In situ imaging of microstructure formation in electronic interconnections

The development of microstructure during melting, reactive wetting and solidification of solder pastes on Cu-plated printed circuit boards has been studied by synchrotron radiography. Using Sn-3.0Ag-0.5Cu/Cu and Sn-0.7Cu/Cu as examples, we show that the interfacial Cu6Sn5 layer is present within 0.05 s of wetting, and explore the kinetics of flux void formation at the interface between the liquid and the Cu6Sn5 layer. Quantification of the nucleation locations and anisotropic growth kinetics of primary Cu6Sn5 crystals reveals a competition between the nucleation of Cu6Sn5 in the liquid versus growth of Cu6Sn5 from the existing Cu6Sn5 layer. Direct imaging confirms that the β-Sn nucleates at/near the Cu6Sn5 layer in Sn-3.0Ag-0.5Cu/Cu joints

Characterization of Thermo-Mechanical Damage in Tin and Sintered Nano-Silver Solders

abstract: Increasing density of microelectronic packages, results in an increase in thermal and mechanical stresses within the various layers of the package. To accommodate the high-performance demands, the materials used in the electronic package would also require improvement. Specifically, the damage that often occurs in solders that function as die-attachment and thermal interfaces need to be addressed. This work evaluates and characterizes thermo-mechanical damage in two material systems – Electroplated Tin and Sintered Nano-Silver solder. Tin plated electrical contacts are prone to formation of single crystalline tin whiskers which can cause short circuiting. A mechanistic model of their formation, evolution and microstructural influence is still not fully understood. In this work, growth of mechanically induced tin whiskers/hillocks is studied using in situ Nano-indentation and Electron Backscatter Diffraction (EBSD). Electroplated tin was indented and monitored in vacuum to study growth of hillocks without the influence of atmosphere. Thermal aging was done to study the effect of intermetallic compounds. Grain orientation of the hillocks and the plastically deformed region surrounding the indent was studied using Focused Ion Beam (FIB) lift-out technique. In addition, micropillars were milled on the surface of electroplated Sn using FIB to evaluate the yield strength and its relation to Sn grain size. High operating temperature power electronics use wide band-gap semiconductor devices (Silicon Carbide/Gallium Nitride). The operating temperature of these devices can exceed 250oC, preventing use of traditional Sn-solders as Thermal Interface materials (TIM). At high temperature, the thermomechanical stresses can severely degrade the reliability and life of the device. In this light, new non-destructive approach is needed to understand the damage mechanism when subjected to reliability tests such as thermal cycling. In this work, sintered nano-Silver was identified as a promising high temperature TIM. Sintered nano-Silver samples were fabricated and their shear strength was evaluated. Thermal cycling tests were conducted and damage evolution was characterized using a lab scale 3D X-ray system to periodically assess changes in the microstructure such as cracks, voids, and porosity in the TIM layer. The evolution of microstructure and the effect of cycling temperature during thermal cycling are discussed.Dissertation/ThesisDoctoral Dissertation Materials Science and Engineering 201

4D Microstructural Characterization of Electromigration and Thermal Aging Damage in Tin-Rich Solder Joints

abstract: As the microelectronics industry continues to decrease the size of solder joints, each joint will have to carry a greater current density, making atom diffusion due to current flow, electromigration (EM), a problem of ever-increasing severity. The rate of EM damage depends on current density, operating temperature, and the original microstructure of the solder joint, including void volume, grain orientation, and grain size. While numerous studies have investigated the post-mortem effects of EM and have tested a range of current densities and temperatures, none have been able to analyze how the same joint evolves from its initial to final microstructure. This thesis focuses on the study of EM, thermal aging, and thermal cycling in Sn-rich solder joints. Solder joints were either of controlled microstructure and orientation or had trace alloying element additions. Sn grain orientation has been linked to a solder joints’ susceptibility to EM damage, but the precise relationship between orientation and intermetallic (IMC) and void growth has not been deduced. In this research x-ray microtomography was used to nondestructively scan samples and generate 3D reconstructions of both surface and internal features such as interfaces, IMC particles, and voids within a solder joint. Combined with controlled fabrication techniques to create comparable samples and electron backscatter diffraction (EBSD) and energy-dispersive spectroscopy (EDS) analysis for grain orientation and composition analysis, this work shows how grain structure plays a critical role in EM damage and how it differs from damage accrued from thermal effects that occur simultaneously. Unique IMC growth and voiding behaviors are characterized and explained in relation to the solder microstructures that cause their formation and the possible IMC-suppression effects of trace alloying element addition are discussed.Dissertation/ThesisDoctoral Dissertation Materials Science and Engineering 201

Microstructure and damage evolution during thermal cycling of Sn-Ag-Cu solders containing antimony

