Chinese Journal of Rare Metals

With the increasing awareness of environmental protection,Sn-0.7Cu eutectic solder is regarded as the best substitute for traditional Sn-Pb solder in the field of wave soldering and flip chip because of its low cost,easy processing,easy recovery and good comprehensive performance.It has broad development prospects.However,the utilization of Sn-0.7Cu eutectic solder is severely re?stricted by its high melting point (227℃)，poor wettability and poor mechanical properties.The addition of trace elements Si can re?duce the surface tension of the melt alloy,improve the wettability,refine the alloy structure and improve the solder properties.At pres?ent,there are few reports on the modification of Sn-0.7Cu solder by doping Si elements.Therefore,Sn-0.7Cu-x Si solder alloy was tak?en as the research object.Sn-0.7Cu solder alloy was doped with Si,and the effects of trace Si on the microstructure,melting character?istics,interface morphology and mechanical properties of Sn-0.7Cu solder were studied,so as to provide reference for obtaining Sn-0.7Cu solder with excellent mechanical properties.Sn-0.7Cu-x Si (x=0,0.1,0.3,0.5,0.75,1.0;%,mass fraction) solder alloys were obtained by induction melting.The microstructure,Sn-0.7Cu-x Si solder/Cu interfacial morphology and micro-area composition of the solder alloys were analyzed by scanning electron microscope (SEM) and energy dispersive spectrometer (EDS).The melting char?acteristics of solders were characterized by STA449F3 comprehensive thermal analyzer.The wettability of solders on copper plate were tested in resistance furnace.The microhardness and the shear strength of the joint were tested with a microhardness tester and a univer?sal tensile tester.The results showed that trace Si element had no significant effect on the melting temperature of Sn-0.7Cu solder alloy and the undercooling of the solder alloy was significantly reduced with the addition of Si element.When the addition of Si was 0.75%,the undercooling was 46.5%lower than that of Sn-0.7Cu solder alloy.Si element was dispersed in the molten solder matrix as heteroge?neous nucleation particles,and the interface between the crystal nucleus and Si particles with low energy replaced the interface be?tween the crystal nucleus and liquid with high energy.The surface energy in the nucleation process was reduced and nucleation was promoted,so the undercooling gradually decreased.The wettability of the solder alloys could be improved by adding Si element.The wettability decreased first and then increased in the range of 0～0.5%Si,and the wettability area decreased continuously in the range of0.5%～1.0%Si.When the addition of Si element was 0.5%,the maximum wetting area of the solder was 93.71 mm²,which was 14.1%higher than that of Sn-0.7Cu solder.Si could improve the oxidation resistance of solder alloy.The surface tension of the solution alloy was reduced by the surface active element Si during the wetting process.It concentrated on the surface of liquid solder and reacted with oxygen atoms to form a protective film,which hindered the further oxidation of the internal solder.However,when the element Si was added excessively,there was too much Si enriched on the surface of the liquid solder,and the oxide layer formed with oxygen atoms became thicker and thicker,which hindered the spreading and wetting of the liquid solder.Therefore,the wetting performance of the solder deteriorated and the wetting area gradually decreased after excessive addition.In addition,the β-Sn phase in the matrix of Sn-0.7Cu-x Si solder alloy was obviously refined,and the growth rate of the compound at the interface was inhibited with the addition of Si.When Si was not added,the microstructure of Sn-0.7Cu solder alloy was mainly composed of gray white β-Sn dendrite,gray black la?mellar Sn-Cu eutectic and dark black spot or rod-shaped Cu_6Sn₅ intermetallic compound.With the addition of 0.1%Si element,the coarse dendrites of β-Sn were broken,the lamellar eutectics of Sn-Cu were increased and the grains were obviously refined.With the continuous increase of Si element,the dendrite of β-Sn increased and the structure became coarser gradually.The particles of Cu_6Sn_5compound began to change from small dot to thin strip and excessive Si elements began to aggregate.The thickness of the interface compound layer decreased first and then increased with the addition of Si.When the content of Si element was 0.1%,the thickness of interfacial intermetallic compounds (IMC) layer at the brazing interface was at least 1.15μm.At this time,the compound at the inter?face changed from continuous to intermittent.Some coarse scallop-shaped Cu_6Sn₅ compounds were inserted into the solder area from the copper matrix to play a pinning role,which was beneficial to the improvement of mechanical properties.With the increase of Si con?tent,the thickness of IMC layer at the interface continued to increase.The mechanical properties of brazed joints deteriorated signifi?cantly.The microhardness and shear strength of Sn-0.7Cu-x Si solder alloys first increased and then decreased with the increase of Si content.At 0.1%Si,the maximum microhardness and shear strength reached HV_0.025 11.38 and 37 MPa,respectively,which were9.95%and 13.28%higher than Sn-0.7Cu solder alloy,respectively.With the addition of 0.1%Si,the microstructure of the solder was fine and uniform,the thickness of the compound layer at the interface was significantly reduced,and the mechanical properties were the best.The mechanical properties of the solder began to deteriorate when the content of Si element was continuously increased.Con?sidering comprehensively,the mechanical properties were better when the content of Si element was 0.1%.