| In recent years,with China’s remarkable progress in lunar landings,military science and technology,and new 5G communication technologies,the growing trend of miniaturization and integration for high-density and high-power device manufacturing has put higher demands on advanced packaging and interconnecting technology and solder joint reliability.Especially in high-power electronic components,lead-free solder joints are prone to high current densities(1×10 4-2.5×10 4A/cm2)under harsh operating conditions,accompanied by a lot of joule heat to form large temperature gradients(1000-3000K/cm),resulting in phase segregation,Kirkendall Void,cracks and other defects within the interconnect structure of microelectronic packages.Its corresponding size effects and microstructural evolution can easily lead to short-circuit failures in solder joints,which has become a prominent issue affecting the reliability of lead-free solder joints with microelectronic joining technology.So far,although some progress has been made in the theoretical study of electromigration,too little is known about the nature of electromigration and its attendant phenomena in lead-free solders to distinguish between thermomigration and its coupling effect under electromigration of solder joints.Therefore,it is in urgent need to study the fundamental issues and key technologies of electrothermal migration related to leadfree solder joints with microelectronic joining technology,and to explore the factors,mechanisms,and regulation approaches that affect electromigration and its coupling effects.This paper implemented relevant research on the use of nickel-reduced graphene oxide(Ni-GNSs)to enhance Sn2.5Ag0.7Cu0.1RE/Cu solder joints.A test platform for electrothermal migration experiments was designed and fabricated to investigate largescale microelectronic interconnection solder joints.The study explored the phenomena of polarity under different current densities,temperature gradients,material factors,and loading processes,as well as the crystallographic characteristics,growth kinetics of intermetallic compound(IMC)at the interfaces,and the electrical and mechanical properties.Additionally,an electrothermal migration diffusion reaction theoretical model was proposed.By studying the correlation between electromigration,thermomigration,and their interactions during the decoupling process,approaches and methods for regulating the electromigration and thermomigration of lead-free solder joints,as well as their coupling effects were proposed.The main conclusions drawn from the research are as follows:1.Following the principle of "thermal" equivalence,an innovative research approach called "amplification,rapidity,and thermal simulation" for studying electromigration and thermomigration under harsh packaging conditions of lead-free solder joints in microelectronic packaging was proposed.A platform for studying electromigration and thermomigration in lead-free solder joints for microelectronic packaging was designed and fabricated.This platform revealed the coupled phenomena of thermomigration induced by electromigration in micro-solder joints.It effectively solved the technical challenges of observing and analyzing electromigration and thermomigration phenomena in high-density microelectronic devices under harsh packaging conditions.2.Criteria for the occurrence of electrothermal migration and its coupling effects were proposed for Sn2.5Ag0.7Cu0.1RExNi-GNSs/Cu solder joints.Ideal electromigration occurred in solder joints under the condition of current density≥7×103A/cm2 in an oil bath environment,with the angle θ between the c-axis of β-Sn grains and the direction of electron flow being ≤53.2°.Constant-temperature thermomigration occurred in solder joints under the condition of temperature gradient≥1000℃/cm,with the angle θ between the c-axis of β-Sn grains and the temperature gradient being ≤43.5°.Electrothermal migration coupling occurred in solder joints under the condition of current density≥7×10 3A/cm2 in ambient atmosphere,with 0 being<32.1°.The misorientation angles of β-Sn crystals were consistently greater than the conditions required for electromigration and thermomigration.Polycrystallineβ-Sn solder joints exhibited larger crystal misorientation angles than single crystal βSn solder joints,suppressing the occurrence of polarity phenomena.3.The growth relationship between β-Sn grains near the solder joint and Cu6Sn5 grains at the interface under the effects of electrothermal migration and coupling effect in Sn2.5Ag0.7Cu0.1 RExNi-GNSs/Cu solder joints was clarified.As the current density increased,under the effects of electrothermal migration and coupling effect,the β-Sn grains in the solder joint exhibited growth towards the<001>axis(c-axis)direction of the {111} crystal plane,leading to an accelerated diffusion rate.Simultaneously,the Cu6Sn5 grains at the electromigration interface underwent recrystallization and grain growth,gradually growing in the<1120>direction of the{1010} crystal plane.However,due to the combined effects of thermomigration and coupling,they also deviated towards the<1010>direction of the {1120} crystal plane,which resulted in stratification and twisting deformation between the β-Sn grains and Cu6Sn5 grains,making them prone to intergranular cracking.4.The corresponding relationships between the four stages of polarity phenomena during