Improvement in thermal reliability of a flip chip interconnection system joined by Pb-free solder and Au bumps

The failure mechanism and improvement in the thermal reliability were studied of a new flip chip interconnection system using Au bumps on Si chips and Sn bumps on substrates. The thermal reliability of this flip chip depended on the diffusion behaviour at the Au/Sn bonded interface. Kirkendall voids, which were formed at the Au/Sn bonded interface, induced cracks and impaired the reliability. By the addition of Ag into the Sn bumps, the reliability was improved, due to the formation of an Ag concentrated layer at the Au/Sn bonded interface which acted as a barrier to Au-Sn further interdiffusion and suppressed the Kirkendall voiding. The AuSn phase grew faster than the other Au-Sn intermetallic compounds did at the Au/Sn bonded interface after annealing. Activation energy for AuSn growth was 29.3 kJ·mol^-1.