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In this study, we reported that effect of solder ball sizes on the intermetallic growth of Sn-Ag-Cu solder balls during thermal aging. Various size (400, 300, 200, and 100 um diameter) solder balls of 98.5 wt.% Sn, 1 wt.% Ag, and 0.5 wt.% Cu(SAC105) were bonded on the Cu/OSP (Organic Solderability Preservative) and ENIG (Electroless Ni Immersion Gold) pads by solder reflowing and then subsequently annealed at 150°C for up to 500 h. In the case of the reaction with Cu/OSP, there was no significant difference of the total Cu3Sn IMC thickness regardless of solder ball size. Unlike the reaction with Cu/OSP, the IMC growth of fine size solder balls on ENIG pads was faster than larger size solder balls during a thermal aging test. The composition of Cu-Ni-Sn IMCs formed at the ENIG pads was changed from Cu-rich phase ((Cu, Ni)6Sn5) to Ni-rich phase ((Ni, Cu)3Sn4), as solder ball sizes decreased and aging time increased. This study showed that the solder ball size significantly affects the IMC thickness, types of IMC phases, and IMC composition at the ENIG pad during a thermal aging test.