Interface Diffusion And Size Effect Of Sn/Cu Solder Joints In Mesoscopic Scale

There is a trend of the miniaturization and multi-functionalization of electronic products which leads to the drastic downsizing of solder joints.Though the size of solder joints is reduced,the mechanical,electrical and thermodynamics load is getting heavier and heavier.The higher requirements on the reliability are put forward for the miniaturization of solder joints.The size of the solder joint has reached mesoscopic dimension which is between micro and macro in microelectronic packaging.The performance parameters,which are obtained relying on traditional methods such as mechanical,electrical,thermal and metallography,all show size dependence in mesoscopic scales,that is to say,these parameters should be constants,but they change over the size of solder joints.It brings bad influence on the engineering design and reliability prediction.Geometric scale effect is an important problem that cannot be ignored in research reliability of solder joint.A lot of researches about size effect on the mechanical properties of solder joint have been performed.Correspondingly,there is less studies about the size effect on the microstructure,the diffusion of interface elements and the formation and evolution of the intermetallic compound(IMC).The main purpose of this study is to reveal solid-liquid,solid-solid interface diffusion law which is depended on the size of micro solder joint in the mesoscopic scale during reflow and aging,and clarify the variation characteristics of the main parameters of solder joints depended on the size and then give mathematical representation.The Cu/solder/Cu sandwich structure solder joint with different thickness(10~50?m)solder layers is the main study carrier and compare with BGA solder joint with different diameters(200~500?m)solder ball.The internal relationship between the thickness of solder layer and the thickness of IMC layer,the concentration distribution of main control elements in diffusion area isanalyzed under solder composition and different processing conditions.The mathematical model about IMC layer thickness and growth rate are set up,and the size factors are introduced in the model.The mechanism of IMC growth evolution and structural transformation is clarified.The solid-liquid diffusion behavior of Cu/SAC305/Cu sandwich structure is investigated.The relationship between the growth rate of IMC and the thickness of solder layer is obtained.Results show that interface structures of all Cu/SAC305/Cu diffusion couples with different thickness of solder layers are Cu/Cu6Sn5/solder/Cu6Sn5/Cu after reflow.The thicker the solder layer thickness is,the smaller the size of IMC grain sizes are,and the thinner IMC layer is.After many times reflow,compared with smaller diffusion couple,the Cu element concentration in solder layer of the larger diffusion couple was increased slowly,and the promote effect on IMC growth was relatively weak.The effect of alloy elements on the growth of the interface IMC of diffusion couple,composition and consumption of Cu layer is investigated.The result shows that adding 0.5%Cu elements could promote the growth of the IMC layer of diffusion couple,its effect increases firstly and then decreases with the decrease of the thickness of solder layer;Adding 3.0% Ag solder element can restrain the growth of the interfacial IMC,at the same time Cu3 Sn proportion in the IMC layer significantly reduced.The thinner the solder layer is,the smaller the proportion of Cu3 Sn in IMC layer is.The correlation between the effect of Ni barrier layer and the size of BGA solder joints on the surface of Cu substrate is investigated after reflow and aging.The BGA joints of Cu/SAC305 and Cu/Ni/SAC305 with different diameter solder balls are analyzed.It was found that the existence of Ni barrier layer had reduced the thickness of the IMC layer regardless of the solder ball size.The results all showed that the smaller the diameter of the solder ball,the thicker the interfacial IMC is after reflow and aging at 160 o C for different hours.This is the same with sandwich structure of 10~50?m solder layer.Compared with Cu/Ni/SAC305/Ni/Cu diffusion couple,the Cu concentration increases in the front of Ni side in Cu/Ni/SAC305/Cu diffusion couple.Ni and Cu can dissolve each other and interchange.The saturated solubility of Cu in the solder changes because of Ni dissolving intosolder,this makes Cu concentration increase.Solid-solid diffusion behavior of Cu/Sn/Cu sandwich diffusion couple is studied under mesoscopic scales,diffusion couple with different thickness solder(10~50?m)solder layer is aged for different hours at 160?.Results show that the growth of the interfacial IMC laws did not obey the traditional parabola law,which was influenced by the size of solder layer.Getting the size factors by fitting a large amount of data and amending traditional diffusion dynamics equation.The growth kinetics model of IMC in Cu/Sn/Cu diffusion is eatablished in mesoscopic scale.The transition rule of the main control element is revealed by analyzing the evolution and growth of IMC in diffusion couple during aging.The main control element of IMC growth can transfer from Cu to Sn.The relative concentration of Cu,Sn plays a leading role.