Microstructure evolution and mechanical properties changes during the formation of full Cu41Sn11 joint in high-temperature electronic packaging

In this study, Cu/Sn/Cu structure under a pressure of 1.5 N was soldered at 390–450 °C. Then, microstructure evolution and changes in mechanical properties of solder joint during the formation of full Cu41Sn11 joint were systematically analyzed. Experimental results showed that Cu6Sn5 and Cu3Sn were observed in the interfacial region at 420 °C in the early stage of soldering. The Cu/Sn/Cu structure completely transformed into Cu/Cu3Sn/Cu at 1 h. With the further progress of the interfacial reaction, Cu41Sn11 appeared and grew at the expense of Cu3Sn. Eventually, the full Cu41Sn11 joint was formed at 10 h. The growth of Cu41Sn11 was controlled by volume-diffusion. Moreover, diffusion activation energy of Cu41Sn11 was about 108.19 kJ/mol. The shear strength of full Cu3Sn joints was 45 MPa, while that of the solder joints with 25, 50, 75 and 100% Cu41Sn11 was 50, 52.2, 59 and 64.5 MPa, respectively. For full Cu3Sn and full Cu41Sn11 joints, respectively, the fracture occurred partly within intermetallic compounds (IMCs) layers and partly along the Cu/IMC interface. However, for the joints composed of Cu3Sn and Cu41Sn11, all fractures occurred in the IMCs layer. Besides, all IMCs joints were brittle fracture, and the fracture of Cu3Sn was mainly intergranular, while the fracture that occurred in Cu41Sn11 was mainly transgranular.