Study On Effects Of Ce On Properties Of Sn-Ag-Cu & Sn-Cu-Ni Solders And Reliability Of Soldered Joints

The application of Pb has been restricted in electronics industry and in household appliances industry according to the"RoHS Regulations"(Restriction of the use of certain Hazardous Substances in electrical and electronic equipment) of the European Union, so the research of lead-free solder is required to replace Sn-Pb solder used widely. As an acceptable substitute for Sn-Pb solder, it has to satisfy not only the mechanical property and wettability, but also the process requirements and the reliability requirements that commonly used traditional Sn-Pb solder possesses, and thus it is imperative to develop and investigate new lead-free solders with better properties. In order to improve the properties of Sn-3.8Ag-0.7Cu and Sn-0.5Cu-0.05Ni solders, small amount of rare earth Ce was added to the base alloys, and the effect of Ce on the properties, microstructure of solder alloy, and reliability of soldered joints were studied.First of all, physical properties and wettability of solder alloys were investigated. It is found that the electrical conductivity and density of Sn-Ag-Cu-Ce and Sn-Cu-Ni-Ce solder is better than Sn-Pb alloys, and the melting temperatures are also acceptable.The wettability results indicate that the wettability of solder on Au/Ni/Cu substrate is better than that on Cu substrate both for Sn-Ag-Cu-Ce and Sn-Cu-Ni-Ce solder. As the oxidation of molten solder and substrate is inhibited in N2 atmosphere, the solderability of solder is enhanced at the temperature of 240℃²80℃. For instance, the wetting times of Sn-Ag-Cu-Ce alloys are reduced by 10²5%, and the wetting times of Sn-Cu-Ni-Ce alloys are reduced by 20⁴5%, respectively.Furthermore, the wettability is improved by adding small amount of Ce, both for Sn-Ag-Cu solder and Sn-Cu-Ni solder. In particular, when the Ce content in Sn-Ag-Cu-Ce alloy is within 0.03%⁰.05%, the wetting time is reduced by 20⁴0% than that of the base solder alloy, and as for Sn-Cu-Ni-Ce solder, the optimal Ce content is in the range of 0.05%⁰.07%. In order to discuss the influence of Ce addition on the wettability of solder, a surface element analysis in the depth direction of the alloy was carried out. It is found that as an active element, Ce tends to accumulate at the Sn-Ag-Cu or Sn-Cu-Ni solder interface in the molten state, thus the surface tension of molten solder is decreased effectively, which plays an important role in the improvement of the wettability.As the reliability of joint depends on the microstructure of solder/ Cu interface, it is important to study the microstructure change of solder/ Cu interface. The mechanical properties of joints on Quad flat pack (QFP) lead and on chip resistor, as well as the reliability of joints in the course of long time thermal cycling were studied in this paper. The mechanical properties of soldered joints are enhanced with the addition of Ce, and the soldered joints possess the peak values of pull forces or shear forces when the Ce addition is about 0.03% in Sn-Ag-Cu-Ce joint, and around 0.05% in Sn-Cu-Ni-Ce joint, respectively. For QFP32, QFP100 and chip resistor, the pull forces or shear forces of Sn-Ag-Cu-0.03Ce joints are 4.56%, 4.06% and 3.81% higher than Sn-Ag-Cu joints, while the pull forces or shear forces of Sn-Cu-Ni-0.05Ce joints are 7.60%, 5.12% and 3.52% higher than Sn-Cu-Ni joints. Ce element tends to combines with Sn element because of the high affinity with Sn in the Sn-Ag-Cu and Sn-Cu-Ni alloy. The size of Ag₃Sn is decreased with the addition of Ce, and at the same time theβ-Sn grains also become finer as the adsorption will decrease the difference of surface energy of the crystal.The influences of thermal cycling on mechanical properties of Sn-Ag-Cu-Ce and Sn-Cu-Ni-Ce joints were studied simultaneously. The results indicate that with the increase of thermal cycling times, the pull forces of QFP joints and shear forces of the chip resistor joints decreased gradually after long time cycling. The mechanical properties of joints are improved with the Ce addition, and thus the reliability of soldered joints is improved. After 2000 times of thermal cycling, the pull forces or shear forces of Sn-Ag-Cu-0.03Ce joints are 14.29%, 12.06% and 10.31% higher than Sn-Ag-Cu joints, 51.01%, 28.16% and 41.87% higher than Sn-Pb joints, for QFP32, QFP100 and chip resistor, while the pull forces or shear forces of Sn-Cu-Ni-0.05Ce joints are 18.71%, 12.27% and 14.03% higher than Sn-Cu-Ni joints, 24.56%, 17.29% and 25.78% higher than Sn-Pb joints.In the course of long time thermal cycling, the small addition of Ce is found to refine the microstructure inside the solder, decrease the thickness of the intermetallic compound (IMC) in soldering, and depress growth of the IMC in cycling. Fatigue failure is found in fracture of Sn-Ag-Cu joint after long time cycling, while the influence of cycling on reliability is decreased owing to Ce addition.In order to study the effect of IMC on mechanical property of joint, the elastic modulus and hardness of IMCs and solder matrix were analyzed with nanoindentation method. From the physical analysis of nanoindentation curves, the elastic modulu of Cu₆Sn₅, Ag₃Sn in Sn-Ag-Cu joints is 114.2±4.6GPa and 79.9±4.2GPa, while the hardness is 5.57±0.28 GPa and 3.05±0.22 GPa, respectively. The mechanical properties of (Cu, Ni)₆Sn₅ in Sn-Cu-Ni joints are the same as Cu₆Sn₅ in Sn-Ag-Cu joints. IMCs, including Cu₆Sn₅ and Ag₃Sn, are the key factors in the reliability of lead-free joints, due to the big contrasts between the elastic modulus as well as hardness of IMCs and solder matrix. The creep strain rate sensitivity of Sn-Ag-Cu, Sn-Ag-Cu-0.03Ce, Sn-Cu-Ni, Sn-Cu-Ni-0.05Ce and Sn-Pb solder matrix is 0.0922, 0.0886, 0.1286, 0.1248, and 0.1832, and the creep stress exponent is 10.8460, 11.2867, 7.7760, 8.0128, and 5.4585, respectively, which indicates the improvement in creep resistance of Sn-Ag-Cu and Sn-Cu-Ni joints due to the addition of Ce.