Electromigration lifetime statistics for Pb-free solder joints with Cu and Ni UBM in plastic flip-chip packages

A series of electromigration tests were performed as a function of temperature and current density to investigate lifetime statistics for Pb-free solder with Cu or Ni under-bump-metallization (UBM). Based on the overall shape of resistance traces, a conservative failure criterion for time-to-failure was defined and the results were compared with those based on the conventional open-failure criterion. Solder joints with Cu UBM had a longer lifetime than with Ni UBM, based on the open-failure criterion; however, the lifetime with Ni UBM became comparable when the conservative criterion was applied. The Joule heating effect was accounted for based on experiments and finite element analysis. The temperature of solder joints was determined to be approximately 15degC higher than that at the Si die surface when 1 A of current was passed. For solder with Cu UBM, voids formed initially at the Cu₆Sn ₅/solder interface while the final open failure occurred at the Cu₃Sn/Cu₆Sn₅ interface. For Ni UBM, voids formed initially at the Ni₃Sn₄/solder interface leading to failure at the Ni₃Sn₄/solder interface