A transmission electron microscopy study of defect generation and microstructure development in ultrasonic wire bonding

Ultrasonic wire bonding is widely used in the electronic industry to connect semiconductor chips to packages. Even though the popularity of the technique has increased in recent times, questions remain about the bonding mechanism, and factors affecting bondability and reliability. In this thesis, answers were provided to many of these questions using TEM to examine bonded cross section and plan view specimens. A detailed investigation of the Al wire and substrate showed dynamically annealed well recovered grains while microstructural observations of other substrates revealed wide varieties of response mechanisms. For example, Ni formed a dislocation cell structure, Cu formed a partially recovered structure, while Cu alloys and stainless steel formed planar dislocation arrays. These observed transformations were correlated with basic material properties and literature reported cyclic deformation studies to determine factors contributing to substrate plastic deformation during bonding. It appeared that the plastic deformation of the substrate is not a requirement for good bonding, but since extensive plastic deformation can occur during bonding, it could have important implications in bond strength and reliability. A model developed to estimate microstructural transformations was effective when applied to different metal substrates but somewhat less effective with Cu alloys. The extent and type of intermetallic phases that formed at the wire-substrate interface after thermal aging, thermal cycling and in the as-bonded conditions were characterized for different Au and NiB plated substrates using EDS. Specimens were also examined for the extent of Kirkendall porosity and the conditions of the unreacted portions of the wire and substrate. It was found that the extent of interfacial reactions depended strongly on substrate metallurgy. For example, in the NiP/immersion Au specimen the original Au layer was still present after bonding, and transformed completely to Al-Au intermetallics after only 1.5 h at 90\sp\circC. The same treatment resulted in no intermetallic phase formation for Ni-B specimens with the interface remaining chemically and structurally sharp. Finally, mechanisms of bonding and microstructural development were proposed

Variable-parameter NiTi ultrasonic spot welding with Cu interlayer

NiTi thin sheets were ultrasonic spot welded with a Cu interlayer, where different welding vibration amplitudes were applied to study the influence on the surface and interface microstructural characteristics, phase transformation behavior and mechanical response of the joints, which aimed to enhance the joint performance by proper optimization of the process parameters. An excellent bonding interface was achieved when an optimized vibration amplitude was applied, with a recrystallized microstructure formed in the Cu foil side near the bonding interface, which helped to improve the mechanical performance of the joints. Joints made with vibration amplitude of 55 mu m had an improved strength compared to the NiTi base material

Accelerated mechanical fatigue testing and lifetime of interconnects in microelectronics

AbstractDue to the rapid development of packaging industry accelerated reliability testing for evaluation of lifetime of electronic components are increasingly utilized. In addition to common active thermal cycling procedures, accelerated mechanical fatigue testing provides a new possibility to assess the reliability of microelectronic components, mainly due to the significantly shorter duration of testing time. In this investigation we have used an ultrasonic fatigue testing system in combination with a special experimental set-up for qualification and lifetime determination of microelectronic interconnects. Using this technique, fatigue life of Al wire bonded interconnects were determined and S-N curves (shear stress as a function of loading cycles) up to N=109 were plotted. Three dimensional elasto-plastic FEM simulations were performed to determine the distributions of shear stress and plastic strain generated during cyclic fatigue in the bond area. Furthermore, the FEM model was employed to predict the lifetime of wire-bonds. The results were correlated to the lifetime curves of similar bonds obtained by power cycling tests. Detailed microstructural investigations were performed by means of EBSD -SEM to study the evolution of microstructure of the interconnects subjected to thermal and mechanical fatigue loading. This study demonstrates the applicability of accelerated mechanical fatigue testing as an alternative to time consuming thermal cycling for qualification of microelectronic interconnects

A study on ultrasonic energy assisted metal processing : its correeltion with microstructure and properties, and its application to additive manufacturing.

