Abstract: The Sn3.0Ag0.5Cu/Cu lead-free solder and Cu joints were isothermally aged at 125,150 and 175℃ for 36,72,216,360 and 720 h. The microstructure and mechanical properties of the solder joints were studied. The results showed, after soldering the interfacial intermetallic compounds (IMC) of solder joints were small serrate. The size of interfacial IMC was found to increase with the aging temperature and aging time. The IMC growth rate increased with aging temperature but decreased with aging time. With the increase of aging time,the amount of IMC serration decreased,and its diameter,height and width became bigger,higher and wider respectively. Through analysis of the interfacial IMC,a scientific method to characterify IMC size was found. When the IMC equivalent thickness was calculated, the quantitative relationship between IMC equivalent thickness and aging time/temperature was established. The IMC activation energy is 88 kJ/mol. In addition,the relationship between isothermal aging and tensile strength was developed. The tensile strength firstly increased and then decreased with the aging time, which could be attributed to the welding residual stress relaxation and the IMC evolution.