The effects of the joining sequence on the interfacial reactions and substrate dissolution behaviors in Ni/solder/Cu joints were studied by using 500-lm (diameter) Sn-3.5Ag solder balls and substrates with a 375-lm (diameter) opening. Three distinct paths for the joining sequence were studied. In path I, a solder ball was first joined to the Cu substrate and then to the Ni substrate. In path II, a solder ball was joined to both the Cu and Ni substrates simultaneously. Path III had the opposite joining sequence to path I. The results of this study indicated that (Cu,Ni) 6 Sn 5 was the predominant reaction product at both the Ni/solder and solder/Cu interfaces regardless of the joining sequence. However, the composition, morphology, and thickness of the (Cu,Ni) 6 Sn 5 varied considerably with the different paths. The dissolution behaviors of Cu and Ni were also different. Dybkov's dissolution kinetics and Cu-Ni-Sn isotherm data were utilized to rationalize these differences.