Study of electro-migration resistivity of micro bump using SnBi solder

There has been a great discussion about electro-migration behavior in semiconductor area. And it has been often discussed that electro-migration behavior of the flip chip package using Sn-Ag bump. However, little study has been done to explore the electro-migration behavior of low temperature solder such as a Sn-Bi solder. In this report, we investigated electro-migration behaviors of micro pillar bump (100 μm diameter) and fine pitch micro bump (25 μm diameter) using Sn57wt%Bi solder. In the case of micro pillar bump, Bi quickly migrated and accumulated on the anode side (Cu pillar) and Sn migrated to the cathode side (substrate pad). And interconnect resistance was quickly increased 80 % from initial during about 150 hours. There was no electrically break failure and it was stabilized at 80% of initial resistance for more than 2800 hours. On the other hand, in the case of fine pitch micro bump, almost of Sn atoms were consumed to form Cu-Sn or Ni-Sn intermetallic compounds (IMCs) after bonding process. The resistance increase was less than 9 %, it is stabilized even for more than 2200 hours and there were no electrically break failure. Additionally, it is evident from electromagnetic field simulation that the maximum current density of the fine pitch micro bump are less than half compared with that of Cu-pillar bumps. Fine pitch micro bump using Sn57 Bi solder is promising candidates for the bonding technology of high performance packages.