The dissolution and reprecipitation behavior of the AuSn₄ intermetallic compound in a solder joint during liquid-state and solid-state reactions

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 °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)₆Sn₅ 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.