Abstract: The effect of annealed Cu substrate on the microstructure of solder joints was investigated. Experimental results showed that only scallop-shaped Cu₆Sn₅ was observed at the interface of Cu/Sn58Bi/Cu after reflowing and aging for 24h. A bi-layer structure consisted of Cu₆Sn₅ and Cu₃Sn was formed at interface and the thickness of IMC layers increased with the increase of aging time. With the long aging process, Bi particle segregation was producedat the interface of solder joint of un-annealed Cu substrates. However, Bi particle segregation was hardly observed at the interface of solder joint of annealed Cu substrates. Compared with the growth rate constants of IMC layer of interface of annealing and un-annealing Cu and solder, it was demonstrated that annealing treatment on Cu substrate could effectively retard the growth of interfacial IMC layers, it mainly attributed to that the annealing treatment on Cu substrate could effectively eliminate inner-stress and defects, reducing the diffusion of Cu atoms and retarding the growth of IMC.