Effect of Ag addition on the mechanical properties of Cu/Ni/Sn/Ni/Cu microbump with a sub-10 micron bump thickness

More recently, the trend of miniaturization in microelectronic devices has led to a continual reduction in bump dimensions, the volume fraction of intermetallic compounds (IMCs) is increasing, significantly highlighting the impact of IMCs on the reliability of microbumps. Ni₃Sn₄ is the most common IMC formed in Ni/Sn/Ni bumps. However, its formation leads to a significant volume shrinkage up to 11.3% due to phase transformation. This volume shrinkage effect is particularly pronounced in microbumps, which may raise significant concerns regarding their reliability. To resolve these issues, the addition of Ag into the solder has been investigated as a method to modify the microstructure and improve the overall performance of microbumps. This study focused on investigating the microstructure and mechanical properties of microbumps with sub-10 micron bump thickness in two systems: Cu/Ni/Sn/Ni/Cu and Cu/Ni/Sn-3.5Ag/Ni/Cu. Various reflow heat treatment processes were employed to achieve different fractions of Ni₃Sn₄ IMC, allowing for a detailed comparison of the volume shrinkage effect between these two systems. The results of shear testing demonstrated that the Cu/Ni/Sn-3.5Ag/Ni/Cu bumps exhibited a significant improvement in shear strength and toughness, with shear strength increasing by approximately 37% compared to the Cu/Ni/Sn/Ni/Cu samples. Microstructure and fracture path analyses were also used to elucidate the strengthening mechanisms in Cu/Ni/Sn-3.5Ag/Ni/Cu bumps, highlighting the effectiveness of Ag addition in improving the reliability of microbumps.