Ternary lead-free SnAgCu micro-bumps for ultra-fine pitch chip-to-chip interconnection

Immersion solder bumping, a mask-less and low cost processing, brings feasibility to the ultra-fine pitch chip-to-chip interconnection; however, how to make the uniform micro-bump is still a great challenge. In this paper, the uniform micro-bumps with very thin Sn-3.0Ag-0.5Cu (SAC305) solder layer on electroless nickel under bump metallization (UBM) are achieved. The test chips we used have 6,507 SAC305 solder micro-bumps with a pitch of 30um. Two test chips are bonded to each other, and the optimum bonding conditions for a sufficient bonding quality are determined. The interfacial microstructure of the bonded samples before and after aging at 150 degrees C for 168 hours is also investigated. The present results shows that the rapid phase growth at the solder/UBM interface would have a great impact on solder joint reliability.