Study Of Geometrical Size Effect On Interfacial Diffusion Kinetics Of Cu/SAC305/Cu Micro Soldered Joints

High performance and continued miniaturization of electronic products aredriving the size of solder joints (BGA joints or Flip-chip joints) shrinks frommillimeters to tens of microns or even a few microns according to the Moore’slaw. At the micro-scale, mechanical parameters and microstructure characteristicsof solder joints exhibit obviously a so-called “size effect”, which have influenceon the structure design, reliability analysis and life assessment of micro-solderjoints. In this paper, IMC layer growth kinetics, metal layer consumption trendand concentration distribution of interfacial diffusion element during aging (t=0h,96h,216h,384h,600h) at different temperatures (120℃,140℃,160℃) inCu/Sn-3.0Ag-0.5Cu(SAC305)/Cu sandwich-structure diffusion couples ofdifferent solder layer (diffusion layer) thickness (δ=15~50μm) obtained byreflow soldering was investigated. The influence of diffusion layer thickness onthe diffusion behavior of interfacial elements was analyzed as well.The results indicated that diffusion behavior of interfacial elements isgreatly dependent on diffusion layer thickness. The scalloped Cu6Sn5layer wasformed in the interface of Cu layer/solder layer after reflow soldering, and thethickness and ratio of interfacial Cu6Sn5layer appeared a decrease with theincrease of diffusion layer thickness after reflow. The Cu6Sn5was in the form oflayer-like and a layer of Cu3Sn has formed between the Cu layer and Cu6Sn5layer after aging at160℃.Diffusion layer thickness significantly affected the evolution of IMC layersduring aging. The thinner the diffusion layer was, the faster the Cu3Sn layer grew,which lead to a thickness ratio decrease of Cu6Sn5/Cu3Sn after aging. And thethickness ratio of Cu6Sn5/Cu3Sn exhibited a decrease firstly then presented aslightly increased trend with the increase of aging time. The growth rate of IMC layer during aging was significantly influenced by diffusion layer thickness. Thethinner the diffusion layer was, the thinner the Cu6Sn5layer and Cu3Sn layergrew after aging. The growth rate of IMC layer (Cu6Sn5layer+Cu3Sn layer)decreased with the decrease of diffusion layer thickness as well. Moreover, thesetrends presented more obviously at higher temperatures.After reflow, the consumed thickness of Cu layer have nearly differenceamong different diffusion layer thickness, whereas after aging the consumed Culayer thickness varied remarkably upon diffusion layer thickness and exhibited indirect proportion to the diffusion layer thickness. Furthermore, it indicated thatthe longer the aging time was, the more significant the solder size effect on theconsumption of Cu layer appeared. The faster consumption rate of Cu layertended to appear in greater thickness of diffusion layer. It could be consideredthat concentration gradient of Cu and Sn played a role in diffusion velocity of Cuand Sn at the micro-scale, which eventually resulted in the difference of IMClayer growth, Cu layer consumption during aging with the different diffusionlayer thickness.There was a trend that Sn concentration increased while Cu concentrationdecreased in diffusion layer with the increase of diffusion layer thickness afterreflow and aging. Since solder joints shrink to the micro-scale, the diffusion ofSn and Cu will lead to a significant variation of Sn and Cu concentration indiffusion layer because of the finite amount of solder, which further determinedthe diffusion coefficient. The hardness and creep performance of diffusion layerafter interfacial reaction presented a downward trend with increasing thediffusion layer thickness. Elements diffusion could cause the composition andorganization change of diffusion layer, the solder size effect on micro-mechanicalparameters further confirmed the “geometrical size effect” on interfacialelements diffusion of micro-joints.Experimental results show that not only aging temperature and soldermaterial influence the diffusion coefficient D, but also the thickness of diffusionlayer was a key factor on interfacial diffusivity at the micro-scale. The growthactivation energy Q of IMC layer was also correlated with the geometric size ofsolder joints. Under conditions of this experiment, further correlated experimentdata indicated the variational regularity between diffusion coefficient D and diffusion layer thickness δ was approximately accordance with paraboliccorrelation:D=4.76×105δ2+1.93×105δ+0.023. The growth activation energyof IMC layer for different diffusion layer thickness (δ=15μm,30μm,50μm)were104.76KJ/mol,114.40KJ/mol,136.59KJ/mol respectively.