Electromigration of Pb-Free Solder Flip Chip Using Electroless Ni-P/Au UBM

In this study, electromigration behaviors of Sn3.0Ag0.5Cu solder flip chip using electroless Ni-P UBM were investigated. From the morphology observation, Ni-Sn intermetallic compounds (IMCs) and Sn were observed at the interface between Al and electroless Ni-P UBM due to movements of Sn atoms against the electron flow. The backward movements of Sn occurred by melting of solder materials. Temperature just before failure rapidly increased up to the melting points of solder materials. For observing the progress of failure, in-situ observation was performed. In in-situ observation, the electrical resistance was stable even though the contact area was reduced due to solder depletion at the UBM/solder interface. After solder material was depleted along the UBM/solder interface, a solder joint was degraded seriously in a short time. By the solder degradation, electroless Ni-P UBM was depleted and a solder joint was opened electrically. To enhance the electromigration reliability, increasing electroless Ni-P UBM thickness was effective. As results, time to failure of the solder joint with 10 mum UBM increased about 3 times than that of the solder joint with 5 mum UBM. Time to failure increased due to the lowered current crowding effect at UBM/solder interface in the solder joint with thicker UBM.