Investigation Of Electromigration On Lead-Free Solder Bump In Flip Chip Packaging

Pb-free solder interconnect applied in flip chip (FC) is a predominant technology forwafer-level chip-scale package (WLCSP). In WLCSP, the die shrinks area for poweravailable, the bump shrinks volume and increase quantity for interconnect efficient. As aresult, the bump current density is now approaching 104A/cm2 level, where EM becomes asignificant reliability issue. In addition, Pb-free solder alloys are looking forward tobecoming the alternative of Pb-Sn alloys of environment protection. Thus, in this thesisSn/3.5Ag/0.5Cu bumps applied in FC technology have been studied on EM regarding theeffects of UBM thickness, current density and temperature. In the thesis, the theories and the current studies on EM have been introduced firstly.And then, a new method of experiment is proposed and done. Finally, the results of theexperiment and the simulation on EM are analyzed deeply. Higher current density above 104A/cm2 level prolongs failure time less than lowerones. It can be explained that more electro-flow impacts atoms to migrate and redistribute,which causes voids easy to propagate and open. Current crowding often occurs at thecorner of the UBM/bump interface when electron-flow enters. Joule heat is associated bytemperature rising at the current crowding area, that deteriorates the EM procedure. Samples with thick Ni/Au diffusion barrier layers have longer failure time than thosewith thin ones. More Ni atoms in the UBM layer delay EM failure occurring. Three stagesare found in EM procedure, which are the stable stage, the fluctuating stage, and the sharpfusion stage. IMCs growth and migration and voids nucleation appear in the stable stage.Voids propagate along the interface in the direction of stress gradient decreasing isincluded in the fluctuating stage. Melts dramatic fusion is observed in the sharp fusionstage. Another unique EM behavior is the polarity effect on IMCs growth at the anode anddissolution at the cathode. That is Ni atoms move from the cathode to the anode, and a lotof IMCs accumulate at the anode. The IMCs evolutions of EM samples are compared with those of thermal agingsamples at 180°C. The results indicate IMC evolutions in thermal aging samples arecompletely different with EM ones. They change slowly and uniform at the both of twosides, not found in the bump.