Investigation On Interfacial Microstructure And Properties Of Sn-based Solder/Aluminum Alloy

Soldering technology of aluminum has a wide application prospect in the field of electronic packaging.Because of the advantages of short cycle,low cost and good adaptability,direct-soldering assisted by flux has much application value in the fields of microelectronic packaging and microwave module manufacturing.As the important alternative for low temperature solder,Sn-Ag and Sn-Zn have received much attention in the field of low temperature soldering for aluminum.However,in the soldering process without auxiliary equipment and coating conditions,the interaction mechanism of Sn-based solder and Al substrate is still unclear.The reliability and strength of the interface are still unknown during service.The interfacial bonding mechanism,microstructure evolution and reliability improvement are the critical issues need to be solved.Sn-based solder/Al joints were studied by scanning electron microscope?SEM?,X-ray analysis?XRD?,X-ray energy spectrum analysis?EDX?,transmission electron microscopy?TEM?,nano-indentation test,etc.The wetting characteristics of Sn,Sn-Zn and Sn-Ag on Al were analyzed.Interfacial bonding and the reaction mechanism of Sn-based solder and Al were systematically researched.Based on this,the evolution of interfacial microstructure and growth kinetics of intermetallic compounds?IMC?during soldering and aging were further studied.The effects of interfacial bonding and microstructure evolution on the mechanical properties of joints were diccussed.Moreover,in-situ shear test and finite element simulation method were employed to investigate the fracture mechanism of different joints in this paper.The wetting time of Sn-Zn solder is the minimum and the wetting force of Sn-Ag solder is the maximum at the same temperature above melting point.The element Zn can significantly reduce the wetting time and the element Ag can improve the wettability of the solder.With the increase of temperature,the wetting time of pure Sn and Sn-Ag solder shortens and the wetting force increases.The wetting time of Sn-Zn solder shortens but the wetting force decreases.After pre-plating Sn-Zn and Sn-Ag,there are inordinately interfacial reactions on the Al surface.The surface tension of the molten solder decreases at the new interface.As a result,the wetting force increases and the wetting time reduces.The effect of coating Sn-Zn on the increase of wetting force is significantly higher than that of Sn-Ag solder,indicating that the contribution of Zn element to the interfacial reaction is stronger than that of Ag element.There is no obvious enrichment and platform of elements at the interface of Sn/Al.Zn element enriches at the interface of Sn-Zn/Al joint.No intermetallic compounds are found in the interface of these two joints.Round corrosion pits appear on Al substrate under the reaction of liquid solder.The corrosion pits on the side of Al in Sn-Zn/Al joint are smaller and deeper than those in Sn/Al joint.A thin and continuous Ag₂Al compound layer is observed in the interface of Sn-Ag/Al joint.The morphology of the substrate after dissolution is similar to that of Sn/A joint,and the IMC appears to be particle shape.Al substrate is corroded into the solder and reaches to the saturated solid solution in three kinds of joints during soldering procrss.As a result,a small amount of fine Al-rich phases are precipitated in the boundary of Sn grain during the solidification process.The interfacial microstructure analysis of Sn/Al,Sn-Zn/Al and Sn-Ag/Al shows that there is a layer of nanometer amorphous Al₂O₃ between the Al substrate and the solder matrix,mixed with a small amount of nanocrystals.The thickness of amorphous layer is:Sn-Zn/Al>Sn/Al>Sn-Ag/Al.The O atoms in the amorphous layer are mainly come from the O₂ dissolved in the liquid solder.Zn is easy to be oxidized and more attracted to oxygen,the amorphous layer of Sn-Zn/Al is the thickest reaching to 250nm.The IMC near the interface prevents the diffusion of O atoms to the interface in Sn-Ag/Al joint.As a result,the amorphous layer is only 5nm.The formation of amorphous layer in the interface can reduce the interfacial energy of the system.The corrosion effect of liquid solder on Al substrate increases with increasing temperature and holding time of Sn/Al,Sn-Zn/Al joints.The diffusion rate of elements to Al substrate decreases with the extension of holding time.The diffusion coefficient of element Zn in Al grain is smaller than that of element Sn,while the saturated solid solubility of Zn in Al is much higher than that of Sn.Therefore,Zn is able to diffuse farther into the Al substrate.The thickness of IMC gradually increases during soldering and the Ag₂Al appears to be hexagonal prism.When the temperature reaches to 300? for 1h,the accumulated grains grow and the IMC turn to be discontinuous.The element Al diffuses into the solder matrix and the?-Al primary phase precipitates out aggregating to the top of the joint.The primary phase constantly increases and enlarges with the increase of soldering temperature and time.The interfacial microstructure of Sn/Al and Sn-Zn/Al solder joint is essentially unchanged exhibiting low sensitivity to temperature during the aging process.The appearance of IMC layer is also unchanged,but the thickness of the layer gradually increases with the rising aging temperature and time.The growth of IMC in Sn-Ag/Al liquid joint presents parabolic law and the growth process can be divided into chemical reaction controled stage and grain boundary diffusion controlled stage.The activation energy of the two stages is84.105±2.571 kJ/mol and 26.385±3.171 kJ/mol,respectively.The growth of IMC in Sn-Ag/Al solid joint can be divided into chemical reaction controled stage and bulk diffusion controled stage.With the increase of aging temperature,the critical thickness increases and the critical time decreases.The activation energy of the two stages is 32.427±3.109 kJ/mol kJ/mol and10.263±1.268 kJ/mol,respectively.With the increase of time in the soldering process,the shear strength of solution-type joints first increases and then maintaines invariable,while the plasticity first increases and then decreases.The shear strength and plasticity of compound-type joints increases first and then decreases.The maximum shear strength of Sn/Al,Sn-Zn/Al and Sn-Ag/Al joint is54.6MPa,82.5MPa and 48.9MPa,respectively.After aging for 1000h,the strength of solution-type joints increases slightly,but the plasticity decreases.The shear strength and plasticity of the compound-type joints both decrease.The fracture occurs at the interface of solder/Al with short insulation time.With the extension of insulation time,the fracture position of solution-type joints is transferred to the inner interface of solder.With the growth of the intermetallic compound,the fracture position of compound-type joints is transferred from the Sn layer between IMC and Al to the solder top of IMC.The shear stress distribution of the flat solution-type joints is uniform,and the stress increases reaching a maximum value in Al substrate.When the interface is scalloped,the shear stress increases in the front of interface and the fracture behavior is affected by the solder strength.The maximum shear stress of flat IMC-type joints appears in the solder layer between IMC and Al.When IMC is block-shaped,the maximum shear stress appears in the front of blocky IMC.The fracture behavior is mainly affected by the interface bonding strength and the distribution of the internal shear stress in the solder.