Research On Alloying Of Tin-zinc Lead-free Solder Based On High-throughput Preparation

Due to the excellent merits such as the pretty mechanical properties,the similar melting point of Sn-37Pb solder,the compatibility of existing process equipment,the abundant resources and low cost of Sn/Zn,the Sn-Zn solder has good prospects of application.However,the poor wettability,oxidation resistance and corrosion resistance of Sn-Zn solder which is harmful to the application need be modified by alloying method.The traditional trial-and-error method is time and labor-consuming,which is hardly to meet the reequipments of the rapid industrial development.Therefore,a new route must be employed to rapidly screen alloying ingredients to accelerate the development of Sn-Zn solder alloy.In this paper,based on Sn-9Zn eutectic alloy,combined fast detection methods such as scanning electron microscopy(SEM),X-ray diffractometer(XRD),digital speckle imaging technique etc.,the dual-target magnetron sputtering co-deposition method was employed to establish the relationship between the composition-structure-property of samples,which is used to predict the variations of solder properties and complete the screening of solder composition.Then,the corresponding block materials are prepared for further verification test to obtain the best range of performance parameters and achieve optimization and screening for the materials.Sn-9Zn system solder alloy with good comprehensive performance was fabricated,and the main research contents and results of this study are as follows:(1)A high throughput preparation technique of the combinatorial material thin films for Sn-based lead-free solders was successfully developed by dual-target magnetron sputtering co-deposition method through adding the Ag and Cu elements,respectively,the corresponding surface density is higher than 1000/cm².Gradient change of composition can be observed in the obtained thin films,in which the Ag content varied from 0.2 wt%to 9.58 wt%,and the Cu content varied from 2.3 wt%to20.2 wt%.The evolution laws of phase structure for the sample with the change of the composition gradient were obtained by Micro area XRD,in which the Sn-Zn-Ag alloying films were composed of Sn matrix and Ag Sn₃,Ag Zn,Ag Zn₃,Ag₅Zn₈intermetallic compounds(IMCs),and the trends for formation of IMCs follow this rule with the increase of Ag content:Ag Zn>Ag₃Sn>(Ag Zn₃ and Ag₅Zn₈);The Sn-Zn-Cu alloying films were composed with Sn matrix and Cu₅Zn₈ and Cu₆Sn₅IMCs,and the Cu₅Zn₈ is easier to form than Cu₆Sn₅with increase of Cu content.The evolution laws of machinal properties for the sample with the change of the composition gradient were obtained by in-situ nanomechanical testing system,the hardness of the Sn-Zn-Ag alloying films increased with the decrease of Ag content,and the hardness of Sn-Zn-Cu alloying films decreased firstly and then increased with the decrease of Cu content,but only showed tiny changes in overall extent;The young's modulus of the samples increased with the decrease of Ag and Cu contents.The evolution laws of oxidation resistance for the samples with the change of the composition gradient were obtained by electron probe micro-analyzer and digital speckle imaging technique,in which the oxidation resistance of Sn-Zn-Ag alloying films decreased with the decrease of Ag content,and serious oxidations can be observed when the Ag loading less than 0.3 wt%;In contrast,the oxidation resistance of Sn-Zn-Cu alloying films increased with the decrease of Cu content,and remarkable increases can be observed when the Cu loading is less than 7 wt%.(2)Based on the results of film tests,some alloying components were optimized and expanded according to the combination of mechanical properties and oxidation resistances to fabricate the corresponding block samples,which were used to further optimize the content of Ag and Cu in solder samples.It was demonstrated that Sn-9Zn-0.35Ag and Sn-9Zn-0.4Cu possessed the best comprehensive properties through the analysis and testing of melting point,microhardness and thermogravimetric properties and so on.According to the microstructure investigations of Sn-9Zn-0.35Ag and Sn-9Zn-0.4Cu,compared with Sn-9Zn master alloy,we can observed that the coarse Zn-rich phases were disappeared,there are only some fine needle-like Zn eutectic structures and dendrite-like Ag-Zn or Cu-Zn IMCs in the Sn matrix.The disappearance of Zn-rich phase and the formation of IMCs will decrease the oxidation of Zn and the surface tension of the solder melts during the process of welding,leading the spreading rate of Sn-9Zn-0.35Ag and Sn-9Zn-0.4Cu increase to 63.2%and 66.05%from 57.76%,respectively.Besides,the selective corrosion of Zn also will be inhibited,resulting a better corrosion resistance of solders.The needle-like Zn phase will modify the mechanical properties of the samples,the specific elongation of two samples are approximately 2 times as high as that of Sn-9Zn,and the tensile strength increased to 80.1 MPa and 71.6 MPa from 63.9 MPa,respectively.However,due to the formation of the IMCs,the effective charges were decreased,leading declines of the electrical conductivity and thermal conductivity.