Electromigration of 300 µm diameter Sn-3.0Ag-0.5Cu lead-free bumps in flip chip package

The electromigration (EM) behavior of 300 μm Sn-3.0Ag-0.5Cu solder bumps on Au/Ni-P substrate metallization in flip chip package was investigated at 150°C with a current density of 5×10³ A/cm². After reflowing for three times, the massive spalling of original intermetallic compound (IMC) of (Cu,Ni)₆Sn₅ was observed on the chip side. After aging and EM at 150°C for 200 h, the initial intermetallic (Cu,Ni)₆Sn₅ at both interfaces transformed into (Ni,Cu)3Sn₄. After aging at 150°C for 200 h, the binary compound AuSn₄ in solder turned into ternary compound (Au,Ni)Sn₄ and formed at the both interfaces. However, after EM at 150°C for 200 h, the (Au,Ni)Sn₄ was observed only at the anode interface but not at the cathode interface. When the electron flowed from the printed circuit board (PCB) substrate side to the chip side, a selective depletion of the Ni-P layer occurred on the PCB substrate side (cathode) after EM for 200 h, leaving the Ni₃P phase with a large number of columnar voids. After EM for 400 h, the damage of Ni-P layer spread over the entire cathode interface. When the electron flowed from the chip side to the PCB substrate side, the voids formed at the interface between solder and IMC layer, and the dissolution of Ni UBM and Cu pad occurred at the higher current density regions after EM for 400 h. Thermal electrical finite element simulation for the flip chip test vehicle showed that the current crowding occurred at the contacts between the solder bump and the Cu trace, and serious Joule heating was generated in the solder bump. It is shown that the simulative results agreed with the experimental results.