The Calphad method has been employed to reassess the thermodynamic description of the Au-Pt-Sn ternary system by remodeling the ternary compound and the solubility of the third elements in relevant binary compounds. A set of consistent parameters was obtained with reproducing most of the experimental results. The microstructures of as-soldered and aged Au-20 wt%Sn|Pt reaction couples were characterized by using scanning electron microscopy with energy-dispersive X-ray microanalysis (EDS), as well as scanning transmission electron microscopy with EDS. During soldering, the (Au1-x,Ptx)Sn (0 ≤ x ≤ 0.5) intermetallic compound layer was formed at solder/Pt interface and evolved with prolonged bonding time. When the as-soldered samples were subsequently annealed at 150 °C for up to 4600 h, only one interfacial intermetallic compound (Au1-x,Ptx)Sn was observed. The average thickness of the (Au1-x,Ptx)Sn layer was reached ∼ 3.3 μm already after 100 h of aging and remained practically constant with the progress of the aging process. When samples were isothermally aged for 6848 h, in addition to the thick (Au1-x,Ptx)Sn interfacial layer, a thin (no more than 100 nm) PtSn layer was observed at the interface of (Au1-x,Ptx)Sn/Pt. These results indicate that the Au-20 wt%Sn|Pt interconnections are thermally stabile at 150 °C. The interfacial reaction mechanism of Au-20 wt%Sn|Pt at 150 °C has been proposed by combining experimental results with the basic thermodynamic considerations. Moreover, the microstructure of Au-20 wt%Sn|Pt was compared with those of Au-20 wt%Sn|Ni and Au-20 wt%Sn|Cu. The advantages and disadvantages of Au-20 wt%Sn solder on Ni, Cu and Pt contact metallizations were discussed.