Study On Solderability And Wetting Mechanisms Of Micro-alloyed Sn-9Zn Lead-free Solder

Due to the toxicity of the lead (Pb), legislative actions around the world have been targeted to eliminate the usage of Pb-bearing solders in electronic assemblies. Researches of lead-free solders for commercial substitutes to Sn-Pb solder have been widely attracted from both industrial and academic concerns. Sn-Zn lead-free solder has the potential to be developed into the next generation of lead-free solder because of its appropriate melting point, low cost and advanced mechanical properties. However, poor performances in wettability, oxidation resistance and corrosion resistance limited its applications.In order to overcome the disadvantages of Sn-Zn lead-free solder, Ag, Al, Ga, and Ce multi-alloying method was adopted to the Sn-9Zn alloy, and the mechanisms of the doping elements were also studied.The elements Al and Ce are extremely oxidizable during processing, which clashes the fact that Zn alloys cannot be smelt via vacuum process. Therefore, in the study, some low melting-point alloys were pre-fabricated, which were subjected to be smelt by the method of smelting multi-element alloys in nitrogen atmosphere with melting salt at a lower temperature. On this basis, multi-element alloys were successfully prepared.The wettability of liquid Sn-Zn solder dramatically relies on its surface tension and oxidability. Ce, Ga and Al were added to improve the wettability and oxidability of the Sn-Zn solder, while the addition of Ag was to improve the microstructure and the interfacial reactions as well as the mechanical properties of soldered joints. The wetting balance tests showed that, Ce, Ga, Al and Ag dosage in Sn-9Zn alloy, was 0.07 wt.%, 1.0wt.%, 0.005 wt.%, and 0.3wt.%, respectively. In the case of multi-addition, the best wetting was achieved from Sn-9Zn-0.25Ag-0.002Al-0.2Ga-0.15Ce.Thermogravimetric analysis (TGA) test indicated that small amounts of Al significantly improved the high temperature oxidation resistance of Sn-9Zn solder. Auger electron spectroscopy (AES) showed that Al enriched at the surface of Sn-9Zn-0.005Al solder in the range of 1-30 nm, which was 2000 times compared with that of substrate. In addition, XPS analysis showed that Al atoms at the solder surface were mainly existed in the form of oxides; the dense Al2O3 film reduces the internal oxidation and further improves the wettibility of the solder.Trace addition of Al has little effect on the interfacial reaction between solder matrix and substrate. However, when the Al content is greater than 0.24wt.%, AlCuZn compound would be formed at the interface, which inhibites the growth of Cu₅Zn₈ compound. Although Al reduces the corrosion potential and deteriorates corrosion resistance of Sn-9Zn solder, the negative effects on corrosion resistance still can be ignored at the amount which meets the need to improve the oxidation resistance.Small amount of Ag enhances the soldered joints, which is ascribed to the formation of AgZn₃ compound particles that uniformly dispersed in the solder matrix and at the interface. The formation of AgZn₃ also improves the corrosion resistance by reducing Zn-rich phase in Sn-Zn solder matrix. During the soldering process, Ag enriches at the interface in the form of serrated AgZn₃ compound, which is the resultant of the peritectic reaction between the liquid solder and Ag5Zn8 that precipitate over the preformed Cu₅Zn₈ surface.On the other hand, the enrichment of Ga on the liquid solder surface reduces its surface tension and improves its oxidation resistance. Ga also improves the passivation of Sn-Zn solder in NaCl solution by forming a continuous corrosive product. Furthermore, the mechanical properties and reliability of soldered joints during high-temperature storage are also improved by the Ga additon. However, when the Ga content exceeded 2wt.%, the solder becomes brittle, and the soldered joints present typical intergranular fracture which is due to the Ga segregation at grain boundary.Studies evidenced that Ce enriched extremely at the surface of liquid Sn-Zn solder, which in turn reduced its surface tension, and improved the wettability of the solder. The wetting balance test suggested that the best wettability was obtained at 0.07wt.% of Ce, and larger amount addition of Ce beyonds 0.07wt.% worsened the wettability instead. This is because Ce is very active; the oxidation of Ce on the surface limits its function in improving the wettability. In this study, the problem was solved by multi-addition of Ce, Al, and Ga. AES and XPS results indicated that the oxidation of Ce was depressed significantly since the multi-addition of Al and Ga. In terms of multi-addition, the optimum dosage of Ce is up to 0.15wt.%, and the concentration of Ce on the surface is up to 45at.%, almost 450 times higher than that in bulk solder.Compared with Sn-9Zn, the wetting force of the optimized Sn-9Zn-0.25Ag-0.002Al-0.2Ga-0.15Ce solder wetted on Cu substrate increases by 38%, while the wetting time reduces by 25%. The shear strength of ceramic resistance micro-joints and the pull strength of QFP chip micro-joints increase by 19% and 35%, respectively, when using Sn-9Zn-0.25Ag-0.002Al-0.2Ga-0.15Ce solder rather than Sn-9Zn. Furthermore, multi-addition considerably improves the reliability of the soldered joints during high-temperature storage.Moreover, wettability, interfacial reactions and reliability of the solders wetted on Cu, Sn-plated copper, and Au/Ni/Cu substrates were also investigated in this paper. Results indicated that Sn-Zn solders showed excellent wettability on Sn-plated copper substrate. Cu₅Zn₈ and AgZn₃ intermetallic compounds form at the interface between Sn-9Zn-0.25Ag-0.2Ga-0.002Al-0.15Ce solder and Cu substrate, while AuZn3 and AuAgZn2 present at the interface between solder and Au/Ni/Cu substrate. During early stages of high temperature annealing, the Cu₅Zn₈ layer becomes thicker due to the continuously diffusion of Zn and Cu atoms at the interface. Because of the higher diffusion constant of Cu in Sn matrix than that of Zn, a Zn depleted zone arises at the interface of Cu₅Zn₈ layer and solder matrix, which splites the Cu₅Zn₈ layer eventually. Meanwhile, Sn atoms diffuse toward to the Cu substrate through Cu₅Zn₈ layer resultes in Cu6Sn5 intermetallic compound at the interface between Cu substrate and Cu-Zn intermetallic layer. Using Ni substrate enable the improvements in the high temperature reliability of solder joints as Ni layer can prevent the diffusion of Cu atoms.Nano-indentation method was seleted to measure the elastic modulus and hardness of the different phases at the interface of the soldered joints. The hardness and modulus values obtained by nano-indentation are remarkably high for Cu₅Zn₈, Ni5Zn21, AgZn₃ and AuZn3; the hardness are 4.92GPa, 5.12GPa, 3.52GPa and 3.40GPa, while the modulus for the IMC phases are 162.1GPa, 159.3GPa, 124.4GPa and 118.2GPa. On the contrary, the Sn-9Zn-0.25Ag-0.2Ga-0.002Al-0.15Ce solder is soft and exhibites good plasticity; the hardness and modulus values of the Sn-9Zn-0.25Ag-0.2Ga-0.002Al-0.15Ce solder are 59.9GPa and 0.36GPa, respectively. This would be the evidence to prove that intermetallic compounds are the key factor affecting the reliability of soldered joints. From the physical analysis of nanoindentation curves, the creep stress exponent of Sn-9Zn and Sn-9Zn-0.25Ag-0.2Ga-0.002Al-0.15Ce solder matrix is 8.12 and 11.56, respectively, which implis the improvement in creep resistance of Sn-9Zn joints from the multi-alloying.