| Sn-58Bi eutectic solder has a low melting point,good wetting properties,and cheap price.Moreover,it has higher creep resistance,fatigue resistance,yield strength and shear strength than Sn-37Pb eutectic solder at room temperature.It is suitable for outer packaging in graded packaging and soldering electronic components with high temperature sensitivity.But the as-cast microstructure of Sn-58Bi alloy is coarse and the brittleness of the alloy is high,resulting in plastic processing difficulty.Therefore,the refinement of grains,reduction of brittleness,and improvement of performance are the major problems faced by Sn-58Bi eutectic solders.It is now difficult to prepare Sn-58Bi eutectic solders and preformed welds of various shapes and sizes conforming to the requirements of microelectronic devices using conventional casting and rolling methods.This project hopes to use rapid solidification technology to prepare Sn-58Bi eutectic solder to replace the conventional casting and rolling process,to achieve the industrial mass production,and to promote the lead-free process of microelectronics packaging solder and even the rapid development of the entire microelectronics industry.The experiment takes Sn-58Bi eutectic alloy as the research object,and the solderribbons are prepared by the casting-rolling method and the twin-roller rapid solidification method,respectively.With the help of optical microscope,scanning electron microscope,electron prober microanalysis,X-ray diffraction analysis differential scanning calorimetry,parallel plate bending method and MTS ceramic tensile test system and other test methods and methods,the microstructure,composition analysis,phase composition,melting characteristics,toughness and tensile strength of the Sn-58Bi solder was analyzed.A small amount of Ag element was added to further improve the microstructure and properties of the rapidly solidified Sn-58Bi eutectic alloy.The results show that Sn-58Bi solder ribbon was successfully prepared using a self-developed twin-roller rapid solidification device.Based on the heat transfer theory,the melt cooling rate during the rapid solidification was estimated to be1.28×10~4 K/s.The suitable process parameters for twin-roll rapid solidification are that double-roll gap is 0.2mm;spraying distance is 15mm;nozzle shape and size is4×Ф2mm porous round nozzle;pouring temperature is 180°C;chilling roller line speed is 7m/s.The as-cast microstructure of the Sn-58Bi eutectic alloy is composed of primaryβ-Sn phase and eutectic structure(β-Sn+Bi).After the hot rolling of the ingot,the primaryβ-Sn phase disappears,but the grain is coarsened.However,the Sn-58Bi solder prepared by rapid solidification has fine and uniform microstructure,the primaryβ-Sn phase isn’t formation,and the solid solution limit of Bi in theβ-Sn phase is increased,but the increase of the cooling rate did not cause the formation of metastable phase.The suitable annealing process of Sn-58Bi solder prepared by rapid solidification method is annealing at 130°C for 1 hour.The toughness of the Sn-58Bi solder is obviously improved after annealing.The Bi element in theβ-Sn phase diffuses outwards,and the grain size is more uniform,but slightly coarsened.Compared with the casting and rolling method,the Sn-58Bi solder prepared by the rapid solidification method has lower solidus and liquidus temperature,the solidus-liquidus temperature difference reduces by 2.6°C.The solder has better wettability on the Cu substrate,and the shear strength increased from 19.93 MPa to22.29 MPa.The fracture mode of Sn-58Bi solder joints prepared by the casting and rolling method is cleavage fracture,and the fracture mode of Sn-58Bi solder joints prepared by rapid solidification is intergranular fracture.Compared with no addition of Ag,the rapidly solidified Sn-58Bi solder with the addition of 0.5%and 1.0%Ag refines the microstructure,and the grain size becomes uniform.The addition of 1.0%Ag improves the effect is more obvious.After adding 1.5%Ag,the microstructure deteriorated.In addition,after the addition of Ag,the solder alloy produced a metastable phase of Ag6.7Sn during rapid solidification.With the increase of Ag addition,the microhardness of the rapidly solidified Sn-58Bi solder increases gradually,and the shear strength of the solder joint increases first and then decreases.The addition of Ag did not change the fracture mode of the welded joints and all belong to the intergranular fracture.However,the addition of Ag has little effect on the solidus and liquidus temperature.During the melting process of solder,Bi tends to segregate on the surface of the melt to reduce the surface tension,but the Ag6.7Sn phase hinders the segregation of Bi on the melt surface,reducing the wettability of the solder.Considering comprehensively,1.0%Ag is the most suitable addition for rapidly solidified Sn-58Bi solder. |
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