Study On Geometrical Size Of Microsolder Joints And Behavior Of Interfacial Element Diffusion

To high density microelectronic devices, the geometrical size of solderjoints were more and more tiny, from millimeters down to dozens of microns.The microstructure, interfacial IMC evolution and element diffusion behavior ofsolder joints have obviously size effect, which were great influence on the designof microsolder joint structure, the reliability analysis of electronic products andlife assessment in the range of mesoscale. In this paper, the study of Sn-0.5Cu/Cu,Sn-1.0Cu/Cu, SAC305/Cu joint interfaces，which have three compositions anddifferent sizes. The research aged(0h,96h,216h,384h and600h) at160℃indicated that the size of solder joints(diffusion distance of bulk solder and CuPad) have a significant effect on interfacial microstructure, consumption of bulksolder, the Cu layer and the diffusion behavior of the elements near the interface.The results show that: Sn-0.5Cu/Cu, Sn-1.0Cu/Cu and SAC305/Cu jointinterfaces of three different compositions and sizes have obviously geometricalsize effect on IMC growth after HTS(High Temperature Storage) aging. IMCstructure changed obviously with the different sizes of bulk solders after HTSaging. The bulk solder have been exhausted, morphology of IMC(Cu3Sn, Cu6Sn5)formed regular wedge-shape at the angle of the wedge-shape sample. With theaging time, wedge IMC area continue to expand. With the increasing diffusiondistance of bulk solder(geometrical size), interface composition changed from Cuand Cu3Sn to Cu, Cu3Sn and Cu6Sn5to Cu, Cu3Sn, Cu6Sn5and bulk solder. IMCgrowth rate is defined to be the thickness of wedge-shape compound generatedper unit time. Cu layer consumption rate is defined to be the thickness of Cuconsumption per unit time, when the bulk solder have been exhausted at theangle of the wedge-shape sample. The growth rate was obviously influenced bythe geometric size of bulk solder. With the increasing geometrical size of bulksolder, the IMC growth rate decreased in Sn-0.5Cu/Cu and SAC305/Cu interfaces,while IMC growth rate of Sn-1.0Cu/Cu increaseed first and then decreased. Three kinds of solder, Sn-0.5Cu/Cu, Sn-1.0Cu/Cu, SAC305/Cu were studied.Experimental results indicated that: the geometrical size of bulk solder have asignificant effect on consumption rate of volume solder and Cu layer. DuringHTS aging process, the consumption rate of volume solder and Cu layer decreasesmaller with bigger size.With the decreaseing geometrical size of bulk solder, the Cu concentrationnear interface increased in the Sn-0.5Cu/Cu and the Sn concentration decreasedin the SAC305/Cu after aged for600hrs at160℃.During the long-term solid-state diffusion process, Sn concentrationdecrease lower, Sn diffusion flux and IMC growth rate increase faster in thesolder near interface with the smaller geometry size, immediately the diffusioncoefficient is larger. The formula of IMC growth kinetics X=X0+(Dt)1/2is nolonger suitable for calculation of the growth of the IMC in the micro-solder joints.during aged. The IMC layer growth rate of Sn-0.5Cu micro-solder joints obtainedfrom experimental data analysis is D=2.82869–0.15582r.