Dramatic advances are taking place in the microelectronic industry. The feature size continues to scale down and it is expected that the minimum feature size on the integrated circuit is expected to reach 9 nm by 2016, and there will be more than 8 billion transistors on a 310 cm² chip, according to various available roadmaps. Subsequently, this reduction in feature size would require the first-level input-output interconnects to decrease in pitch size to meet the increased number of transistors on the chip. Also, to minimize the on-chip interconnect delay, development of low-K dielectric/copper will become increasingly common in future devices. However, due to the low fracture strength of low-K dielectric, it is essential that the first-level interconnects exert minimal force on the die pads and therefore, do not crack or delaminate the low-K dielectric material. It is also preferable to have a wafer-level packaging approach to facilitate test-and-burn in and to produce known-good dies. Based on these growing demands from the microelectronics industry, there is a compelling need to develop innovative interconnect technologies. This thesis aims to develop one such innovative interconnect — G-Helix interconnect. G-Helix is a scalable lithography-based wafer-level electroplated compliant interconnect that has the potential to meet the fine-pitch first-level chip-to-substrate interconnect requirements. The three-mask fabrication of G-Helix is based on lithography, electroplating and molding (LIGA-like) technologies, and this fabrication can be easily integrated into large-area wafer-level fine-pitch batch processing. In this work, the fabrication, assembly, experimental reliability testing, and numerical physics-based modeling of the G-Helix interconnects will be presented. The fabrication of the interconnects will be demonstrated at 100μm pitch on a 20 x 20 mm die in a class 10/1000 cleanroom facility. The wafers with compliant interconnects will be singulated into individual dies and assembled on substrates using Pb/Sn eutectic solder. The assembly will then be subjected to air-to-air thermal cycling between 0℃and 100℃ and the reliability of the compliant interconnect will be assessed. In addition to the thermo-mechanical reliability testing, some of the dies with free-standing interconnects will also be used for measuring the compliance of the interconnects by compressing with a nanoindenter. In parallel to the experimental research, a numerical analysis study will also be carried out. The numerical model will use direction-, temperature, time-dependent, and time independent material constitutive properties as appropriate. The thermo-mechanical fatigue life of the compliant interconnect assembly will be determined and compared with the experimental data. Recommendations will be developed for further enhancement of reliability and reduction in pitch size.

The usage of lead-­free solder joints in electronic packaging is of greatest concern to the electronic industry due to the health and environmental hazards arising with the use of lead. As a consequence, lead is legally prohibited in the European Union and the industry is aiming to produce lead-free products.            The reliability of solder joints is an important issue as the failure could destroy the whole function of a product. SnAgCu is a commonly used alloy for lead-­free solders. Compared to solders containing lead, tin-­rich solders react more rapidly with the copper substrate. The reaction results in formation of brittle intermetallic compounds and in poor mechanical reliability. The formation can be slowed down by the addition of nickel in the under bump metallization.     In this project the objective was to evaluate the mechanical reliability of solder joints in high temperature applications. An alloy of nickel and phosphorus was plated on copper plates by electroless plating. The plates were joined together using SnAgCu solder. The samples were then thermally aged at 180°C for different durations (100, 200, 300, 400 and 500 hours). Tensile tests were performed on the samples. The result from the tensile test showed a decrease in mechanical strength with increasing aging duration. The fracture path shifted from being in the bulk solder to being at the interfaced.

This study mainly evaluated the thermo-mechanical reliability of lead-free packaging techniques for attaching large-area chip. With 3 MPa pressure, a low-temperature (<300oC) sintering technique enabled by a nano-scale silver paste was developed for attaching 100 mm2 silicon die. This new lead-free packaging technique for die-attachment was compared with soldering by vacuum reflow. Lead-free solder SAC305 and SN100C were selected and used in this work since they were widely used in electronic packaging industry. Inspection of as-prepared die-attachments by X-ray and optical microscopy (observation of cross-section) showed that the voids percentage in solder joint was less than 5% and no voids was observed at the scale of hundreds of micron in sintered silver joint. Then these die-attachment were thermal cycled with the temperature range from -40oC to 125oC. Deduction of curvature and residual stresses were found for both soldered and sintered die-attachment. After 800 cycles, the residual stresses in silicon-solder-copper sample already decreased to around 0. The SEM images of solder and silver joint after 800 thermal cycles showed that cracks longer than 2.5 mm already grew in both kinds of solder joint (die-attachment of Si-Solder-Copper). In contrast, no cracks or voids at the scale of hundreds of micron were defected in silver joint. Based on these observation, different mode of stress-relaxation were proposed for sintered silver and solder, respectively. While solder joint released stresses by crack growth, the silver joint relied on the deformation of porous structure, and plastic deformation may occur. The pressure-sintering process with double printing and drying was proved to be a reliable process to produce sintered - silver bonding with high strength. The reliability of silver joint was better than that of SAC305 or SN100C. Besides, the technique of measuring the curvature by laser scanning, introduced in this work, showed its significance by directly reflecting the bonding integrity of die-attachment. As a nondestructive testing technique, It was a cheaper and faster way to examine the die-attachment. Additionally, it overcame the disadvantage of X-ray Inspection: it was of the ability to differentiate between layers of die-attachment. / Master of Science

