Leslie, David Buchanan (Ph.D.)
Length-Scale Dependence of Viscoplastic Properties of Silver Sinter Revealed by Indentation Testing and Modeling
This doctoral dissertation research focuses on using a combination of indentation testing and modeling to characterize the creep behavior of heterogeneous silver sinter at different temperatures, using multiple indenter sizes to interrogate length-scale effects. The measured steady-state creep deformation is characterized with three different modeling approaches, that rely on: (i) conventional deviatoric creep potential; (ii) pressure-sensitive Drucker-Prager creep potential; and (iii) length-scale dependent deviatoric creep potential. The creep flow rule for all three cases is Norton’s power-law creep.
The materials in this study are from a family of sintered silver materials used for interconnects and die-attach in high-temperature electronics and for conductor traces in printed electronics. The dissertation focuses on identifying and quantifying the length-scale dependence presented by sintered materials due to their non-homogeneous morphology. Testing consists of constant-force indentations using spherical indenters of two different radii at three different temperatures: 25°C, 75°C, and 125°C.
The indentation results were first analyzed using two different post-processing methods: an empirical approach with closed-form models (CFM) and a computational FEA approach based on classical continuum mechanics. Differences found between the CFM and numerical (FEA) analyses, while significant at room temperature, reduce with temperature. Both models reveal that indenters of different radii cause significantly different viscoplastic behavior. This dependence on tip radius increases with temperature.
The research was extended to examine two second-order influences of the metallic agglomerated phase and the discontinuous compliant phase of the microstructure of sintered silver on its viscoplastic behavior: (i) dependence on hydrostatic stress; (ii) dependence on microstructural length-scale. The aim of incorporating the pressure-sensitive modeling was to investigate what effect the intrinsic compressive hydrostatic stress in indentation tests might have on the measured viscoplastic properties. Results from using the Drucker-Prager creep model further confirmed the increasing dependence on length-scale with temperature.
The length-scale dependence seen in all the above results is investigated and quantified with the help of a simplified strain-gradient viscoplastic model. This modeling approach is motivated by the conventional mechanism-based strain gradient (CMSG) model that is widely used in plasticity theory to quantify length-scale effects. The characteristic length-scale metric in this problem is presented by the agglomerate size distribution in the sintered material and is quantified in this study with ‘watershed analysis’ of cross-sectional features observed via electron microscopy. This discrete length scale is believed to cause the variations in the observed creep response when queried with indenters of different radii, because of the different strain gradients produced by the two different indenters. The length-scale dependence is incorporated in a strain-gradient viscoplastic constitutive model suitable for finite element modeling of deformation fields containing strong strain-gradients (e.g. in the die attach layer in microelectronics chip assembly). Finally, a procedure is proposed, to incorporate the scale-dependence in the empirical closed-form approach, currently available in the literature, for extracting viscoplastic properties from indentation tests. This approach provides corrected model constants for the strain-gradient viscoplastic model, using simple closed-form equations instead of expensive finite element modeling.
Phansalkar, Sukrut Prashant (Ph.D.)
