Design And Preparation Of Sn58Bi-xCu Solder And The Mechanism Of Influence On The Performance Of Micro Solder Joints

With the development of lead-free solders in electronic packaging area,low-temperature solders have been widely investigated and applied in the packaging of temperature-sensitive components and step soldering due to their low melting temperature and favorable mechanical properties.Light-emitting diode?LED?is a typical step soldering package,so low temperature solders are widely used in LED packaging.Due to the low efficiency of electro-optical conversion of LED products,most of the electrical energy is converted into heat.Therefore,it is of great significance to improve the thermal conductivity and mechanical properties of the solder layer in order to enhance the reliability of the LED package.Among which,Sn58Bi eutectic solder alloy has been widely used in step soldering.Because this solder alloy has a low melting point?139??,low cost,good solderability,and high creep resistance.However,there are a large number of Bi-rich phases in the microstructure,which result in the high brittleness of the Sn58Bi solder.Besides,the thermal conductivity of Sn58Bi solder is quite low,which can't meet the heat dissipation requirements of high power LEDs.Therefore,the micro copper particles and copper foam added Sn58Bi solder joints were designed and prepared in this thesis.The purpose is to modify the performance of the solderability,thermal resistance of the package,micro indentation properties and the resistance to thermal aging of Sn58Bi solder.Meanwhile,the corresponding mechanisms were also analyzed.The effects of micron copper particles addition on the solderability of the composite solder paste were studied.When the mass fraction?wt%?of the micro copper particles increased from 0wt%to 5wt%,the spreading rate of the composite solder paste on the copper substrate was slightly reduced.However,as the mass fraction of micro copper particles exceeded 5wt%,the spreading rate decreased obviously.The composite solder joint has a comparable shear strength with the Sn58Bi solder joint when the mass fraction of micro copper particles is lower than 5wt%.The shear strength of composite solder joint decreased gradually as the mass fraction of micro copper particels increased from 5wt%to20wt%.The model of pores formation in composite solder joint was established.For solder joints with more than 5wt%micro copper particles addition,the evaporation of organics was suppressed by excessively formed intermetallic compound?IMC?This phenomenon led to the formation of pores inside solder joint,which finally resulted in the decrease of shear strength.Besides,the coarse Bi rich and Sn rich phases are refined after adding 5wt%micro copper particles,but no further refinement was found with more than 5wt%addition.The effects of micro copper particles addition?0wt%?20wt%?on the thermal conductivity of composite solder paste were investigated through the thermal performance measurement of LED package.The thermal conductivity of 5wt%micro copper particles added Sn58Bi solder layer was greatly improved,which increased from 18.89 W?m^-1?K^-1to 26.60 W?m^-1?K^-1.The finite element analysis of the 3D model of LED package was conducted to obtain the effects of the thermal conductivity of the solder layer on the junction temperature of the LED chip.The simulation resulted was also verified by the infrared thermal imaging method.The simulation results matched well with the measured value.The junction temperature of the LED chip can be effectively decreased after adding5wt%micro copper particles.In addition,after analyzing the distribution of micro copper particles in the solder layer,the heat transfer model of micro copper particles was established.The corresponding heat transfer mechanisms and the effects of the mass fraction of micro copper particles on heat transfer were analyzed.The added micro copper particles acted as the highly conductive cores in the heat transfer route and fastened the heat transfer rate of solder layer.The evolution of the microstructure and interfacial IMC during the thermal aging process were studied through the thermal aging test.The addition of micro copper particles depressed the coarsening of Sn58Bi bulk solder microstructure.The interfacical IMC growth rate of the Sn58Bi/Cu micro solder joints is 0.035?m/h^1/2.However,the interfacical IMC growth rate of 5wt%and 10wt%micro copper particles added micro solder joints were 0.024,0.027?m/h^1/2,respectively.This result indicated that the growth of interfacial IMC is inhibited during thermal aging.With the increase of thermal aging time,the shear strength of the three micro solder joints showed a decrease trend,which was attributed to the coarsening of bulk microstructure and the growth of interfacial IMC.Besides,the creep rate of 5wt%micro copper particles added solder joint was lower than Sn58Bi before and after thermal aging.The coarsening of Sn58Bi bulk solder microstructure was effectively suppressed with the addition of 5wt%micro copper particles,thus the creep resistance of Sn58Bi solder joint was significantly improved.In order to solve the problem that excessive addition of mciro copper particles will cause the formation of pores,the copper foam was selected to improve the performance of Sn58Bi solder.The bulk solder microstructure of the solder joint was refined after adding copper foam into Sn58Bi solder.The creep resistance of the copper foam added solder joints was notably improved when comparing with the Sn58Bi joints.This was due to the refinement of bulk solder microstructure after adding copper foam.Meanwhile,the composite solder joint showed excellent plastic deformation performance during the shear test,and no brittle fracture occurred after the shear test.The copper foam consists of a large number of ligaments in its structure,large plastic deformation capability can be achieved without brittle fracture during shear testing,thus the plastic deformation performance of Sn58Bi was improved.Limited amount of copper foam was consumed in the bulk solder after thermal aging at 100?for 672 h.The copper foam can exist stably in the bulk solder.In addition,the refining effect of copper foam on the Sn58Bi bulk solder microstructure was more obvious when comparing with the micro copper particles added ones.The consumption of coper foam was much slower than the micro copper particles during thermal aging process,which implied that the stability of copper foam was much better than micro copper particles.