Flip chip bump formation of Sn-1.8Bi-0.8Cu-0.6In solder by stencil printing

Flip chip bumping by stencil printing method using a new composition of solder paste, Sn-1.8%Bi-0.8%Cu-0.6%In, all in mass%, was investigated. Sn-3.5%Ag, Sn-37%Pb and Sn-36%Pb-2%Ag were selected as references for the experiment. The solder pastes were printed on the under bump metallization (UBM) of a Si-wafer using a stencil, where diameter and thickness of the stencil opening were 400 and 150 μm, respectively. The UBM deposit comprised 0.4 μm each of Al, Ni and Cu, and 20 nm of Au from bottom to top of the metallization, sequentially. The printed paste bumps were reflow soldered in air, and the peak soldering temperature of Sn-1.7Bi-0.8Cu-0.6In and Sn-3.5Ag was 523 K and of Sn-37Pb and Sn-36Pb-2Ag was 503 K. From the experimental results the solder bumps of Sn-1.8Bi-0.8Cu-0.6In alloys were well-formed with a mean height of 260 μm. The shear strength of Sn-1.8Bi-0.8Cu-0.6In at 523 K (as-reflowed) showed the highest value of 6.5N followed by those of Sn-3.5Ag, Sn-37Pb and Sn-36Pb-2Ag solders. After 1000 h aging, while the shear strength of the Sn-1.8Bi-0.8Cu-0.6In showed 27% decrease compared to as-reflowed conditions, it was still 15-30% higher than those of Sn-37Pb, Sn-36Pb-2Ag and Sn-3.5Ag solders. Intermetallic compounds (IMCs) formed on the interface between solder and UBM were (Cu,Ni)₆Sn₅. As aging time went on up to 1000 h, the content of Ni in the IMC changed from 6.6% at initial stage (as-reflowed) to 13.5% at final stage (1000 h aging).