Modeling Of The Coalescence Of Micro-voids In The Intermediate Phase At The Interface Between Solder-joints And Interconnects

Due to the tendency of miniaturization of electronic products, the size of solder-joints becomes smaller and smaller. Thus, its realibility has become a major concern. The realibility of solder-joints is related to many factors. Among them, an important one is the growth and coalescence of micro- voids at interfaces of intermediate phases between solder balls and interconnects. Thus it is important to investigate the envolution and morphology of those voids to improve the realibility of solder joints and finally increase the service life time of electronic products.The evolut ion of voids depends on the interfacial energy, electric field and elastic stress fields and so on. Previous work mainly focused on the influence of interfacial energy, while the influences of electric field or elastic stress fields are not considered. To enable the evolut ion of voids to be simulated more accurately, in this thesis, we take the effect of the electric field and elastic stress fie lds into account. By using a diffuse interface model, simulation results show that:(1) When only the effect of interfacial energy is considered, i.e., no electric currents or elastic stresses are applied, the result shows that under the influence of interfacial energy, the voids coalesce and the rate of void coalescence decreases with time. Because, in the initial stage, the total interfacial energy is relatively large, which results in a large rate of void coalescence. As time goes on, with the continuous coalescence of voids, the total interfacial energy becomes smaller, so the rate of void coalescence slows down.(2) When both interfacial energy and tensile stresses are taken into account, the voids coalesce with a larger rate compared with the results in case(1). Furthermore, in the init ial stage of coalescence of the voids, the interfacial energy plays a dominant role. As time goes on, the influence of elastic stresses gradually becomes dominant, which not only leads to a greater rate of void coalescence, but also enables the voids to envolve into an elliptic shape with its long axis lying in the direction perpendicular to that of tensile stresses.(3) When shear stresses are applied, the morphology and evolut ion of voids are similar to those in case(2) in the init ial stage. While as time goes on, the morphology of voids will becomes greatly different. The morphology of voids becomes rectangular with four corners developed in the directions where there are maximum stresses, which is in agreement with the analysis of the principal stresses according to the theory of elasticity.(4) When both electric currents and tensile stresses are applied in our model, the voids not only coalesce with a higher rate, but also migrate along the direction of electric currents,and finally evolve into an el iptic shape.(5) When both electric currents and shear stresses are applied, the voids will migrate along the direction of currents. In addition, the morphology of voids will be similar to that in case(3) with a rectangular shape.