Additive manufacturing or 3d printing is the process of constructing a 3-dimensional object layer-by-layer. This additive approach to manufacturing has enabled fabrication of complex components directly from a computer model (or a CAD model). The process has now matured from its earlier version of being a rapid prototyping tool to a technology that can fabricate service-ready components. Development of low-cost polymer additive manufacturing printers enabled by open source Fused Deposition Modeling (FDM) printers and printers of other technologies like SLA and binder jetting has made polymer additive manufacturing accessible and affordable. But the metal additive manufacturing technologies are still expensive in terms of initial system cost and operating costs. With this motivation, this dissertation aims to develop and study a novel metal additive manufacturing approach called Acoustoplastic Metal Direct-Write (AMD) that promises to make metal additive manufacturing accessible and affordable. The process is a voxel based additive manufacturing approach which uses ultrasonic energy to manipulate and deposit material. This dissertation demonstrates that the process can fabricate near-net shape metal components in ambient conditions. This dissertation investigates two key phenomenon that govern the process. The first phenomenon investigated is ultrasonic/acoustic softening. It is the reduction in yield stress of the metals when being deformed under simultaneous application of ultrasonic energy. A detailed analysis of the stress and microstructure evolution during ultrasonic assisted deformation has been presented in this dissertation. Crystal plasticity model modified on the basis of microstructure analysis has been developed to predict the stress evolution. The 2nd phenomenon investigated is ultrasonic energy assisted diffusion that enables the bonding of voxels during the AMD process. High resolution Transmission Electron Microscopy (HRTEM) and Energy Dispersive Spectroscopy (EDS) analysis has been used to quantify this phenomenon and also distinguish the process mechanics from other foil or sheet based ultrasonic joining processes

The Effect of Ultrasonic Power in Aluminum Wire Bonding Hardness Profiles

Ultrasonic wire bonding is a critical process used widely across the microelectronics industry. Despite its ubiquity, there is a breadth of literature and ongoing active research into the basic principles of wire bonding. In particular, the effects of ultrasonic bonding on material properties are not fully understood. This thesis presents the effects of different ultrasonic bond powers on material properties. The changes in mechanical properties were measured by collecting Vickers microhardness data and nanoindentation data. The hardness in the bonded wire varied with two parameters: the distance from the bond interface, and the applied ultrasonic power. The hardness varied 5 HV across the profile of a bond and a 5 HV difference was also measured due to change in bond power. In addition, the measured hardness of the bonds was lower by up to 10 HV than calculated hardness values based on strain hardening only. These trends were found with the microhardness data and corroborated by nanoindentation results. This work provides a method to further study the effects of additional bonding parameters on mechanical properties

Calibration of a novel microstructural damage model for wire bonds

In a previous paper, a new time-domain damage-based physics model was proposed for the lifetime prediction of wire bond interconnects in power electronic modules. Unlike cycle-dependent life prediction methodologies, this model innovatively incorporates temperature- and time-dependent properties so that rate-sensitive processes associated with the bond degradation can be accurately represented. This paper presents the work on the development and calibration of the damage model by linking its core parameter, i.e., “damage,” to the strain energy density, which is a physically quantifiable materials property. Isothermal uniaxial tensile data for unbonded pure aluminum wires (99.999%) have been used to develop constitutive functions, and the model has been calibrated by the derived values of the strain energy density

Stability of Microstructure in Al3003 Builds Made by Very High Power Ultrasonic Additive Manufacturing

Very High Power Ultrasonic Additive Manufacturing (VHP-UAM) system was used to produce aluminum parts from 150 µm thick Al3003-H18 foils. The build was processed at 36 μm vibration amplitude, 8 kN normal load, and 35.6 mm/s weld speed at 20 kHz frequency. Almost 100% linear weld density was achieved. A deformation-interaction volume of ~20 μm was observed below the bonded interface. The microstructural stability including grain boundary structures, and crystallographic orientations was evaluated after annealing these samples at 343oC for 2 hours and 450oC for 2 hours. After heat treatment, small grains persisted at the interfaces with sluggish grain growth kinetics. In contrast, normal grain growth kinetics was observed in the middle of the foils. Possible mechanisms for such phenomena are discussed.Mechanical Engineerin

