Mechanical Engineering
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Item ASSESSMENT OF PROPERTIES OF TRANSIENT LIQUID PHASE SINTERED (TLPS) INTERCONNECTS BY SIMULATION AND EXPERIMENTS(2017) Greve, Hannes Martin Hinrich; McCluskey, Patrick; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Growing power densities of electronic products and application of electronic systems in high temperature environment increase the temperature requirements on electronic packaging systems. Conventional interconnect technology was designed for devices based on silicon semiconductor technology limited to 175 °C and below. The introduction of wide bandgap semiconductor materials such as silicon carbide and gallium nitride expands the potential application temperature range to 500 °C beyond the range of conventional electronic packaging solutions. Transient Liquid Phase Sintering (TLPS) is a promising high temperature, high strength, low cost interconnect technology solution. TLPS is a liquid-assisted sintering process during which a low melting temperature constituent melts, surrounds, and diffuses with a high melting temperature constituent. A shift towards higher melting temperatures occurs as the low melting temperature phase is transformed into high melting temperature intermetallic compounds (IMCs). In this work, three TLPS sinter paste systems based on the copper-tin (Cu-Sn), nickel-tin (Ni-Sn), and copper-nickel-tin (Cu-Ni-Sn) material systems are designed. A novel process for their application as electronic interconnects is developed. Processing and thermal aging studies are performed to determine times to process completion characterized by high-temperature capability of the joints. Microstructural convergence durations are studied for each of the material systems. A modeling approach is developed to model realistic joint geometries with varying types, sizes, and distributions of metal particles and voids in intermetallic matrices. These are used to predict the constitutive (elastic-plastic) stress-strain responses and thermal properties of these systems by simulation. The constitutive models derived by this approach are compared to constitutive properties determined experimentally by Iosipescu shear samples with TLPS joints. The thermal properties of TLPS joints are determined experimentally by transient thermal response analyses. Failure mechanisms driven by thermal and thermo-mechanical stressors are predicted and verified, and mitigation techniques are developed.Item Multi-Scale Dynamic Study of Secondary Impact During Drop Testing of Surface Mount Packages(2016) Meng, Jingshi; Dasgupta, Abhijit; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation focuses on design challenges caused by secondary impacts to printed wiring assemblies (PWAs) within hand-held electronics due to accidental drop or impact loading. The continuing increase of functionality, miniaturization and affordability has resulted in a decrease in the size and weight of handheld electronic products. As a result, PWAs have become thinner and the clearances between surrounding structures have decreased. The resulting increase in flexibility of the PWAs in combination with the reduced clearances requires new design rules to minimize and survive possible internal collisions impacts between PWAs and surrounding structures. Such collisions are being termed ‘secondary impact’ in this study. The effect of secondary impact on board-level drop reliability of printed wiring boards (PWBs) assembled with MEMS microphone components, is investigated using a combination of testing, response and stress analysis, and damage modeling. The response analysis is conducted using a combination of numerical finite element modeling and simplified analytic models for additional parametric sensitivity studies.Item Effect of Dynamic Flexural Loading on the Durability and Failure Site of Solder Interconnects in Printed Wiring Assemblies(2007-12-04) Varghese, Joseph; Dasgupta, Abhijit; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation investigates the durability of solder interconnects of area array packages mounted on Printed Wiring Assemblies (PWAs) subjected to dynamic flexural loads, using a combination of testing, empirical curve fitting and mechanistic modeling. Dynamic 4-point bend tests are conducted on a drop tower and with an impact pendulum. Failure data is collected and an empirical rate-dependent durability model, based on mechanistic considerations, is developed to estimate the fatigue failure envelopes of the solder, as a function of solder strain and strain-rate. The solder plastic strain histories are obtained from the PWA flexural strain and strain rate, using transfer functions developed from 3D transient Finite Element Analysis (FEA) with rate-dependent solder material properties. The test data also shows the existence of multiple competing failure sites: solder, copper trace, PWB under solder pads, and layers of intermetallic compound (IMC) between the solder and solder pads. The failures in the IMC layers are found to be either in the bulk of the IMC layers or at the interface between different species of IMC layers. The dominant failure site is found to be strongly dependent on the loading conditions. The empirical model is demonstrated for solder failures as well as Cu trace failures, and the transition between their competing failure envelopes is identified. This dissertation then focuses in detail on two of these competing failure sites: (i) the solder and (ii) the interface between two IMC layers. A strain-range fatigue damage model, based on strain-rate hardening and exhaustion of ductility, is used to quantify the durability and estimate the fatigue constants of the solder for high strain rates of loading. Interfacial fracture mechanics is used to estimate the damage accumulation rates at the IMC interface. The IMC failure model and the solder failure model provide a mechanistic perspective on the failure site transitions. Durability metrics, based on the mechanics of these two failure mechanisms, are used to quantify the competing damage accumulation rates at the two failure sites for a given loading condition. The results not only identify which failure site dominates but also provide estimate of the durability of the solder interconnect. The test data shows good correlation with the model predictions. The test vehicles used in this study consist of PWAs with Sn37Pb solder interconnects. But the proposed test methodologies and mechanistic models are generic enough to be easily extended to other emerging lead free solder materials. Wherever possible, suggestions are provided for the development of test techniques or phenomenological models which can be used for engineering applications. A methodology is proposed in the appendix to implement the findings of this thesis in real-world applications. Damage in the solder interconnect is quantified in terms of generic empirical metrics, PWA flexural strain and strain rate. It is shown that the proposed metrics (PWA strain and strain rate) can quantify the durability of the solder interconnect, irrespective of the loading orientation or the PWA boundary conditions.Item DEVELOPMENT OF MOIRÉ INTERFEROMETRY FOR REAL-TIME OBSERVATION OF NONLINEAR THERMAL DEFORMATIONS OF SOLDER AND SOLDER ASSEMBLY(2005-04-20) Cho, Seungmin; Han, Bongtae; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)An experimental apparatus using moiré interferometry is developed to characterize the thermo-mechanical behavior of solder joints. A compact moiré interferometer is combined with an environmental chamber to allow real-time observation of non-linear and time-dependent solder and solder assemblies. The first apparatus is based on convection heating and cooling to simulate an accelerated thermal cycling (ATC) condition. Vibrations caused by an environmental chamber are circumvented by unique rigid links that connect the specimen to the moiré interferometer. Displacement fields are documented while the chamber is being operated. The system is utilized to analyze thermo-mechanical behavior of a ceramic ball grid array package assembly and a plastic ball grid array package assembly. The effect of thermal cycling on the accumulated permanent deformation is documented, which reveals the temperature-dependent non-linearity of solder joints. The second apparatus is based on conduction heating and cooling to achieve a high ramp rate. A special chamber is designed and fabricated using a high power thermoelectric cooler to achieve the desired ramp rate. The system is utilized to investigate the time-dependent behavior of solder joints. A new solder joint configuration is designed and fabricated to be tested with the conduction based apparatus. The specimen is an extension of the conventional bi-material joint configuration but the unique design offers two important features; it negates the inherent shortcoming from cross sectioning required in moiré interferometry and produces a virtually uniform shear strain field at the solder joint. The deformation of solder joint is documented at a controlled ramp rate over several thermal cycles. The experimental results are analyzed and compared with those of Finite Element analysis to investigate the validity of solder constitutive models available in the literatures.