Kinetics of a Layer of Bismuth at the Anode of a Sn-Bi Based Solder Joint
Electrical resistance of low temperature solder during current stressing was shown to have the thickness of continuous layers of Bi accumulated at the anode.
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Authored By:
Javier Flores, Faramarz Hadian, Sitaram Panta, Eric Cotts, Ph.D.
Binghamton University
Summary
Measurements of the electrical resistance of low temperature solder, SnBi microelectronic interconnects, during current stressing were shown to provide sensitive indicators of the thickness of continuous layers of Bi accumulated at the anode. A linear rate of Bi accumulation at the anode of a SnBi based solder joint (for a constant temperature and current density) was observed. A Black’s equation for failure (based upon a criterion of a set increase in electrical resistance of the solder joint, e.g., twenty percent) yielded good fits over a temperature range extending from 90 to 125oC.
Conclusions
Utilizing the JEDEC current stressing failure criterion that depends upon a twenty percent increase in the electrical resistance of a SnBi based solder joint, and the relation between this change in electrical resistance and the thickness of a Bi layer accumulated at the anode of the solder joint during current stressing, a quantitative expression for MTF as a function of j and T was identified. Fits of this Arrhenius expression yielded a relation between MTF and current density and temperature with an activation energy of 0.8 eV. This single Black’s equation was observed to fit observations obtained at operating temperatures, and at significantly higher temperatures. Thus, this formalism provides clear acceleration factors for laboratory current stressing of such low temperature solder joints.
Initially Published in the SMTA Proceedings
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