Electromigration Reliability of Solder Balls

The electromigration performance of solder balls has been characterized in terms of its failure mechanism, process legs and current distribution. Through extensive failure analysis, a step-by-step mechanism of electromigration failure has been identified, which begins with the formation of CuSn at the expense of the under-bump metallurgy and nearby solder, followed by the nucleation and growth of voids along the CuSn-Sn interface until open circuit failure. Lifetime gains proportional to the thickness of the under-bump metallurgy and/or the redistribution layer have been measured. Finally, current distribution was observed to play a large role in EM lifetime, in which splitting the current equally between balls greatly outlived those configured serially.