A solder/intermetallic layers/copper joint assembly was prepared by the dipping process and then aged thermally for an accelerating aging test. The phase transformation of the assembly was compared to the phase diagram of binary alloy, Cu-Sn, and it showed an agreement with the resultant intermetallic phases formed between the pure tin and pure copper. Two theoretical models proposed by Gosele and Shatynski were developed and then employed to characterize the assembly. The Gosele＇s model was used to predict whether the intermetallic layers grew or shrank during aging, while the Shatynski＇s model was employed to estimate the related reactive thicknesses and hence the ratios of the interdiffusivities in the joint assembly. After a series of calculations, the Gosele＇s model predicted that Cu6Sn5 and Cu3Sn intermetallic layers became thicker; the orders of the intermetallic interdiffusivities were also proven to approach theoretical values from the Shatynski＇s model.