Joint properties of micro Sn-58Bi solder bumps on flexible substrate

The mechanical bending properties of the Sn-Bi bumps on flexible circuit board were investigated in this study. Cu pillar bumps were fabricated on a Si wafer and then Sn-58wt%Bi bumps were formed on the Cu pillar bumps with electroplating after fabrication of Si chip with bump, Si chips were bonded on flexible substrate by flip chip process. Flip chip bonding conditions were bonding force of 1N, 2N, and 3N with bonding temperature of 170°C and 15s of bonding time. After bonding, we performed shear test, thermal shock test, and bending test to evaluate mechanical property and reliability of joint. Shear strength of 1N and 3N were 6.70gf/bump and 6.75gf/bump respectively. In the case of 2N, shear strength was 7.16gf/bump. After shear test, we observed fracture surface by SEM and fracture were occurred at solder bump/Cu₆Sn₅ IMC layer, solder matrix, and UBM/Cu pillar bump side, respectively. After thermal shock test, crack initiated interface between Cu₆Sn₅ IMCs and Sn-Bi solder and then propagated within Sn-Bi solder and/or in interface between IMCs and solder. On the other hands, crack initiated interface between Cu₆Sn₅ IMCs and Sn-Bi solder and then propagated within Ni₃Sn₄ IMCs/solder and solder matrix in bending test.