Solder Material And Size Effect On Fracture Properties Of Interconnects In Electronic Packaging

The miniaturization and high density of the next generation electronic packaging make it possible to have different sizes of solder interconnects in the packaged device.Solder interconnects with differernt size may show different mechanical performance and failure characteristics,which leads to size effect of solder interconnects.With the application of high-power components in electronic packaging,the commonly used tin-silver-copper-based lead-free solder can hardly achieve the high-temperature reliability of solder interconnectes.Therefore,it is urgent to find alternative solder material with good reliability.Sintered nano-silver paste is one of the packaging materials that has the potential to be applied for high power components.In this work,the size effect of lead-free solder interconnects and sintered nano-silver solder interconnects is studied,focusing on the variation of solder interconnect fracture performance from macroscopic specimen to microstructure size.Firstly,the effect of solder layer thickness on the microstructure and tensile fracture properties of Sn3.0Ag0.5Cu solder interconnects is investigated.Microscopic analysis and tensile fracture experiments were performed on solder interconnects with different thickness from 50?m to 2mm.The growth rate of intermetallic compound(IMC)layer in the solder interconnect is found to be closely related to the thickness of the solder layer.The IMC thickness of smaller size solder interconnects is thinner than large size ones.The average IMC grain size increases as the solder interconnect size decreases.The effect of s 210? solid state and 300? molten state thermal aging on IMC growth rate are investigated.It is found that under solid thermal aging,the IMC grows faster in small size solder interconnects than that of large size interconnects.Under the molten state thermal aging,the IMC grows slower in the smaller size solder interconnect.The ultimate tensile strength of solder interconnects with different thickness of solder layer were measured.The solder interconnects exhibit a transition from ductile to brittle fracture as the solder layer decreasing.When the thickness of the solder interconnect layer is below 1mm,the H-field fracture criterion can be adopted to predict the tensile strength of SAC305 solder joints of different thicknesses.Solder interconnects with different macro sizes have various IMC dimensions,while IMC dimensions have important influences on the fracture properties of solder interconnect.The IMC size of Sn3.0Ag0.5Cu solder interconnects are controlled by solid state and molten thermal aging.In this work,the fracture toughness of solder interconnects after heat treatment was measured.Three types of fracture modes are observed,including solder fracture,intermetallic/solder interfacial fracture,and IMC fracture.Generally,the fracture toughness in the IMC fracture mode decreases with longer heat treatment time in the molten and solid state.After different time of thermal aging in the molten state,the IMC fracture mode always keeps the main failure type.The intermetallic layer morphology evolution and IMC grains after different heat treatments are analyzed.After solid state thermal aging,the IMC layer tends to change from an initial scallop-type to a lamellar-type.After heat treatment at 230?,the IMC layer tends to remain scalloped.IMC morphology and grain size are key factors affecting the fracture toughness and failure mode.Based on the size effect theory,an expression of IMC grain size effects on fracture toughness is derived.The fracture toughness of the solder interconnects after 230? molten aging is consistent with the theoretical prediction.The derived size effect model on fracture toughness of solder interconnect contains two parts:the microscopic IMC grain size and the macro dimension of the solder interconnects.When the IMC grain size keeps a constant,only the influence of the macro size of the solder interconnect on the fracture toughness needs to be considerd.The derived model is experimentally verified.It shows that the prediction of developed size effect theory matches the experimental data considering the discreteness of the fracture toughness data.The measured slope between the macro-scale of the solder interconnect and the fracture toughness in the logarithmic coordinate system is 0.38.The dispersion of solder interconnect fracture toughness data is mainly due to the holes caused by the flux residue in the solder layer.According to the porous materials fracture analysis,the relationship between internal pores and fracture toughness of welded interconnects was analyzed.The logarithmic average method was adopted to quantify the pore size.The relationship between effective section ratio and fracture strength of interconnects are defined in the logarithmic coordinate system.The critical fracture stress of the solder interconnect containing holes is derived.The fracture toughness of sintered composite nano-silver interconnects(AgNPs)are investigated.The effects of different sintering temperatures and Au/Cu substrates on the interface fracture toughness of nano-silver interconnects are considered.When the sintering temperature is raised from 200? to 250?,the interfacial fracture toughness of the nano silver solder interconnects increases.As the sintering temperature increases from 250? to 300?,the fracture strength tends to deteriorate.The interfacial microstructure and porosity of Au/AgNPs and Cu/AgNPs are observed.An Au-Aginterfacial diffusion layer is observed at the interface of the Au/AgNPs interconnect,and an oxide layer is found at the interface of the Cu/AgNP interconnect.The porosity and pore size of nano-silver interconnects are quantitatively analyzed on both microscopic and macroscopic scales.A theoretical model of porous materials is employed to derive the expression among the interfacial fracture toughness,the average pore size and porosity of nano-silver interconnects.The predictons meet with the experimental results well.Based on the expression of the fracture toughness size effect of lead-free solder interconnects,it can be inferred that the nano-silver solder interconnects also have macroscopic size effects.The fracture toughness of nano-silver interconnects at 200? and 250? were measured.In general,the fracture toughness of nano-silver interconnects is more strongly dependent on the macro-size effect than that of lead-free solder interconnects.The slope between the fracture toughness of the 200? sintered nano-silver interconnect and the macro dimension of the specimen in the logarithmic coordinate system is 1.01,while for the lead-free solder interconnect of IMC fracture,the slope is measured to be 0.38.The nano-silver solder interconnect with Cu substrate sintered at 250? has obvious oxide layer,which leads to oxide fracture mode.The fracture toughness of the interconnects decrease with the decreasing of the macro size of the specimen.