Study On Preparation Of Sn-58Bi Solder Powders By Solution Dispersion Method And Its Brazing Performance

Sn-58Bi,as a kind of lead-free solder,is widely used in the welding of lightning protection elements,radiators and other temperature-sensitive components due to its low melting point,good wettability,excellent tensile strength and yield strength.At present,the most frequently used method to prepare solder powder is the gas atomization,but this method requires special equipment,which is difficult to control the atmosphere,besides the energy conversion rate is low,and it’s hard to produce powder with particle size less than 15μm.Therefore,solution dispersion method was adopted in this paper to prepare Sn-58Bi solder powders with core shell structure whose particle size distribution range was from 1 to 30μm and the average particle size was about 15μm.Homemade solder paste,consisting of Sn-58Bi powders and commercial paste flux,was used to compare with the commercial Sn-58Bi paste on shear strength under 9groups of different brazing process parameters.This paper has analyzed the weldability of homemade solder paste,the rule of IMC growth and the rule of the shear strength of the joints.At last,this paper studied the rule of IMC growth and the rule of shear strength after aging.The main conclusions are as follows.By means of high-speed mechanical stirring of molten metals and dispersants,bulk metals can be dispersed into small core-shell structure particles wrapped by organics and oxide.Organic solvent B plays a big role in the process of solution dispersion.The ether bond of the organic solvent B has strong affinity with the precursor of precipitation particles surface,which makes it easier for particle surface adsorption of organic solvent B.Thus a high molecular film is formed to reduce the attraction between tiny metal droplet and avoid droplets from aggregating due to the space steric effect of the film.The larger the stirring rate（the longer the stirring time）,the smaller the average particle size of the solder powder,and the narrower the particle size distribution range.Sn-58Bi powders are the finest when the stirring rate is 15000 rpm,the stirring time is 5 min,and the cooling medium is water or ice water.The oxygen content of the prepared Sn-58Bi powders is low and the melting temperature starts from139.43℃.Sn-58Bi solder powders obtained by solution dispersion have a layer of organic shell on its surface.In the process of brazing,it is necessary to apply certain pressure on the solder powders to destroy the organic shell,so that the melted solder powder metal can be connected with each other and react with the substrate to realize the combination.The joint organization is consisted of Bi rich phase,Sn rich phase and Cu₆Sn₅ under 9groups of parameters（for the brazing temperature of 160℃190℃and 220℃,the brazing time of 10 min,20min and 40min）.The thickness of IMC is from 0.87².62μm,and the shear fracture mode of the fracture is brittle fracture.The shear strength of the joint obtained by using homemade Sn-58Bi solder paste and commercial sn-58bi solder paste has advantages and disadvantages under different brazing time and temperature.The shear strength of the joints made by homemade Sn-58Bi reaches the maximum（31.25 MPa）when the brazing temperature reaches 190℃and the brazing time reaches 10 min.The IMC of the joint made by homemade Sn-58Bi solder paste,consisting of Cu₆Sn₅,is scalloped and it’s roughness becomes larger after 190℃-10min soldering and 1.3,5 days’aging,but the Bi rich phase segregate severely.The thickness of IMC grows from 1.16μm to 2.06μm after 5 days’aging.The linear relationship between IMC thickness and the square root of aging time is satisfied.The diffusion coefficient of IMC at the interface is 1.14×10^-22m²/s.After the shear strength test of joints with 1,3,5,days’aging,there are many small planes in the fracture,and there are obvious river patterns in local areas.All of the fracture forms are brittle fracture.After aging for 1 day,the shear strength of the joint rapidly decreased,and the shear strength firstly decreased and then slightly increased with the extension of aging time.After aging for 5 days,the shear strength of the joint is 25.98MPa.