Abstract: Flip-Chip LEDs were reflow soldered to Cu/Ag and Cu/Ni/Au substrates using SAC305 solder paste. The microstructure evolutions of the two kinds of sample during high-temperature aging were investigated in this study. Experimental results indicated that there was some interactions between the microstructure on both side of the solder joints. Au layer on Cu/Ni/Au substrate was melted into the bulk solder during the reflow which depressed the growth of Au-Sn intermetallic compounds during aging. Meanwhile, the fast growth of Au-Sn layer on the chip side decreased the relative concentration of Sn in the bulk solder. Consequently, the relative concentration of other elements, including the elements having positive effect for the IMC growth on the substrate, was increased.Compared with the two kinds of sample, the samples on Cu/Ag substrates show poorer thermal aging resistance.