Fundamental Understanding of Bond Formation During Solid State Welding of Dissimilar Metals

Dissimilar metal welds are used in a wide range of applications to effect light weighting and for corrosion resistance. While fusion welding techniques are limited in their ability to fabricate dissimilar metal welds, solid state welding techniques are limited in their ability to fabricate complex geometries with dissimilar metal combinations. Hence alternative techniques need to be explored to fabricate complex geometries with dissimilar metals welds in the solid state. Ultrasonic additive manufacturing in a solid state additive manufacturing process that combines ultrasonic welding with mechanized tape layering to fabricate dissimilar metal welds in the solid state. Though extensive feasibility studies have been performed to fabricate dissimilar metal welds using ultrasonic additive manufacturing, the fundamental mechanisms related to the bond formation mechanism are not fully understood. In this work multi scale characterization using scanning electron microscopy, electron backscatter diffraction, nano indentation and atom probe tomography was performed to rationalize the mechanism of bond formation in dissimilar metal welds. The fundamental questions that needed to be answered were Is possible for a solid state bond to form with extensive plastic deformation occurring only on one metal in a dissimilar metal combination The effect of plastic deformation on the oxide layer at the interface of dissimilar metal welds. To answer the above questions various dissimilar metal combinations (Steel-Ta) BCC-BCC, (Al-Ti) FCC-HCP, (Al-Steel) FCC-BCC were fabricated using ultrasonic additive manufacturing and characterized using the above-mentioned techniques. Bonded regions were characterized to study the role of plastic deformation by analyzing the micro texture developed at the interface. The general conclusion is the presence of a strong shear texture in the softer metal while the harder metal did not show any evidence of change in texture. To understand the effect of plastic deformation on the oxide dispersion atom probe tomography analysis was performed and the results indicate the possibility of oxide breakdown resulting in oxygen super saturation in the lattice. The bond formation is hypothesized to occur as a result of plastic deformation localized in the softer metal alone

Effects of Plastic Deformation From Ultrasonic Additive Manufacturing

Nuclear energy technology can be exponentially advanced using advanced manufacturing, which can drastically transform how materials, structures, and designs can be built. Ultrasonic Additive Manufacturing (UAM) represents one of the four main additive manufacturing methods, although it is also the newest. As UAM technology and applications develop, a fundamental understanding of the bonding mechanism is crucial to fully realize its potential. Currently UAM bonding is considered to occur through breaking down surface asperities and removing surface oxides. Plastic deformation occurs although its role is currently unclear. This research analyzes material configurations in a variety of geometries, with similar and dissimilar material interfaces, and with pure metals and complex engineered materials. A variety of characterization techniques were used to develop a general description that UAM bonding requires plastic deformation. First, we analyzed various dissimilar material interfaces created between UAM foils and the coating of embedded optical fibers. Enhanced interdiffusion of elements was found beyond that expected from the thermal profile experienced during bonding. This interdiffusion was rationalized based on enhanced point defect vacancies creating additional diffusion pathways. Following on this study, we analyzed the local strengthening at one of these interfaces. These interfaces strengthened through a complex interaction dominated by dislocation forest hardening, reduced grain sizes, and vacancy clusters created by the agglomeration of vacancies. UAM bonding of pre-treated Al 6061 was also performed and analyzed using multi-length scale characterization. Macroscale strengthening was observed as well as foil-foil interface strengthening. This was a result of dynamic recrystallization, dynamic recovery, adiabatic heating, and precipitate dissolution (as the vacancies allowed enhanced diffusion of elements). Finally, UAM bonding of titanium was analyzed. The HCP phase of titanium significantly resisted plastic deformation, which resulted in a phase transformation to the BCC phase, which was stabilized by the introduction of certain stabilizing elements. The strain induced phase transformation and enhanced vacancy driven interdiffusion were utilized to demonstrate a viable method of improving UAM bonding by focusing on the plastic deformation requirement. The phenomena outlined in this research demonstrates an improvement in our understanding of the fundamental bonding requirements of UAM, and deformation induced vacancy formation