Les céramiques macroporeuses sont largement utilisées dans des applications telles que la filtration, l'isolation thermique, les scaffolds pour la croissance de tissus biologiques, les SOFC, ou encore les OTM. En plus d’une bonne stabilité mécanique, ces céramiques doivent généralement posséder une autre propriété fonctionnelle, comme une perméabilité élevée, une faible conductivité thermique, ou une biocompatibilité. Cependant, la résistance mécanique est généralement augmentée en diminuant le volume total des pores, même si cela peut dégrader d’autres propriétés fonctionnelles. Au-delà de la porosité, des paramètres morphologiques tels que la taille des pores, leur forme ou la tortuosité, peuvent devenir cruciaux pour maximiser les performances, tout en conservant une grande résistance mécanique. Une importante amélioration des propriétés mécaniques peut par exemple être obtenue par des structures anisotropes. Celles-ci renforcent les structures dans la direction de la contrainte principale, de manière similaire aux matériaux naturels tels que l'os trabéculaire, le liège ou le bois. Néanmoins, la plupart des techniques classiquement utilisées pour produire des céramiques macroporeuses ne proposent pas ce niveau de flexibilité. L’ ice-templating est une technique de mise en forme appropriée pour obtenir des matériaux macroporeux anisotropes. Elle est basée sur la congélation de suspensions colloïdales et la séparation ultérieure des particules par le front de solidification. Le solvant congelé est ensuite éliminé, en laissant des pores dont les morphologies sont une réplique des cristaux sublimés. Enfin, le matériau cru est fritté pour consolider la microstructure. Ce processus assure un contrôle indépendant de l'architecture des pores (volume des pores, la taille et la morphologie) à travers la fraction de solides initiale, la vitesse de refroidissement, ou les éventuels additifs. Par conséquent, une bonne compréhension de ces paramètres est essentielle afin d’établir un lien entre les procédés de mise en forme, la microstructure, et les performances de ce type de matériaux et d'étendre leur utilisation dans les applications mentionnées précédemment. L’objectif de ce travail est premièrement, d’adapter l'architecture des pores faits par l’ice-templating pour ensuite, déterminer les principaux paramètres des microstructures qui contrôlent la résistance à la compression, la fiabilité mécanique, et la perméabilité de ces matériaux poreux unidirectionnels. En outre, l'applicabilité des modèles de flux mécanistique et de flux de gaz sera discutée dans le contexte des morphologies de pores structurés. Enfin, nous allons fournir des lignes directrices pour produire des échantillons tubulaires produits par ice-templating. / Macroporous ceramics are widely used in applications such as filtration, thermal insulation, scaffolds for tissue engineering, SOFCs, or OTM’s. They must combine mechanical stability with at least one other functional property such as high permeability, low thermal conductivity, or biocompatibility. However, strength is usually increased by decreasing the total pore volume even though this may degrade the other functional properties. Beyond porosity content, morphological parameters such as pore size, shape, or tortuosity, can become crucial to maximize the performance while maintaining high strength. For example, a significant improvement can be achieved by engineering anisotropic structures to mechanically reinforce the direction of the main stress, similarly to natural materials such as trabecular bone, cork, or wood. Unfortunately, most of the techniques conventionally used to produce macroporous ceramics do not offer this level of flexibility. Ice-templating is a processing technique suitable to obtain anisotropic macroporous materials. It is based on the freezing of colloidal suspensions and the subsequent segregation of particles by the solidification front. After solidification, the frozen solvent is removed, leaving pores whose morphologies are a replica of the sublimated crystals. Finally, the green body is sintered to consolidate the microstructure. This process provides independent control of the pore architecture (pore volume, size, and morphology) through initial solids loading, cooling rate, or additives. Therefore, a good understanding of these parameters is essential to understand the relationship between processing, microstructure, and performance of this type of materials and extend their use in the aforementioned applications.The purpose of this work is first, tailor the pore architecture of specimens processed by ice-templating to then, determine the main microstructural parameters that control the compressive strength, mechanical reliability, and air permeability of unidirectional porous materials. Furthermore, the applicability of mechanistic and gas flow models will be discussed in the context of the structured pore morphologies. Finally, we will provide some guidelines to produce tubular ice-templated samples with controlled porosity.The purpose of this work is first, tailor the pore architecture of specimens processed by ice-templating to then, determine the main microstructural parameters that control the compressive strength, mechanical reliability, and air permeability of unidirectional porous materials. Furthermore, the applicability of mechanistic and gas flow models will be discussed in the context of the structured pore morphologies. Finally, we will provide some guidelines to produce tubular ice-templated samples with controlled porosity.

