Interface Diffusion And Electromigration Regularity Of Cu/SnAgCu/Cu Micro Solder Joints Under Electro-thermal Coupling

With the development of microelectronics products development follow Moore’s law, micro solder joints bearing electrical and thermal load was getting bigger in electronic components. The problem of solder joint failure had become the bottleneck of the microelectronics industry development under electrothermal coupling aging. In this paper, interface diffusion and electromigration regularity of Cu/Sn Ag Cu/Cu micro solder joints were investigated under electrothermal coupling aging.The regular of element diffusion and interface IMC growth were investigated under electro-thermal coupling. The influence of temperature, current density, interfacial IMC grain size on the properties of solder joint electromigration resistance were analysised. The differences and relations between solid-liquid diffusion and solid-solid diffusion were obtained. Experimental results indicated that the growth of IMC showed parabola dependence on time under thermal-electrical coupling at the anode, and the cathode IMC thickness increased at first and then decreased. Meanwhile, the morphology of IMC changed obviously at the cathode. The diffusion of the elements at the soldering interface could be divided into two stages. At the initial stage, the high element concentration gradient in the soldering interface played a leading role to the element diffusion. The IMC showed similar growth trend at the anode and cathode. At the second stage, the concentration gradient in the soldering interface decreased. The electron-wind force played a crucial role to the element diffusion. Accordingly, the thickness of IMC decreased at the cathode but increased at the anode. The solder joints electromigration failure process will be accelerated if the solder joint temperature or current density pass solder joints increase. And the microstructure of interface IMC was closely related to the solder joint temperature and current density.The effect of solder joint geometrical sizes(solder layer thickness, solder joint volume, solder joint height) on thermal aging and electro-thermal coupling aging were investigated. During electro-thermal coupling condition, the electron wind force provides the dominating driving force for diffusion, and diffusion coefficient and diffusion flux of thin δ are significantly larger than the thick ones. The Cu concentration of the area near interface is relatively high for a thin solder layer in this test. Under the same experimental temperature, the effects of δ on the electro-thermal coupling aging are more obvious than thermal aging. During electro-thermal coupling aging, the smaller solder ball diameter was(the diameter ratio of solder ball and solder pad was in range of 1.17~1.50), the stronger electromigration resistance was, when the solder joints had the same size solder pad. And the higher solder joint is, the worse electromigration resistance was, when the solder joints had the same volum solder ball.The constitutive equations of cathode Cu pad consumption and anode interface IMC growth were established, respectively. During thermal aging, the cathode Cu pad consumption(δ) increased linearly with the loading time. The consumption rate showed parabolic curve relationships with sample temperature and showed linear relationships with current density. The anode interface IMC thickness(δ1) increased linearly with the square root of loading time. The interface IMC growth coefficient showed parabolic curve relationships with sample temperature and showed showed linear relationships with current density.The influences of trace elements on electro-thermal coupling aging resistance were analysised. During reflow soldering, the addition of Ag content in solder promoted the Cu6Sn5 grain nucleation and growth. The Bi element didn’t participate in the interface reaction. The addition of Ni in solder the IMC from Cu6Sn5 transfrom(Cu, Ni)6Sn5, and the IMC grain was refined but the microstructures change was not obvious. After thermal aging, the grain was columnar after the addition of Ag content in solder. The addition of Ni in solder, the IMC grain shows two kinds of microstructure of slender cylindrical and vermicular, and the IMC grain was refined. During electro-thermal coupling aging, the addition of Ag, Bi, Ni in solder increase the nucleation rate of the cathode interface IMC, the IMC grain was refined, meanwhile, improved electrothermal coupling aging resistance of solder joints.