Antimony is attracting interest as an addition to Pb-free solders to improve thermal cycling performance in harsher conditions. Here, we investigate microstructure evolution and failure in harsh accelerated thermal cycling (ATC) of a Sn-3.8Ag-0.9Cu solder with 5.5 wt.% antimony as the major addition in two ball grid array (BGA) packages. SbSn particles are shown to precipitate on both Cu6Sn5 and as cuboids in β-Sn, with reproducible orientation relationships and a good lattice match. Similar to Sn-Ag-Cu solders, the microstructure and damage evolution were generally localised in the β-Sn near the component side where localised β-Sn misorientations and subgrains, accelerated SbSn and Ag3Sn particle coarsening, and β-Sn recrystallisation occurred. Cracks grew along the network of recrystallised grain boundaries to failure. The improved ATC performance is mostly attributed to SbSn solid-state precipitation within β-Sn dendrites, which supplements the Ag3Sn that formed in a eutectic reaction between β-Sn dendrites, providing populations of strengthening particles in both the dendritic and eutectic β-Sn

The nucleation and growth of Cu6Sn5 in solders

Microstructure formation and evolution in Pb-free solder alloys and solder joints on Cu substrates depend on the nucleation and growth of primary Cu6Sn5 and beta-Sn during solidification and thermal cycling in service. This thesis explores the mechanisms responsible for microstructure evolution at different stages during the lifetime of a solder joint. Cu6Sn5, a common intermetallic in Pb-free soldering, is usually first to nucleate and past work showed that aluminium additions can cause significant refinement of primary Cu6Sn5. In this work, it is showed that the mechanism of refinement is heterogeneous nucleation of Cu6Sn5 on either deltaCu33Al17 or gamma1Cu9Al4 coupled with significant constitutional supercooling ahead of growing Cu6Sn5 crystals. Cu-Al particles are shown to be effective catalytic nucleant particles in both hyper-eutectic Sn-4Cu-0.02Al and hypo-eutectic Sn-0.7Cu-0.05Al/Cu joints and share reproducible orientation relationships with Cu6Sn5. The growth of primary Cu6Sn5 also plays a role in determining the final microstructure. A deeper understanding of crystal growth mechanisms and transitions between different Cu6Sn5 morphologies is developed. It is shown that, for different composition and cooling rate combinations, Cu6Sn5 crystals undergo a faceted to non-faceted growth transition as a result of a kinetic interface roughening transition and a gradual change in mechanism from lateral growth governed by anisotropic attachment kinetics to continuous growth governed by diffusion and curvature. As the majority phase in most solder joints, betaSn nucleates at a later stage of solidification after Cu6Sn5 has nucleated. The nucleation of betaSn on the Cu6Sn5 layer in solder joints is studied in detail. It is shown that primary Cu6Sn5 is not a potent nucleant for Sn, but the Cu6Sn5 layer plays a key role in betaSn nucleation and microstructure formation in solder joints. Thermal contraction of Cu6Sn5, betaSn and other common phases in soldering is another important phenomenon that affects the performance of solder joints in service. Directional data on the anisotropic coefficient of thermal expansion (CTE) of Cu6Sn5 and other non-cubic intermetallics are measured and correlated with the directional Young’s modulus.Open Acces

Applications of Crystal Plasticity in Forming Technologies

In this Special Issue, we have gathered work on simulations of polycrystalline metals and alloys at various length scales to model multiscale localization phenomena such as slip bands, cracks, and twins. The series highlights innovative techniques that combine simulation and experiments to capture material production and guide the development of forming theories. The published work helps to understand the effect of microstructure characteristics on deformation and damage behavior under multiaxial load conditions. Furthermore, these models and the studies can be used with machine learning technologies to optimize microstructure functions for materials application and process paths

Determination of the creep properties of Pb-free solders for harsh environments using meso-scale testing

Solder joints in electronic packages are prone to failure due to the evolution of thermal expansion mismatch strains during thermal cycling. The comparatively wide operating temperature range and long lifetimes of aerospace electronics require high reliability solder joints. Since 2006, high reliability industries (aerospace and military amongst others) that are exempt from lead-free RoHS regulation on account of concerns over the reliability of Pb-free solders have found it increasingly difficult and expensive to continue using traditional Sn-Pb-based solders. Hence there is a pressing need to find a suitable alternative that can match the manufacturing and reliability performance of Sn-Pb. There remains a dearth of data for the constitutive behaviour of Pb-free solders under harsh environment scenarios. Unfortunately, conventional test approaches, particularly in the case of creep behaviour which is critical to solder lifetimes, are expensive and time-consuming. High temperature nanoindentation has been recently developed as a quick method for the determination of creep properties of solder alloys. This paper compares and contrasts nanoindentation creep results for bulk Pb-Sn and lead-free solders. However, there are limits to nanoindentation creep, in particular the load-dependence of the technique. A new meso-scale test approach that lies between nanoindentation and bulk creep testing has been developed. Real ball grid arrays using Pb-free solders have been creep tested in the temperature and stress ranges of operating solder joints. High temperature creep constitutive data has been obtained. The technique offers promising time and materials savings in obtaining important mechanical property data for subsequent use in life-prediction models

Fragilisation des brasures d'interconnexions pour la reprise de puces microélectroniques.

