Electro-migration behavior in low temperature flip chip bonding

Abstract

In this report, we investigated electro-migration behavior of two types of low temperature bonding. One was Sn-57 Bi using conventional C4 process. The other was Au-In Transient Liquid Phase bonding (TLP). Electron flow to induce the electro-migration was from substrate side (Ni pad) to chip side (Cu post) with current density of 40000A/cm2 at 150 degree C. In the case of Sn-57 Bi conventional C4 process, Bi quickly migrated to accumulate on the anode side (Cu post) and Sn migrated to the cathode side (substrate Ni pad). And the interconnect resistance increased until about 150 hours. Although this temperature was higher than the melting point of Sn57 Bi solder, there was no electrically break failure and the resistance was stabilized at 80% increase of initial resistance for more than 2800 hours, that was 10 times longer life of the Sn3.0wt%Ag0.5wt%Cu (SAC305) solder joint. From the cross-sectional analyses of Sn-57 Bi solder joints after the test, it was found that Bi layer and intermetallic compound (IMC) behaved as the barriers of the Cu atom migration into Sn solder. In the case of Au-In TLP bonding, remarkable change was not observed in metallic structure. And resistance was stabilized at 0.5% increase of initial for more than 1300 hours. Sn57 Bi solder joining and Au-In TLP bonding are promising candidates for the bonding technique of high density Flip Chip packages and 3D packages.