No description available.

京都大学 / 新制・課程博士 / 博士(工学) / 甲第24228号 / 工博第5056号 / 新制||工||1789(附属図書館) / 京都大学大学院工学研究科マイクロエンジニアリング専攻 / (主査)教授 土屋 智由, 教授 平方 寛之, 教授 江利口 浩二 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

Lithium alloys with metallic or semi-metallic elements are attractive candidate materials for the next-generation rechargeable Li-ion battery anodes, thanks to their large specific and volumetric capacities. The key challenge, however, has been the large volume changes, and the associated stress buildup and failure during cycling. The chemo-mechanics of alloy-based electrode materials entail interactions among diffusion, chemical reactions, plastic flow, and material property evolutions. In this study, a continuum theory of two-way coupling between diffusion and deformation is formulated and numerically implemented. Analyses based on this framework reveal three major conclusions. First, the stress-to-diffusion coupling in Li/Si is much stronger than what has been known in other electrode materials. Practically, since the beneficial effect of stress-enhanced diffusion is more pronounced at intermediate or higher concentrations, lower charging rates should be used during the initial stages of charging. Second, when plastic deformation and lithiation-induced softening take place, the effect of stress-enhanced diffusion is neutralized. Because the mechanical driving forces tend to retard diffusion when constraints are strong, even in terms of operational charging rate alone, Li/Si nano-particles are superior to Li/Si thin films or bulk materials. Third, the diffusion of the host atoms can lead to significant stress relaxation even when the stress levels are below the yield threshold of the material, a beneficial effect that can be leveraged to reduce stresses because the host diffusivity in Li/Si can be non-negligible at higher Li concentrations. A theory of coupled chemo-mechanical fracture driving forces is formulated in order to capture the effect of deformation-diffusion coupling and lithiation-induced softening on fracture. It is shown that under tensile loading, Li accumulates in front of crack tips, leading to an anti-shielding effect on the energy release rate. For a pre-cracked Li/Si thin-film electrode, it is found that the driving force for fracture is significantly lower when the electrode is operated at higher Li concentrations -- a result of more effective stress relaxation via global yielding. The results indicate that operation at higher concentrations is an effective means to minimize failure of thin-film Li/Si alloy electrodes.

