for the second test, the solder ball pitches were 25 micrometers with 32,096 solder balls within each flip-chip and 35 micrometers with 77,520 solder balls within each flip-chip. Resistance of all samples was monitored continuously by automating the test equipment using a special Labview program, and the resistance did not change. With electromigration, an increase in the samples' resistance was expected to occur. No test samples exhibited any increases in their resistance, validating the notion that it is very unlikely for electromigration to occur at liquid helium temperatures, thereby ensuring the reliability of the flip chip design having indium solder balls. |
