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 electoless Ni-P UBM were investigated. In anode solder joint, UBM was completely depleted and crack was observed. In the top view of consumed UBM, we could observe several holes on Ni-Sn IMC backfilled by Sn. Anode solder joint was disconnected by the large cavity under this consumed UBM. In the in-situ observation, UBM was preserved its own shape in spite of crack propagation. And resistance was started to increase with fluctuation after crack propagation was ended. To understand the electromigraition behaviors, effect of applied current level and temperature on time to failure was investigated. Calculated activation energy for Black´s equation from experimental data was low because of the difference of failure criteria. As a method for enhancing the resistibility to the electromigration failure, effect of UBM thickness was investigated. As results, time to failure of solder joint with 10 mum UBM increased about 5 times than that of solder joint with 5 mum UBM. And UBM consumption rate was reduced due to the low current density at interface in the solder joint with thicker UBM.