In-situ observation of electromigration induced failure modes and intermetallic compound growth mechanisms for Chip Scale Packages with different structures

In this study we observed the electromigration-induced (EM-induced) failure and intermetallic compound (IMC) growth mechanism in-situ with four structures: (1) 8.6 μm Cu UBM on 4 μm Cu RDL (2) 9 μm Cu RDL without UBM (3) 3 μm Cu/ 2 μm Ni UBM on 4 μm Cu RDL (4) 7 μm Cu/ 2 μm Ni RDL without UBM The Chip Scale Packages (CSPs) were mounted with printed circuit boards (PCB) then tested in EM testing system and monitored the resistances with 0.7 A at 160°C. Two kinds of typical failure modes were observed for the samples without UBM after current stressing. One is the Cu trace dissolution and the other is the voids formation at IMC and SnAgCu solder ball (SAC) interface due to unbalance atomic fluxes. Some UBM consumed for the samples with UBM after current stressing and there were no any failures even after 3000 h current stressing. The current were spread by UBM and reduced 54%~73% current crowding effect as the simulation results. The Cu₆Sn₅ and Cu₃Sn IMCs growth mechanism is quite different when Ni atoms participate in the interfacial reaction. The lower chemical potential (Cu,Ni)₆Sn₅ IMC formed and induced more Cu atoms diffused from Cu₃Sn into (Cu,Ni)₆Sn₅ IMC then retarded the growth rate of Cu₃Sn IMC. The current spreading simulation, detailed mechanisms of two failure modes and Cu₆Sn₅/Cu₃Sn IMCs growth mechanisms were investigated in this study.