<p><a>Low reflow temperature solder interconnect technology based on Sn-Bi alloys is currently being considered as an alternative for Sn-Ag-Cu solder alloys to form solder interconnects at significantly lower melting temperatures than required for Sn-Ag-Cu alloys. </a></p> <p>A new low temperature interconnect technology based on Sn-Bi alloys is being considered for attaching Sn-Ag-Cu (SAC) solder BGAs to circuit boards at temperatures significantly lower than for homogeneous SAC joints. Microstructure development studies of reflow and annealing, including Bi diffusion and precipitation, are important in understanding mechanical reliability and failures paths in the resulting heterogeneous joints. Experiments in several SAC-SnBi geometries revealed that Bi concentration profiles deviate from local equilibrium expected from the phase diagram, with much higher local concentrations and lower volume fractions of liquid than expected during short-time high temperature anneals in the two-phase region. As annealing time increased and Sn grain coarsening occurred, the compositions and fractions revert to the phase diagram, suggesting an “anti-Scheil” effect. A Bi interface segregation model based on Bi segregation at Sn grain boundaries was developed to explain the Bi distribution characteristics in Sn during two-phase annealing process. </p> <p>Besides hybrid joints, microstructural evolution after reflow and aging, especially of intermetallic compound (IMC) growth at solder/pad surface finish interfaces in homogeneous SnBi LTS joints, is also important to understanding fatigue life and crack paths in the solder joints. This study describes intermetallic growth in homogeneous solder joints of Sn-Bi eutectic alloy and Sn-Bi-Ag alloys formed with electroless nickel-immersion gold (ENIG) and Cu-organic surface protection (Cu-OSP) surface finishes. Experimental observations revealed that, during solid state annealing following reflow, the 50nm Au from the ENIG surface finish catalyzed rapid (Au,Ni)Sn₄ intermetallic growth at the Ni-solder interface in both Sn-Bi and Sn-Bi-Ag homogeneous joints, which led to significant solder joint embrittlement during fatigue testing. Further study found that the growth rate of (Au,Ni)Sn₄ intermetallic could be reduced by In and Sb alloying of SnBi solders and is totally eliminated with Cu addition. Fatigue testing revealed Au embrittlement is always present in solder joints without Cu, even with In and Sb additions due to (Au,Ni)Sn₄ formation. The fatigue reliability of Cu-containing alloys is better on ENIG due to the formation of (Ni,Cu,Au)₆Sn₅ at the solder-surface finish interface instead of (Au,Ni)Sn₄.</p> <p>With the development of SnBi LTSs, a new generation alloy called HRL1 stands out for its outstanding reliability during thermal cycling and drop shock testing. This study focused on microstructure evolution in SnBi eutectic, SnBiAg eutectic and HRL1 solders (MacDermid Alpha) homogeneous joints and hybrid joints with SAC305 formed with ENIG and Cu-OSP surface finishes. Experimental results revealed that with more microalloying elements, HRL1 has significantly refined microstructure and slower Sn grain growth rate during solid-state aging compared with SnBi and SnBiAg eutectic alloys. Intermetallic compound growth study showed that during solid state annealing following reflow, the (50nm) Au from the ENIG finish catalyzed rapid (Au,Ni)Sn₄ intermetallic growth at the Ni-solder interface in both Sn-Bi and Sn-Bi-Ag homogeneous joints, which led to significant solder joint embrittlement during creep and fatigue loading. However, (Au,Ni)Sn₄ growth and gold embrittlement was completely eliminated for HRL1 due to Cu additions in it, and HRL1 has significantly better fatigue reliability than SnBi and SnBiAg eutectic alloys on both OSP and ENIG surface finishes.</p>

The main objective of this dissertation is to explore and develop computer-aided-design methodologies and optimization techniques for reliability, performance, and power of through-silicon-via-based 3D IC designs. Through-silicon-via (TSV), a vertical interconnect element between dies, is the key enabling technology in 3D ICs. This new design element provides unprecedented design freedom as well as challenges. To maximize benefits and overcome challenges in TSV-based 3D ICs, new analysis methodologies and optimization techniques should be developed. In this dissertation, first, the robustness of 3D power delivery network is assessed under different power/ground TSV placement schemes and TSV RC variations. Next, thermo-mechanical stress and reliability problems are examined in full-chip/stack scale using the principle of linear superposition of stress tensors. Finally, physical design methods for low power 3D designs are explored to enhance the 3D power benefit over the 2D counterpart.

The focus of the research is on the development of the concept of floating flexible thin film arrays for renewable electricity generation, in marine and lacustrine application areas. This research was motivated by reliability issues from wave energy converters which are prone to large loads due to the environment which they are exposed in; a flexible system would not need to withstand these loads but simply yield to them. The solid state power take off is an advantage of photovoltaic (PV) technology which removes failure risks associated with mechanical machinery, and also potential environmental hazards such as hydraulic oil spillage. The novelty of this technology requires some development before it could even be considered feasible for large scale installation. Techno-economics are a big issue in electricity developments and need to be scoped in order to ensure that they would be cost-competitive in the market and with other technologies. Other more technical issues relate to the change in expected electrical yield due to the modulation of the PV array according to the waves and the electrical performance of the PVs when in wet conditions. Results from numerical modelling of the modulating arrays show that there is not expected variation in electrical yield at central latitudes (slightly positive), although at higher latitudes there could be considerable depreciation. With regards to the electrical performance a notable improvement was measured due to the cooling effect, slight decrease in performance was also estimated due to water absorption (of ~ 1.4%) within the panels. Overall results from both economic and technical analysis show the feasibility of the concept and that it is a possibility for future commercialisation.