| Zn-Sn-based alloy is considered as a kind of high-temperature lead-free solder alloy with development prospect because of its excellent electrical and thermal conductivity and low price.However,poor corrosion resistance and lack of toughness limit its application.Therefore,in this paper,a Zn-Sn-based multi-component high-temperature lead-free solder alloy with excellent performance was designed and developed by the microalloying.Aerosolization was used to prepare the solder alloy into a spherical powder with excellent flowability,from which industrial-grade solder pastes were prepared.The growth kinetics of the intermetallic compound（IMC）layer and the fracture mechanism were investigated by analyzing the interfacial reaction of the welded joint and the microstructural evolution.Besides,the mechanical reliability of the welded joint was evaluated.The research in this paper included:1.Multicomponent Zn-30Sn-2Cu-x Ni（x=0,0.5,1.0,1.5 wt.%）high-temperature solder alloys were designed by microalloying and the effect of Ni on the corrosion resistance and mechanical properties of solder alloys was investigated.The results showed that trace Ni has a significant effect on the corrosion resistance and mechanical properties of the alloy.With the addition of 0.5 wt.%Ni,the solder alloy formed a pseudo-passivation film during the corrosion process,which could improve the corrosion resistance of the alloy.When the Ni content reached 1.0 wt.%or more,the corrosion resistance of the alloy decreased due to the presence of galvanic corrosion between（Ni,Cu）5Zn21 phase and the Zn-rich phase.The ultimate tensile strength（UTS）and elongation of the Zn-30Sn-2Cu-x Ni alloy showed a trend of increasing and then decreasing with increasing Ni content.The Zn-30Sn-2Cu-0.5Ni alloy has the best corrosion resistance and mechanical properties,with maximum values of tensile strength and elongation（65.71 MPa and 26.1%,respectively）.Compared with the tensile strength and elongation of Zn-30Sn alloy（67 MPa and 20%,respectively）,there is a slight decrease in tensile strength and a significant increase in elongation.2.In order to enhance the corrosion resistance of Zn-30Sn-2Cu-0.5Ni alloy,the influence of Al element on corrosion resistance was further investigated in this paper.The results showed that the Zn-30Sn-2Cu-0.5Ni-0.2Al alloy significantly improved the corrosion resistance on the basis of maintaining the mechanical properties of the Zn-30Sn-2Cu-0.5Ni alloy（tensile strength and elongation are 73.32 MPa and 25.0%,respectively）.The corrosion current density(1.61μA·cm-2)of the solder alloy decreased by 71.6%and the total impedance（1991.0Ω·cm2）increased by 42.5%with the addition of Al.The reason was that trace Al element caused the corrosion products on the alloy surface to contain Al2O3,which promoted the formation of a denser pseudo passivation film on its surface.At the same time,the addition of trace Al elements reduced the heat of melting of the Zn-30Sn-2Cu-0.5Ni alloy,which was conducive to the rapid melting of brazing material in reflow soldering and the metallurgical bonding of brazing.3.Zn-30Sn-2Cu-0.5Ni-0.2Al solder alloy powder was prepared by aerosolization method.The optimum atomization process was determined through systematic study of the influence of melt superheat and atomization pressure on the sphericity,morphology and particle size distribution of the powder.The alloy powder prepared at an atomization temperature of 589℃（superheat degree of 200℃）and an atomization pressure of 2.5 MPa has the advantages of fine particle size,good sphericity and homogeneous composition.4.The homemade Zn-30Sn-2Cu-0.5Ni-0.2Al high-temperature solder paste was brazed into solder joints on Cu substrates using reflow soldering.And the interfacial reaction of the solder joints and the growth kinetics of the interfacial IMC layer during brazing were investigated.The results showed that the interface reaction between the liquid brazing paste and the solid Cu substrate formed a thin irregular scallop-shapedε-Cu Zn5 and a thicker layeredγ-Cu5Zn8 compound.The growth of the IMC layer at the solder joint interface was all controlled by bulk diffusion,and its thickness had a linear relationship with the square root of the aging time.In addition,the higher growth activation energy of the IMC layer at the solder joint（77.78 k J/mol）was favorable to reduce its growth rate and decrease its thickness.5.The effect of aging conditions on the mechanical properties of Cu/Zn-30Sn-2Cu-0.5Ni-0.2Al/Cu（Cu/Zn Sn Cu Ni Al/Cu）lap joints was investigated by means of a solid-state isothermal aging test,and the failure and fracture modes of the joints under solid-state isothermal aging were also thoroughly discussed.The results showed that the mechanical reliability and failure fracture mode of Cu/Zn Sn Cu Ni Al/Cu lap joints were related to the thickness and morphology of the IMC layer.With the increase of aging temperature and time,the thickness of the IMC layer gradually increased and the shear strength of the Cu/Zn Sn Cu Ni Al/Cu lap joints gradually decreased.Besides,with the increase of the thickness of the IMC layer,the shear fracture modes of the joints were in the order of through three stages of ductile fracture,mixed fracture and brittle fracture.Compared with the thermal cycling test results of Pb-5Sn high-temperature solder joints,Zn-30Sn-2Cu-0.5Ni-0.2Al solder alloy had better mechanical properties and excellent high-temperature service reliability. |
