Abstract
In recent years, many studies have aimed to improve the characteristics of SAC solders by adding fourth element in SAC solder, changing to another solder alloy system, or modifying the material of under bump materialization (UBM). Besides the material selection in both solder ball and UBM, the effect caused by different substrate orientation and the reaction between column structure and intertwined structure was also demonstrated. However, the mechanism of interfacial reaction owing to various substrate structure after reflow was not discussed clearly. It is noted that the intermetallic compounds (IMC) growth and the substrate consumption are affected by factors, such as substrate fabrication, substrate orientation, and substrate structure. Hence, the purpose of this study is to investigate the effect of Cu substrate structural difference on interfacial reaction after reflow with pure Sn solder with various time. To simplify the material systems, polycrystalline Cu substrate with different grain size was applied in this study and reflowed with pure Sn solder. Both bulk Cu substrate and electroplated Cu substrate were used in this study. Bulk Cu substrate was annealed at 400oC for various time and electroplated Cu substrate was prepared by different duty cycle to achieve different grain size of Cu substrate. Electron Back-Scatter Diffraction (EBSD) was provided to evaluate the grain size of Cu substrate. The pole figure and inverse pole figure by EBSD and X-Ray Diffraction (XRD) were used to confirm that there was not any preferred orientation in the Cu substrate. After Cu substrate reflow with pure Sn at 250oC, a Field-Emission Scanning Electron Microscope (FE-SEM JSM-7600F, JEOL, Japan) was applied to observe the interfacial reaction. The thickness of IMCs was measured and averaged to evaluate the consumption rate of Cu substrate. The reaction order of IMCs was also computed to illustrate the growth of IMCs. The Kirkendall void formation and its influence was also discussed. Combining the experiment verification and theoretical discussion, the effect of Cu substrate with different grain size on interfacial reaction after reflow with pure Sn solder was demonstrated.