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 30 um. 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 150degC 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.