Avec l’avènement des nouvelles technologies et la demande croissante en équipements toujours plus performants, on assiste de plus en plus à la miniaturisation et la complexification des dispositifs microélectroniques (lois de Moore). Cette miniaturisation passe par l’optimisation des modes d’assemblages de composants permettant la densification et l’intégration de puces de fonctionnalités diverses dans un même module. Toutefois, la complexité de ces dispositifs – dits d’intégration hétérogène – introduit un réel besoin d’un procédé de reprise de puces pour la réparation de potentielles défectuosités induites lors de leur assemblage. Les procédés actuels de reprise de puces microélectroniques utilisent des méthodes thermiques pour ramollir (~ 200°C) les brasures d’interconnexions afin de retirer la puce défectueuse, et ce au risque d’endommager le substrat organique thermosensible ou les bonnes puces adjacentes. De plus, ces procédés traditionnels ont montré leurs limites lorsqu’il s’agit de puces à interconnexions à pas fins où les interconnexions possèdent une plus grande quantité de composés intermétalliques (IMC) ayant un point de fusion très élevé (~ 400°C). Il devient donc important de mettre sur pied des procédés alternatifs de reprises de puces défectueuses qui seraient compatibles avec les nouvelles générations de puces microélectroniques à densité élevé d’interconnexions (ex. interconnexions à pas fins ou ultrafins). À cette fin, cette thèse développe une nouvelle méthode de séparation de brasures d’interconnexions à plus faible température (< 100°C) qui exploite un des modes de défaillance connus des métaux solides (la fragilisation par métal liquide (FML)) pour faciliter le retrait de la puce défectueuse. La FML est le phénomène qui définit la perte de ductilité (ou une dégradation des propriétés mécaniques) d’un métal solide lorsqu’il est au contact d’un métal liquide donné. Les résultats obtenus dans cette thèse ont permis de confirmer la fragilisation de l’alliage de brasure étain-argent-cuivre (SAC) par le gallium (Ga) liquide, et une analyse complète de l’évolution de la microstructure des brasures SAC a permis d’établir le modèle de fragilisation en présence. La cinétique de diffusion du Ga liquide dans les brasures d’interconnexions a été établie et s’est avérée être en accord avec les mécanismes de fragilisation mis en jeu lors de l’exposition des brasures d’interconnexions au Ga liquide. De plus, la particularité des puces Flip-Chip – où les brasures sont inaccessibles individuellement et confinées dans un gap – a motivé la création d’émulsions à base de Ga liquide. L’optimisation de l’action capillaire du Ga liquide dans le gap et de son adhérence sur la surface des brasures a permis d’effectuer avec succès le retrait d’une puce Flip-Chip avec une réduction (de plus de 50%) de sa résistance mécanique et aucun résidu de Ga restant sur le module. Une étude préliminaire de l’action du Ga liquide sur les métallisations sous-jacentes des brasures a permis de démontrer le risque minimal de la méthode sur la fiabilité du module final (pour des substrats avec contacts en cuivre renforcé d’une finition de surface au nickel). Finalement, les résultats présentés dans cette thèse démontrent la faisabilité de l’utilisation de la fragilisation des brasures d’interconnexions comme base d’un nouveau procédé de reprise de puces microélectroniques à faible température

Micromechanics of oxides - From complex scales to single crystals

Protective oxide scales shield high temperature materials from corrosion, thus ensuring safety and long material life under adverse operating conditions. Cracking and spallation of such scales can lead to fatigue crack initiation and expose the material to further oxidation. It is therefore imperative to measure the fracture properties of oxides so that they can be incorporated in the life estimation models of high temperature materials. Existing models require inputs on oxide properties such as fracture strain and elastic modulus. The established measurement methods are mainly applied for thick (several microns) scales, but for many materials such as superalloys the oxides are thinner (&lt; 1 \ub5m), and the results would be affected by the influence of substrate and residual stresses. Focused ion beam machining (FIB) enables the preparation of micro sized specimens in the size range of these scales. \ua0In this work, a modified microcantilever geometry with partially removed substrate is proposed for testing of oxide scales. Room temperature microcantilever bending of thermally grown superalloy oxide (complex oxide with an upper layer of spinel and lower layer of Cr2O3) revealed the presence of plasticity, which is attributed to the deformation of the upper cubic spinel layer and low defect density of the volume being probed. Due to difficulties in isolating Cr2O3 from the complex oxide layer, dedicated oxidation exposures are performed on pure chromium to generate Cr2O3 which is tested using the same cantilever geometry at room temperature and 600 \ub0C. Results show lower fracture strain at 600 \ub0C in comparison to room temperature and presence of cleavage type of transgranular fracture in both cases, pointing to a need for studying cleavage fracture of Cr2O3. This was analysed using microcantilever bending of single crystal Cr2O3 to identify the preferential cleavage planes. Finally, fracture toughness was also measured through microcantilever bending and micropillar splitting. \ua0Thus, it is shown that micromechanical testing is an effective tool for measuring fracture properties of oxide scales. The fracture study of Cr2O3 scales show that it is a complex process in which the crystallographic texture also plays a role. Surface energy and fracture toughness criterion was unable to explain the fracture behaviour of single crystal Cr2O3 observed from experiments. Such a comprehensive analysis can contribute towards the development of reliable models for oxidation assisted failure