Research On The Micro-scale Mechanical Properties Of Lead-free Solder Joint In Electronic Packaging

Since the lead-free development of electronic packaging,lead-free soldershave been the major interconnected materials.With the miniaturization and multi-functionalization of electronic products,the size of interconnected materials in electronic packaging is becoming smaller?micron level?,the reliability of the solder joints in electrical and mechanical connection of faces a great challenge.Therefore,developing the measurement method with high resolution in small scale and characterizing the mechanical properties of IMC from the mesoscopic view have an important operation significance in establishing the mechanical properties database and industry design.Intermetallic compound?IMC?,which is formed between the solder and the substrate metal,have considerable effect on the reliability of the solder joint.In this dissertation,nanoindentation tests were carried out on SAC305 solder and intermetallic compound?Cu₆Sn₅ and Cu₃Sn?,the effect of indentation strain rate and creep of the solder joint were investigated,then the constitutive relation of materials under different temperature were obtained by using the representative strain method.The related research contents and conclusions are as follows:1.The preparation process of the sample is introduced,the interfacial compounds were detected by scanning electron microscope?SEM?and its morphology was obtained.Results of the line scan using energy spectrometer?EDS?show that the layer material behaves a step change rather than a gradual transition,Cu3 Sn and Cu₆Sn₅ compound were then identified by point scan.2.The mechanical properties of Sn-3.0Ag-0.5Cu?SAC305?lead-free solder and solder joint interconnect interface material of the solder and copper pad after welding were investigated by nanoindentation,the mechanical properties of SAC305 solder interface and two kinds of intermetallic compounds were measured at different strain rate target(0.01s^-1,0.05s^-1,0.25s^-1and 0.50s^-1),the effect of indentation strain rate on hardness,elastic modulus and yield strength were analysed.Through the comparison of elastic modulus and hardness of the four layer generated by reflowing,results show that the hardness and elastic modulus of the solder joint interface layer are higher than those of the SAC305 solder.Given the clear distinction on material hardness between IMCs and Cu pad,the real existence of intermetallic compound was further verified.The hardness of SAC305,Cu₆Sn₅ and Cu3 Sn increased with the increasing indentation strain rate,indicating the strain rate hardening effect.Hard and brittle mechanical properties of interfacial compound is evidently different from that of lead-free solder,solder joint interconnected interface layer must be key part of the solder joint failure.3.The temperature has great influence on the mechanical properties of SAC305 solder and interfacial compound.With increasing temperature,the indentation depth and creep displacement of the two materials increase,but the hardness and modulus decrease.4.Nanoindentation tests were carried out by using Berkovich tip and Cube corner tip.Through model analysis and data processing,two groups of representative stress and representative strain were identified,further the plastic parameters of the material were identified by the representative strain method and the power-law constitutive relation function of the two materials were obtained at different temperature.The variation law of elastic modulus,strength coefficient and strain hardening exponent with temperature were analyzed,the relationship between them and the absolute temperature were fitted by a linear equation,then the power-law constitutive relation functions in the temperature range?25?-150??were established for SAC305 solder and IMC.5.Based on the power-law constitutive relation and constitutive relationship model of Johnson-cook,the five coefficients related to the strain,strain rate and temperature were obtained,constitutive relation of Johnson-cook expression for two kinds of materials were presented.6.The creep behavior of SAC305 solder and interfacial compound in the temperature range of 25?-150? was studied,and the parameters such as creep strain rate sensitivity index,creep activation energy and so on were given.The creep rate sensitivity index increased from 0.106 to 0.258 with the increase of temperature for SAC305 solder,the creep activation energy of SAC305 solder is55.3KJ/mol;the creep rate sensitivity index increased from 0.064 to 0.154 with the increasing temperature for the interfacial compound.The creep activation energy of the interfacial compound is 37.08 KJ/mol,which is close to the self diffusion activation energy of ?-Sn.Therefore,the Sn element in the solder will continue to diffuse to the Cu pad and the interfacial compound layer is continuously thickened during service.The creep mechanism of interfacial compound layer is dominated by dislocation climb in the temperature range.