electromigration and its coupled effect in Sn2.5Ag0.7Cu0.1RE0.05NiGNSs/Cu solder joints were proposed,and a diffusion reaction model from cathode to anode was established.The process of electromigration and its coupled effect in solder joints could be divided into four stages:nucleation stage of cathodic cracks,propagation stage of microcracks,steady stage of macroscopic crack propagation,and failure stage of fracture,corresponding to the "formation","extension","stability",and ultimate "failure" of anodic IMCs.At the anode interface of the solder joint,the primary phase was Cu6Sn5 in the IMC layer,which gradually thickened and formed Cu3Sn phase.Microcracks propagated and gradually developed into macroscopic cracks.At the hot end interface,the Cu6Sn5 phase of the IMC layer gradually dissolved,while the growth of the Cu3Sn phase accompanied the formation of "Kirkendall voids"and gradually formed cracks in the vicinity of the interface and solder joint,ultimately leading to the formation of a "full IMC solder joint".The growth rate of IMC at the cold end after the propagation stage was significantly faster under thermomigration than under electromigration at the anode.In the stable stage of electromigration and thermomigration coupling,the anodic interface IMC layer continued to thicken,accompanied by microcrack propagation,exhibiting phenomena different from pure electromigration and thermomigration.The 0 gradually decreased in the four stages of the solder joint,accompanied by an increase in the flux of Cu atoms and the reaction flux at the interface,resulting in an overall increase in substances migration towards the anode.5.The IMC growth mechanisms were revealed on the interfacial Cu6Sn5 and Cu3Sn under the electrothermal migration and coupling effect of Sn2.5Ag0.7Cu0.1RExNi-GNSs/Cu solder joints.As the current density or temperature gradient of the solder joint increases,the interfacial Cu6Sn5 IMC gradually changes from volume diffusion-controlled growth to co-controlled growth with surface reactions,while the Cu3Sn IMC changes from grain boundary-controlled growth to volume diffusion-controlled growth,with the growth of interfacial Cu6Sn5 IMC at the solder joint taking precedence over Cu3Sn IMC,near the substrate interface,Cu3Sn was formed due to Cu atom supersaturation,while near the solder joint interface,Cu3Sn was formed due to the transformation of Cu6Sn5.The addition of 0.05 wt.%NiGNSs resulted in grain refinement and improved growth activation for Cu6Sn5 and Cu3Sn IMC by 14.7%and 11.2%,respectively.The growth of the Cu3Sn IMC is correlation at different average temperatures and temperature gradients of the solder joint.When the temperature gradient becomes smaller,require higher temperature and accumulation of load time.6.The crack propagation pathways and fracture mechanisms were elucidated under the electrothermal migration and coupling effect of Sn2.5Ag0.7Cu0.1RE0.05NiGNSs/Cu solder joints.The shear fracture path of the thermomigration moves from the cathode interface IMC/brazing seam junction position to the Cu6Sn5/Cu3Sn interface,and the shear fracture mechanism changes from ductile transcrystalline fracture dominated by the dimple of β-Sn to intergranular brittle fracture dominated by the cleavage of the interface IMC and slip bands;The shear fracture path of the electromigration and coupling effect moves from the cathode interface IMC/brazing seam junction position to the Cu3Sn interface,and the shear fracture mechanism changes from ductile transcrystalline fracture dominated by the dimple of β-Sn to intergranular brittle fracture dominated by the cleavage of the interface IMC and glide steps.The solder joint experienced failure due to electromigration and its coupled effect at distinct time intervals:at 250 hours,between 250 to 350 hours,and between 230 to 280 hours respectively.During the loading process of the solder joint,microcracks gradually extended from the brazing seam towards the interface between the brazing seam and the cathodic IMC.Subsequently,they propagated gradually from the end of the cathodic IMC interface towards the root.7.A quantitative analysis method for decoupling electromigration and thermo migration in Sn2.5Ag0.7Cu0.1RExNi-GNSs/Cu solder joints was proposed,elucidating the composition and correlation of electromigration,thermomigration,and their interaction.In the "induction period" of the solder joint under electrothermal migration decoupling,the effect proportion of electromigration,thermomigration,and their interaction is all about 33%,with a relation of mutual promotion;in the"expansion period",the effect of electromigration is dominated,with the proportion of 67.5-76.4%and a relation of mutual promotion;in the "stabilization period",the effect of thermomigration is dominant,with the proportion of 63.3-72.6%and a relation of mutual inhibition;in the“failure period”,the effect of all three is dominant,with the larger proportion of thermomigration being 38.2-45.7%and a relation of mutual promotion.The effect proportion of thermomigration and electrothermal migration interaction can be effectively inhibited by the addition of the reinforced Ni-GNSs phase under,achieving the clearest effect with an additive amount of 0.05 wt%.The research results provided a basis for the regulation of electrothermal migration and coupling effect. |
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