Study On The Properties Of SnBi-SAC Low Temperature Composite Solder Paste By Solid-state Mixing Method

With the increasingly miniaturization of solder joints in the packaging field,the increasing demand for the size.Sn-58Bi(SnBi)materials are widely used in the low temperature packaging,duing to its low melting point?wettability and tensile strength,however SnBi solder is seriously restrict used because of the brittleness of rich Bi phase,thus how to optimize SnBi materials become a hot point in the low temperature packaging field.Therefore,in this paper,composite solder paste was prepared by means of adding Sn-3.0Ag-0.5Cu(SAC),Sn-0.7Ag-0.5Cu-3.5Bi-0.5Ni(SACBN)alloy particles to SnBi solder paste by using mechanical mixing,the melting properties,wettability,microstructure,hardness and shear properties of composite solder paste were analyzed,to provide a theoretical evidence for the further improving the property of SnBi materials.The melting characteristic test showed that when the SnBi-SAC composite solder paste heating at 180 ?,the solid SAC alloy powder is dissolved and diffused into the molten SnBi alloy to form the solder joint.With adding SAC alloy particles,the wettability of composite solder paste has been reduced.The reason is that the added micron particles is solid when soldering,which impedes the flow of molten SnBi alloy and reduces its wetting property.By using scanning electron microscopy(SEM)analysis obtain that owing to the dissolution and diffusion of SAC particle,the content of ?-Sn phase is increasing and Bi rich phase content is relatively reduced in the conposite solder paste asreflowed,the solder paste composition from eutectic composition to sub eutectic transformation leads to the melting range larger,then the grain size of ?-Sn grow.When the concentration of SAC particles increase to 15 wt%,the grain size of the ?-Sn phase increases from 1.3 ?m to 22.1 ?m;However,due to the refinement effect of Ni on the microstructure of solder alloy,the grain size of ?-Sn increases first then decreases,when the contention of SACBN particles reaches 8 wt%,the grain size of ?-Sn is the largest in composite solder paste as-reflowed.In the SnBi-SAC/Cu interface reaction process,due to the Sn element of SAC particles dissolve into the melten SnBi alloy which can reduce activation energy of diffusion of Cu atoms in the interfacial reaction,then speed up the interfacial reaction,lead to SnBi-SAC/Cu interface compound thickness increases with the addition of SAC particles droped.However,when the addition amount of SAC alloy particles in composite solder paste is no less than 8 wt%,the increase of solid SAC powder seriously hinders the evaporation of flux,resulting in the increase of porosity at the interface after soldering.When the contention of SAC particles in SnBi-SAC composite solder paste droped from 0 wt% to 8 wt%,the hardness of solder bulks decreased from 213.9 MPa to 117 MPa.However,when the SAC particles continued to increase,the hardness of the solder bulks increased,the contention and the size of grain of ?-Sn phase increases in the solder bulks leads to hardness decreases.however,when SAC particules adding to 15 wt%,the content of Ag and Cu elements increases in solder bulks,because the hardness of SAC particles was higher than SnBi solder,lead to hardness increased.Similarly,when SACBN particles droped from 0 wt% to 8 wt% in SnBi-SACBN composite solder paste,the solder bulks hardness decreased,but when the doped percentage rises,due to the above two points and SnBi-SACBN structure refinement.after soldering,lead the hardness of SnBi-SACBN solder bulks increases.With the addition of SAC particles,the shear fracture mode of SnBi-SAC composite solder paste is changed from typical brittle fracture to ductile fracture,when the amount of SAC particles is added to 5 wt%,the shear force of the composite solder paste increases from 6.253 N to 7.120 N,but when the SAC particles continues to add to 15 wt%,the shear force of the composite solder paste decreases because of the flux volatilize is hindered,which cause the porosity increases,lead to shear force and connection strength of SnBi-SAC composite solder paste decreases;After isothermal aging 600 h,the shear force of composite solder paste decreased obviously,the shear force of SnBi solder paste decreased from 6.230 N to 5.300 N,reducing by about 15%.The shear force of SnBi-5Sn-3.0Ag-0.5Cu(SnBi-SAC)is reduced to 5.725 N after aging 600 h,reducing by about 20%.The shearproperty of SnBi-5SAC composite solder paste is still better than SnBi solder paste.