Study On Microstructure Of Sn-Au-Ag(Ni) Solders And Their Mechanical Properties On Cu And Ni Substrates

Au-20 wt.% Sn eutectic alloy is widely used in electronic packaging as solder material for its excellent mechanical properties, outstanding thermal conductivity, and can be reflow soldering without flux. The high melting point, excellent creep resistance and thermal conductivity make Au-20 wt.% Sn alloy particularly suitable for power device packaging. However, the cost using the Au-20 wt.% Sn eutectic alloy is very high. Besides the high cost, it is difficult to produce using conventional preparation method. Therefore, the new lead-free solders of Sn-rich Sn-Au-Ag(Ni) systems were designed using cluser-plus-glum-atom model, and the microstructure, melting behavior, wettability and mechanical properties of the designed Sn-Au-Ag(Ni) solders were systematically investigated.The main contents of this thesis are as follows:(1) Novel Sn-Au-Ag(Ni) solder alloys are designed using the cluster-plus-glue-atom model. The Sn-9.2Au-5.1Ag, Sn-4.7Au-7.8Ag, Sn-5.1Au-2.8Ag, Sn-2.6Au-4.3Ag and Sn-2.6Au-2.4Ni solders have a near-eutectic composition with a narrow melting temperature ranged from 206.89℃ to 224.14℃.(2) The melting temperature of Sn-Au-Ag solders ranged from 206.29 to 208.59℃. The microstructure of the Sn-Au-Ag solders consists of Ag3Sn and AuSn4 phases dispersed in β-Sn matrix. In Ni/Sn-Au-Ag/Cu solder joints, the interfacial intermetallic compounds (IMCs) of Ni/Sn-Au-Ag interface are (Cu,Ni,Au)6Sn5 compounds, while the interfacial IMCs of Sn-Au-Ag/Cu interface are (Cu,Au)6Sn5 compounds.(3) The melting temperature of Sn-2.6Au-2.4Ni solder is 224.14℃ and has a near-eutectic composition. The microstructure of the Sn-Au-Ni solders is composed of Ag3Sn and AuSn4 phases dispersed in β-Sn matrix. In Ni/Sn-Au-Ni/Cu solder joints, the interfacial IMCs of Sn-Au-Ni solders on both Ni and Cu substrate are (Cu,Ni,Au)6Sn5 compounds.(4). The mechanical properties of Ni/Sn-Au-Ag(Ni)/Cu solder joints were studied and found:the fracture location of all solder joints lay inside the bulk solders, and Sn-5.1Au-2.8Ag. Sn-2.6Au-4.3Ag and Sn-5.2Au-1.5Ni solders had a significantly high shear strength, which is higher than the eutectic Au-20 wt.% Sn solder joint.(5) The wettability of all Sn-Au-Ag(Ni) alloys on Ni substrates was better than that on Cu substrates, due to the severe interfacial reaction on Cu substrates.