The effects of Ag content in the solder on electromigration behavior

As the volume shrinking of electronic products becomes a trend, the dimensions of solder joints have devoted to scale down. With a smaller solder bump, the current density of solder bump gets higher. Electromigration becomes a very serious reliability issue. The two Ag-contents of SnAg solder bump influenced on electromigration lifetime in the fine pitch of the flip chip package. The tri-layer Ti/Cu/Ni UBM has a 75 μm in diameter and the bump pitch is 150 μm. The passivation opening is defined 50 μm on the Al trace of the chip. The metallization of 2-layer substrate is pre-solder on the Cu pad (SOP) and the diameter of the Cu pad is 105 μm. Two solder compositions, Sn2.4Ag and Sn1.3Ag, were stressed with 4.5×10⁴ A/cm² at 150°C and 160°C. However, the lifetime at 150°C with 4.5×10⁴ A/cm² is 2.35 times than one at 160°C. The electrical resistance changes with current stressing time have three modes of variation relation. The crack and void were observed at the cathode of solder bump. Therefore, the failure mode should correspond with the relation of resistance versus current stressing time. Before current stressing, it was not easy to distinguish the microstructure of Sn1.3Ag from Sn2.4Ag solder bump. The same kind of IMC was found at the chip side and substrate side in both two solder bumps. After current stressing of 4928h at 150°C with 4.5×10⁴ A/cm², all solder of the current bypassed solder bump (Sn1.3Ag) turned into IMC. The UBM has not been consumed completely. A lot of Cu₆Sn₅ was observed at the cathode in the failed solder bump. After a prolonged time of current stressing, the electromigration induced not only the consumption of the UBM but also the dissolution of the Cu pad.