Surface Diffusion and the Interfacial Reaction in Cu/Sn/Ni Micro-Pillars

An electroplated Cu/Sn/Ni micro-bump was fabricated for analysis. After aging for different times, both the morphology and microstructure show significant changes, particularly after long-term aging. Because of the small aspect ratio of the micro-bump, surface diffusion has a strong effect on the morphology and roughness of the Cu/Sn and Cu/IMCs interfaces. A focused ion beam was applied for 3D reconstruction and used for calculation of the voiding rate of the micro-joint after 720 h of aging. In this way, the relationship between the voiding rate redistribution and the position along the radius direction was obtained. It is concluded that the redistribution of voids after long time aging is controlled by surface diffusion when almost all of the intermetallic compounds (IMCs) in the Cu/Sn/Ni micro-bump were transformed to Cu3Sn. Furthermore, as observed from the relationship between the voiding rate of the 5 μm and 10 μm samples and the position along the radius, when the diameter of the micro-bump decreases to 5 μm or even less than 5 μm, the total voiding rate of the sample after long-term aging rapidly increases, and a higher voiding rate has a strong influence on the reliability of the micro-joint and may lead to the failure of the electronic device.