Micro-mechanical characteristics and dimensional change of Cu-Sn interconnects due to growth of interfacial intermetallic compounds

Sn-based solder alloys are extensively used in electronic devices to form interconnects between different components to provide mechanical support and electrical path. The formation of a reliable solder interconnects fundamentally relies on the metallurgic reaction between the molten solder and solid pad metallization in reflowing. The resultant IMC layer at the solder/pad metallization interface can grow continuously during service or aging at an elevated temperature, uplifting the proportion of IMCs in the entire solder joint. However, the essential mechanical properties of interfacial IMC (i.e. Cu6Sn5, Cu3Sn) layers, such as Young s modulus and hardness, are drastically different in comparison with Sn-based solder and substrate. Therefore, the increasing fraction of interfacial IMCs in the solder joint can lead to significant deformation incompatibility under exterior load, which becomes an important reliability concern in the uses of solder joints for electronic interconnects. In the past decades, extensive research works were implemented and reported regarding the growth of interfacial IMC layers and its effect on the mechanical integrity of solder joints. But, the following fundamental issues in terms of mechanical and microstructural evolution in the uses of solder joints still remain unclear, demanding further research to elaborate: (1) The protrusion of IMCs: Though the growth of interfacial IMC layers along the diffusion direction in solder joints were studied extensively, the growth of IMCs perpendicular to the diffusion direction were reported in only a few papers without any further detailed investigation. This phenomena can crucially govern the long-term reliability of solder interconnects, in particular, in the applications that require a robust microstructural integrity from a solder joint. (2) Fracture behaviour of interfacial IMC layers: The fracture behaviour of interfacial IMC layers is a vital factor in determining the failure mechanism of solder joints, but this was scarcely investigated due to numerous challenges to enable a potential in-situ micro-scale tests. It is therefore highly imperative to carry out such study in order to reveal the fracture behaviour of interfacial IMC layers which can eventually provide better understanding of the influence of interfacial IMC layers on the mechanical integrity of solder joints. (3) Volume shrinkage: The volume shrinkage (or solder joint collapse) induced by the growth of interfacial IMC layers was frequently ascribed as one of the main causes of the degradation of mechanical reliability during aging due to the potentially resulted voids and residual stress at the solder/substrate interface. However, very few experimental works on the characterisation of such type of volume shrinkage can be found in literatures, primarily due to the difficulties of observing the small dimensional changes that can be encountered in the course of IMCs growth. (4) Residual stress: The residual stress within solder joints is another key factor that contributes to the failure of solder joints under external loads. However, the stress evolution in solder joints as aging progresses and the potential correlation between the residual stress and the growth of interfacial IMC layers is yet to be fully understood, as stress/strain status can fundamentally alter the course of total failure of a solder joint. (5) Crack initiation and propagation in solder joints: Modelling on the mechanical behaviour of solder joints is often undertaken primarily on the stress distribution within solder joints, for instance, under a given external loading. But there is lack of utilising numerical analysis to simulate the crack initiation and propagation within solder joints, thus the effect of interfacial IMC layers on the fracture behaviour of the solder joints can be elaborated in further details. In this thesis, the growth of interfacial IMCs in parallel and perpendicular to the interdiffusion direction in the Sn99Cu1/Cu solder joints after aging was investigated and followed by observation with SEM, with an intention of correlating the growth of IMCs along these two directions with aging durations based on the measured thickness of IMC layer and height of perpendicular IMCs. The mechanism of the protrusion of IMCs and the mutual effect between the growth of IMCs along these two directions was also discussed. The tensile fracture behaviour of interfacial Cu6Sn5 and Cu3Sn layers at the Sn99Cu1/Cu interface was characterised by implementing cantilever bending tests on micro Cu6Sn5 and Cu3Sn pillars prepared by focused ion beam (FIB). The fracture stress and strain were evaluated by finite element modelling using Abaqus. The tensile fracture mechanism of both Cu6Sn5 and Cu3Sn can then be proposed and discussed based on the observed fracture surface of the micro IMC pillars. The volume shrinkage of solder joints induced by the growth of interfacial IMC layers in parallel to the interdiffusion direction in solder joint was also studied by specifically designed specimens, to enable the collapse of the solder joint to be estimated by surface profiling with Zygo Newview after increased durations of aging. Finite element modelling was also carried out to understand the residual stress potentially induced due to the volume shrinkage. The volume shrinkage in solder joints is likely to be subjected to the constraint from both the attached solder and substrate, which can lead to the build-up of residual stress at the solder/Cu interface. Depth-controlled nanoindentation tests were therefore carried out in the Sn99Cu1 solder, interfacial Cu6Sn5 layer, Cu3Sn layer and Cu with Vickers indenter after aging. The residual stress was then evaluated in the correlation with aging durations, different interlayers and the locations in the solder joint. Finally, finite element models incorporated with factors that may contribute to the failure of solder joints, including microstructure of solder joints, residual stress and the fracture of interfacial IMC, were built using Abaqus to reveal the effect of these factors on the fracture behaviour of solder joints under applied load. The effect of growth of IMC layer during aging on the fracture behaviour was then discussed to provide a better understanding of the degradation of mechanical integrity of solder joints due to aging. The results from this thesis can facilitate the understanding of the influence of interfacial IMC layers on the mechanical behaviour of solder joints due to long-term exposure to high temperatures

Etude de l’impact de micro-cavités (voids) dans les attaches de puces des modules électroniques de puissance

