The present study investigated the reaction behavior of the AuSn4 intermetallic compound in the Ni/Sn1.8Ag+Sn3Ag0.5Cu/Au/Pd/Cu solder joint when subjected to multiple reflow for five cycles and solid-state aging at 150 degrees C for 1000 h. Multiple reflow induced grain refining of the AuSn4 in the solder matrix. The grain refining was ascribed to the repeated dissolution and reprecipitation of the compound that occurred during multiple reflow. The dissolution of the AuSn4 was driven by the chemical potential gradient at the AuSn4/molten solder interface. The dissolution behavior enriched the solder matrix with the constituent elements that were partly involved in the formation of the Cu6Sn5 at the solder/Cu interface and the subsequent reprecipitation as the refined AuSn4 particles when the solder was solidified. Solid-state aging, however, induced complete dissolution of the AuSn4 and the migration of the constituent elements toward the solder/Cu interface to form a more stable (Cu,Ni,Pd,Au)(6)Sn-5 phase due to the thermodynamic stability competition that occurred among the compounds. Consequently, no AuSn4 reprecipitation behavior occurred during solid-state aging. An understanding of the reaction behavior of the AuSn4 intermetallic compound is important for practical applications due to its brittle characteristics. The refining of the compound throughout the solder matrix may also strengthen the solder joint, leading to a better performance.