Microstructure Control Of β-Sn Grains And Interfacial Reactions In Sn-based Micro Solder Joints

With the developing trends of electronic packaging technology towards miniaturization and lead-free,the size of micro solder joints downsizes to micron scale,resulting in the reduction of the number ofβ-Sn grain,which has drawn extensive attentions.The micro solder joints with limitedβ-Sn grains will exhibit significant differences in physical and mechanical properties due to the strong anisotropy ofβ-Sn with body-centered tetragonal crystal structure.This brings increasing challenges for the reliability of micro solder joints,because not only the growth of interfacial intermetallic compound（IMC）and the dissolution of substrates will be affected,but also cracks may form in micro solder joints.In fact,during a whole packaging process,Sn-based micro solder joints generally need to go through several reflows.Thus,the investigation of the grain orientation feature and evolution ofβ-Sn grains during the whole process has become a key issue in advanced packaging manufacturing.Meanwhile,since the reliability of micro solder joint is closely related to the thickness of interfacial IMC,it is very important to systematically explore the effect ofβ-Sn grain orientation on the interfacial reactions in micro solder joints under service conditions based on the diffusion anisotropy of Cu atoms inβ-Sn.In this thesis,the effect of Ag content onβ-Sn grain orientation in the micro solder joints after reflow and the heredity of highly preferredβ-Sn grains during multi-reflow process were firstly studied based on the directional solidification induced by temperature gradient（TG）.Then,the interfacial reactions in Cu/Sn-3.0Ag-0.5Cu（SAC305）/Cu micro solder joints with differentβ-Sn grain orientation during aging under TG were systematically studied,and a theoretical model for predicting IMC thickness and morphology at the cold end was proposed.The main results are summarized as follows:（1）The number and orientation ofβ-Sn grains in Cu/Sn-x Ag/Cu micro solder joints are closely related to Ag content and TG.There are manyβ-Sn grains with small size in the Cu/Sn/Cu and Cu/Sn-0.5Ag/Cu micro solder joints whatever after isothermal reflow or reflow under TG,while theβ-Sn grains in the Cu/Sn-0.5Ag/Cu solder joints present more twinning structures.In the Cu/Sn-2.0Ag/Cu solder joints,there is one or twoβ-Sn grains after isothermal reflow and oneβ-Sn grain after reflow under TG.For the Cu/Sn-3.5Ag/Cu solder joints,β-Sn grains exhibit cyclic twinning structure after isothermal reflow,while there is oneβ-Sn grain structure after reflow under TG.The Cu/Sn-5.0Ag/Cu solder joints always contain a singleβ-Sn grain after both isothermal reflow or reflow under TG.（2）Ag element can affect the solidification behavior ofβ-Sn to change grain orientation through influence the type and size of the clusters in Cu/Sn-x Ag/Cu micro solder joints.There are tetrahedral metastable short-range order（SRO）structures in Cu/Sn/Cu and Cu/Sn-0.5Ag/Cu micro solder joints.During reflow,multiple SRO structures as nuclei nucleate and grow to form multipleβ-Sn grains.While for the Cu/Sn-x Ag（x≥2.0 wt.%）/Cu micro solder joints,limitedβ-Sn grains are easily formed during isothermal reflow based on{101}type cyclic twin configurations clusters as nucleus.With the introduction of TG,β-Sn can directionally solidify into single or highly preferredβ-Sn grains.（3）The highly preferredβ-Sn grains formed by directional solidification under TG in Cu/SAC305/Cu micro solder joints present microstructure heredity during subsequent multi-reflow.During the first reflow under TG,gene clusters（{101}type cyclic twin configurations clusters with special structure）which contains heredity factor is generated and deep and narrow cavities between neighbor Cu₆Sn₅ grains at the cold end as favorable nucleation sites are formed in the solder joints.With the gene clusters and favorable nucleation sites,the initial highly preferredβ-Sn grains appear repeatedly during the subsequent multi-reflow.That is,the inheritance of highly preferredβ-Sn grains structure can be realized during a multi-reflow process.The critical reflow temperature is 255°C and the critical cooling rate is 10°C/s for maintaining the microstructure heredity.（4）β-Sn grain orientation and grain boundaries play little effect on the interfacial reactions in the Cu/SAC305/Cu micro solder joints during isothermal aging.However,during aging under TG,theβ-Sn grain orientation significantly affects the thickness and morphology of the IMC at both hot and cold ends.The results of quasi-in study show that the interfacial IMC growth is dominated by Cu₆Sn₅ phase,and a little amount of Cu₃Sn phase and no Kirkendall void formed during aging under TG.In the joints containingβ-Sn grains with small and mediumθangle（≤55.1°）,the diffusion anisotropy of Cu atoms leads to the asymmetrical growth of interfacial IMCs with faster IMC growth at the cold end accompanying with severe Cu substrate dissolution at the hot end during aging under TG.In addition,step-like Cu₆Sn₅ formed at the cold end interface in the solder joints with mediumθangle.In the largeθangle joints,there was no asymmetrical IMC growth or serious Cu substrate dissolution,showing similar phenomena to the isothermal aging cases.By analyzing the growth kinetics of the Cu₆Sn₅ at the cold end,a model for describing the growth behavior of the IMC at the cold end was established based on the TG-induced thermomigration and the diffusion anisotropy of Cu atoms inβ-Sn grain.The developed model has been verified to be viable for accurately predicting the thickness and morphology evolution of the IMC at the cold end of the micro solder joints with differentβ-Sn grain orientations during aging under TG.