Investigation On The Relationship Between The Microstructure Evolution Of The Intermetallic Compounds In Solder Joints And The Micro-and Macro-mechanical Behavior

Solder joints serve as electrical connections and mechanical supports inelectronic packages. The failure of the solder joint can cause electronic devices tomalfunction. The solder joint is usually fabricated by the reflow process, in which aliquid solder alloy reacts with a solid metal substrate, such as a Cu pad, to form a thinlayer of intermetallic compounds (IMC) at the bonding interface. The IMC layer isessential for the formation of a functional and robust metallurgical bond in a solderjoint interface. However, an excessively grown IMC layer becomes vulnerable andseverely degrades the mechanical strength of the solder joint. Nowadays, the adoptionof Pb-free solders has become an inevitable trend in the electronic packaging industrydue to the environmental and health concern. The higher Sn content in Pb-free soldersresults in more IMC formation between the solder and the Cu substrate. Therefore,further understanding of the IMC growth and its effect on the mechanical behavior ofthe solder joint is significant for the implementation of Pb-free solders in electronicsindustry.The samples of Sn3.0Ag0.5Cu/Cu solder joints are aged isothermally at150Cfor0,72,288and500h, and the thickness of the IMC layer and the roughness of thesolder/IMC interface are measured and used to characterize the microstructureevolution of the IMC layer. The results show that the growth of the IMC follows theFick’s law that predicts the total IMC thickness increases linearly with the square rootof the aging time. With the increase of the isothermal aging time, the thickness of theIMC layer increases and the roughness of the solder/IMC interface decreases,indicating the initial scallop morphology of the solder/IMC interface changes to amore planar type.To investigate the effects of the IMC microstructure and the strain rate on thetensile strength and failure mode of solder joints, the tensile tests of the aged solderjoints are conducted under the strain rates of2×104,2×102and2s1. The resultsindicate that under the low strain rates, both the thickness and roughness of the IMClayer have influence on the strength and failure mode of the solder joint. There is anegative correlation between the IMC thickness and the tensile strength and a positivecorrelation between the IMC roughness and the tensile strength of the solder joint.With the increase of the aging time, the thickness of the IMC layer increases and theroughness of the solder/IMC interface decreases, the dominant failure mode migratesfrom the ductile fracture in the bulk solder to the brittle fracture in the IMC layer.Under the high strain rates, the thickness plays a more important role, the tensilestrength decreases with the increase of the IMC thickness, and the dominant failure mode is crittle fracture in the IMC layer. There is a positive correlation between thetensile strength of the solder joint and the stain rate applied during the test. With theincrease of the strain rate, the failure mode migrates from the ductile fracture in thebulk solder to the brittle fracture in the IMC layer.The diffusion-induced stress in the IMC layer between the Cu pad and the solderis investigated. The analytical model with a single IMC layer is constructed. Theconcentration of the Cu atoms in the IMC layer is calculated by solving the diffusionequation using Laplace transformation technique, and then the diffusion-inducedstress in the IMC layer is obtained analytically. A practical finite element modeling isdeveloped to calculate the Cu atoms concentration and the diffusion-induced stress inthe scallop-edged IMC layer. It is found that the IMC layer is subjected tocompressive stress due to the Cu atoms diffusion. As the diffusion time is long enough,the diffusion-induced stress increases and gradually stabilized. The interfacialmorphology of the solder/IMC has great influence on the evolution of the Cu atomsconcentration, and the diffusion-induced stress in the IMC layer with the scallop edgeis less than that with the flat edge.A finite element model is developed to study the microcracking behavior of solidmaterials. Voronoi tessellations are used to divide the plane of the material intoirregular polygons. The four-node cohesive interface elements implement via the UELsubroutine capability of ABAQUS are embedded along all of the polygonalboundaries to simulate the initiation, propagation and coalescence of microcracks.The effect of the polygons shapes, the material defects and the sensitivity of thevarious CZM parameters in predicting the overall mechanical response is investigated.The overall strength is determined predominantly by weak interfaces; both thepolygons shapes and the weak interfaces control the crack configuration; differentnormal and tangential strengths of interfaces result in different cracking behavior, andplay a critical role in determining the macroscopic mechanical response of the system.The effect of the diffusion-induced stress, the thickness of the IMC layer and theroughness of the solder/IMC on the overall response and failure mode of the solderjoint is studied using proposed finite element models. The numerical simulationresults suggest that the diffusion-induced stress in the IMC layer has little influenceon the overall strength and failure mode of the solder joint. A thicker IMC layer and arougher solder/IMC interface reduce the overall strength of the solder joint, and thedominant failure mode migrates to the fracture within the IMC layer when the IMClayer is thick and the solder/IMC interface is planar.