Interfacial Reactions Between Sn-Ag-Cu Solder And Sputtered Cu-Ti(Cr) Alloy Film Study

With the rapid development of electronic technology,electronic products are developing in the direction of high performance and microcosmic.In flip chip technology,solder and under bump metallization(UBM)are two significant research directions.Despite its toxicity and health hazards,lead(Pb)solder has been widely used due to its low melting point and costeffectiveness.However,because lead can cause great harm to the human body,since the European Union,countries have adopted legislative means to promote lead-free electronics,and lead-free solder has become a general trend.For instance,Sn-Ag-Cu solder is preferred due to its excellent fatigue resistance and good wettability.In the flip-chip packaging,UBM is used to prevent direct contact between the solder and Al pad,which also facilitates the solder wetting and IMC forming during reflow.In current research,there are relatively few UBM designs.As an important part of determining the reliability of solder joints,the improvement and design of UBM are worthy of research and discussion.In this dissertation,Closed Field Unbalanced Magnetron Sputter Ion Plating(CFUMSIP)technology is used to deposit Cu-Cr films with different Cr contents,Cu-Ti films with different Ti contents,and Cu-Ti films with multilayer structure and nano multilayer structure.UBM and Sn-Ag-Cu(SAC305)solder were reflowed to form a solder joint structure at 240°C,and then the solder joint samples were placed in a 180°C vacuum heat treatment furnace for aging heat treatment.In order to study the feasibility of Cu-Cr and Cu-Ti films as under-bump metallization(UBM).A scanning electron microscope was used to observe the cross-sectional morphology of IMC.An energy dispersive spectrometer(EDS)and X-ray diffraction were used to quantitatively analyze the elemental composition of the interface and intermetallic compounds.In order to estimate the average thickness of IMCs,the image analysis software was used to calculate the area of IMC.The shear strength test of the soldered joints were completed by the multifunction bond tester.The investigative results indicated that:(1)The content of titanium and chromium in the Cu-Ti and Cu-Cr UBM films increases with the increase of the Ti and Cr target currents.The cross-sectional morphology of the film is mainly composed of columnar crystals,and the thickness increases with the increase of the target current,but the growth rate becomes slower.The surface morphology of the film is dense,the grain boundary is obvious,no shedding and large-area pores are observed;the average grain size increases with the increase of the Ti and Cr target current.(2)Scallop-shaped Cu6Sn5 intermetallic compound is formed at the interface of SAC/Cu solder joint reflow,SAC/Cu-Ti and SAC/Cu-Cr solder joints form needle-shaped Cu6Sn5 intermetallic compound.The scallop-like IMC growth is mainly controlled by the IMC ripening rate and interface reaction rate in SAC/Cu samples,and the needle-like morphology of IMC at the interface can be ascribed to the influence of interface reaction rate on IMC growth.(3)The temperature provides the driving force for the diffusion of atoms during thermal aging,so the morphology of the intermetallic compounds of the samples becomes lamellar,and the thickness increases with the increase of aging time.The intermetallic compound diffusion coefficient of each sample is obtained by calculation.The coefficients of SAC/Cu-Ti and SAC/Cu-Cr solder joints are all smaller than SAC/Cu solder joints.This is because Ti and Cr will segregate in the lower layer of IMC after aging to form a Ti-rich and Cr-rich layer.It acts as a diffusion barrier to inhibit the formation of intermetallic compounds and avoid the continuous formation of brittle phases and Kirkendall voids.(4)In the solder joint reliability analysis test,the fracture mode of the reflowed SAC/CuCr and SAC/Cu-Ti solder joint samples is ductile fracture,the strength of the solder joint after aging heat treatment is reduced,and the fracture mode is ductile-brittle Composite mode is the main.The fracture mode of the reflowed SAC/Cu solder joint sample is ductile-brittle composite mode;the fracture mode of the solder joint after aging is brittle fracture.The intermetallic compounds of SAC/Cu-Cr and SAC/Cu-Ti solder joint samples are thinner and there is no formation of defects such as Kirkendall voids.Therefore,SAC/Cu-Cr and SAC/Cu-Ti solder joints have higher shear strength before and after aging heat treatment than SAC/Cu solder joints.The solder joints formed when the Cu-Cr and Cu-Ti films are used as the under-bump metallization layer have the effect of enhancing the reliability of the soldering.