Mechanical properties of Sn-Bi bumps on flexible substrate

The future electronics will be flexible, bendable, and wearable device. The damages like thermal stress and warpage occurred generally during bonding process between flexible substrate and chip used Pb-free solder for wearable electronic device. Though the wearable electronic devices have been researched, there is few study on bonding technology and reliability of joint between chip with solder bump and flexible substrate. In this study, we investigated joint properties of eutectic Sn-58Bi solder bump formed on flexible substrate finished with ENIG by flip chip process. The Si chip with Sn-58Bi solder bumps and Cu pillar bumps for interconnection were formed by electroplating method. The diameter and height of Sn-58Bi bumps were 25μm and 16μm, respectively. The Sn-58Bi solder bumps were bonded on flexible substrate at the conditions of bonding temperature of 170°C, bonding time for 15sec, and bonding pressure of 1N, 2N, and 3N. The Si chip and flexible substrate have been designed to daisy chain for measurement of electric resistance. After flip chip bonding, we observed cross-sectioned images by SEM. The Cu₆Sn₅ IMC was formed at the Cu pillar bump side and Ni₃Sn₄ IMC was formed at the ENIG substrate side. And then, the thermal reliability of joints was evaluated by thermal shock test. The test conditions were in the range or -40°C to 105°C and dwell time for 15 min. After thermal shock test, we observe that crack initiated between Cu₆Sn₅ IMCs and Sn-Bi solder and then propagated within Sn-Bi solder and/or in interface between IMCs and solder. And we also evaluated reliability of bending fatigue. We observed that failure occurred at Ni₃Sn₄ IMCs/solder and in solder matrix after bending test.