Power converters nowadays are required to function under harsh conditions in meeting energy efficiency and reliability requirement. Whereas, industrial specifications tend toward a higher level of power integration in respect to the cost constraint. As a result, the die attach is one of the key elements in power module packaging because of high current densities and high heat flow which are transported through. Void formation in the die attach may lead to performance degradation and premature aging of the component. This study introduces a methodology based on the comparison of numerical simulations and experimental campaigns. The obtained results help to improve our understanding on the electro-thermal behaviour of MOSFETs with solder voids. In this thesis, we depict a finite element model in which electro-thermal coupling of a MOSFET active layer is taken in to account. Simulation results will be correlated to the experimental responses. Later on, a parametric numerical study based on the response surface method (RSM) which minimizes the number of simulations and future tests will be exploited to quantify the impact of void position and size on several selective performance criteria. A future serial experimental study in respect to the same RSM design is expected in prospect, in order to fulfil the complementarity for this approach.Les convertisseurs électroniques de puissance sont voués à fonctionner sous des conditions applicatives de plus en plus sévères tout en respectant les impératifs d’efficacité énergétique et de fiabilité. Or, les besoins industriels tendent vers un plus haut niveau d’intégration fonctionnelle tout en améliorant le rapport qualité-prix. Dès lors, la solution utilisée pour le report des puces semi-conductrices est le siège de densités de courant importantes et d’un flux thermique élevé. La présence de défauts dans cette couche d’interconnexion peut conduire à la dégradation de ses performances et au vieillissement prématuré du composant. L’objectif de nos recherches est d’évaluer la pertinence d’une méthodologie basée sur la confrontation de simulations numériques et de campagnes expérimentales. L’objectif est d’améliorer la compréhension du comportement électrothermique en régime de conduction d’un transistor MOSFET en présence d’un void dans sa brasure. Dans cette manuscrite, nous présenterons la construction d’un modèle intégrant le couplage électrothermique de la partie active qui sera confronté à la réponse de résultats expérimentaux. Puis, une étude numérique basée sur la théorie des plans fractionnaires, qui minimise le nombre de simulations, sera exploitée afin de quantifier l’impact de la taille et de la position du défaut sur la réponse électrothermique du composant et de ses liaisons électriques. Les détails de la mise en place d’une étude expérimentale analogue permettront de mettre en perspective la complémentarité de cette approche

Etude et caractérisation d'interconnexions intermétalliques à partir de plot de cuivre et d'alliage SnAgCu pour l'empilement tridimentionnel de composants actifs

Technological roadmap of the microelectronic industry is mainly described by Moore'slaw which aims a constant reduction of transistors size. Three-dimensional integration ofactive chips appears more and more as an alternative way to Moore's law. According to thisstrategy, chips are interconnected along the vertical axis thanks to copper pillars and a tinbased alloy (SnAgCu).The joining is then performed through eutectic bonding using aSnAgCu solder alloy which is at the origin of intermetallic compounds growing at the copperalloy interface. These intermetallic compounds are sometimes described in literature asweakening factor of the interconnection mechanical reliability. Moreover this interfacialreactivity leads also to the formation of Kirkendall microvoids potentially causinginterconnections breakings, mostly noticed during ageing tests.This report is dedicated to the study and metallurgical characterization of theinterconnection system with a size close to that of the actual prototypes which is 25μm. Thestudy is successively focused on SnAgCu alloy microstructure, Cu/SnAgCu and Ni/SnAgCuinterfacial reactivity and on the mechanical reliability of interconnection system. These topicsare investigated in function of thermal constraints and during different integration steps untilchips packaging. The main critical aspect is related to the fact that system dimensions, alreadysmall, are planned to be reduced, leading to a more important proportion of the solder alloyconsumed by interfacial reaction.Les objectifs technologiques de l'industrie de la microélectronique sont largement dictés par la loi de Moore qui vise une réduction permanente de la taille des transistors. Depuis peu l'intégration tridimensionnel de composant actif se présente comme une voie d'intégration alternative à la loi de Moore. Selon cette stratégie, les composants sont interconnectés selon l'axe verticale au moyen de plots de cuivre et d'un alliage à base d'étain (SnAgCu). L'assemblage est alors réalisé par un brasage eutectique de l'alliage SnAgCu qui génère une formation de composés intermétalliques (Cu6Sn6 et Cu3Sn) à l'interface entre les plots de cuivre et l'alliage. Or, ces composés intermétalliques sont parfois décrits dans la littérature comme facteur affaiblissant la fiabilité mécanique de l'interconnexion. Par ailleurs cette réactivité interfaciale s'accompagne de l'apparition microcavités de type trous Kirkendall susceptibles d'être à l'origine de ruptures d'interconnexions notée lors de tests de vieillissement. Ce mémoire est consacré à la caractérisation métallurgique du système d'interconnexion par brasage dont les dimensions sont celles des prototypes actuels c'est-à-dire 25µm. L'étude se concentrera successivement sur les aspects relatifs à la microstructure de l'alliage SnAgCu, à la réactivité interfaciale des systèmes Cu/SnAgCu et Ni/SnAgCu puis à la fiabilité mécanique du système d'interconnexion. Ces thématiques seront investiguées en fonction de la contrainte thermique et au cours des différentes étapes d'intégration jusqu'à l'assemblage de composant. Le caractère critique de la problématique réside dans le fait que les dimensions du système, déjà faibles, ont vocation à se réduire, rendant de plus en plus importante la proportion du volume de l'alliage occupée par ces formations interfaciales