Mold Process Induced Residual Stress Prediction using Cure Extent Dependent Viscoelastic
Behavior
Epoxy molding compounds (EMC) are widely used in encapsulation of semiconductor packages. Encapsulation protects the package from physical damage or corrosion due to harsh environments. Molding processes produce residual stresses in encapsulated components. They are combined with the stresses caused by the coefficient of thermal expansion (CTE) mismatch to dictate the final warpage at room and reflow temperatures, which must be controlled for fabrication of redistribution layer (RDL) as well as yield during assembly.During molding process, EMC is continuously curing and the mechanical properties continue to evolve; more specifically, the equilibrium modulus and the relaxation modulus. The former defines behavior after complete relaxation while the latter describes the transient behavior. It is thus necessary to measure cure-dependent viscoelastic properties of EMC to be able to determine mold induced residual stresses accurately. This is the motivation for this thesis.In this thesis, a set of novel methodologies are developed and implemented to quantify a complete set of cure-dependent viscoelastic properties, including the fully cured properties. Firstly, an advanced numerical scheme has been developed to quantify cure kinetics of thermosets with both single and dual-reaction systems. Secondly, a unique methodology is proposed to measure the viscoelastic bulk modulus - of EMC using hydrostatic testing. The methodology is implemented with a unique test setup based on inert gas. The results of viscoelastic testing along with the shear modulus () and bulk modulus () master curves and temperature-dependent shift factors () of fully-cured EMC are presented.Thirdly, a novel test methodology utilizing monotonic testing has been developed to measure two sets of equilibrium moduli - and of EMC as a function of cure extent . The main challenge for the measurements is that the equilibrium moduli could only be measured accurately only when the EMC has fully relaxed. The temperatures for complete relaxation typically occur above the glass transition temperature, , where the curing rate is also high. A special measurement procedure is developed, through which the EMC moduli above can be determined quickly at a near isocure state. Viscoelastic testing of partially-cured EMC is followed to determine the cure-dependent shift factors of EMC. The test durations have to be very long (several hours) and it is conducted below of the EMC where the reaction is slow (under diffusion-control).Finally, a numerical scheme that can utilize the measured cure-dependent viscoelastic properties is developed. It is implemented to predict the residual stress evolution of molded packages during and after molding processes.
Akhavantaheri, Hirbod (Ph.D.)
Sociotechnical Network Modeling to Quantitatively Analyze the Impact of Blockchain on the Risk of Procuring Counterfeit Electronics
This dissertation develops sociotechnical agent-based network modeling to quantitatively analyze the impact of blockchain and other related policies on the supply-chain risk associated with the procurement of counterfeit electronics in critical systems. Safety-critical, mission-critical, and infrastructure-critical systems (e.g., aerospace, transportation, defense, and power generation) are forced to source parts from a supply chain that they do not control over exceptionally long periods of time. Critical systems are exposed to the dual risks of the impacts of system failure and the exposure to the vagaries of the marketplace over decades. Therefore, critical-systems operators, manufacturers, and sustainers, must implement policies and technologies to reduce the risk of obtaining counterfeit parts. Several policies, ranging from debarment and claw back to “hop counting,” have been considered and used to mitigate such risks. One technology that critical-system operators, manufacturers, and sustainers could adopt is distributed digital ledger (i.e., blockchain) for the supply chain. This dissertation does not focus on how such a blockchain could be implemented but rather on how (and if) blockchain for the supply chain can provide value for verifying the authenticity of parts when prolonged periods of time (decades) elapse between part manufacturing and part sourcing. Additionally, during a part’s ownership changes, supply-chain actors may choose to participate in the distributed ledger based on individual incentives and can recuse themselves from such participation later. The lack of complete participation may affect the designed functionality, and the consequences of lack of participation need to be understood. Using a comprehensive supply-chain model, it can be shown that blockchain for supply chain can reduce the prevalence of counterfeit electronics in the supply chain of critical systems by up to 70%. However, such a reduction requires near complete participation by all supply-chain stakeholders, which is not likely. Due to the relatively high cost of ownership transfer on a blockchain, and the indirect cost of supply-chain information disclosure, a high participation rate is not anticipated. Although blockchain can have benefits in other aspects of supply chain, it may not be a viable solution to combat the counterfeit electronics problem.
Bascolo, Manuel (M.S.)
Fatigue Degradation Sensing with Surface Mounted Conjugate-Stress (CS) Sensor
This study advances a unique dual-stiffness mechanical sensor concept in the literature (termed Conjugate-stress sensor), to evaluate and validate the effectiveness of surface-mounted versions of the Conjugate Stress (CS) sensor in detecting cyclic fatigue progression under both quasi-static axial cycling and dynamic flexural loading conditions. The CS sensor’s capability to recognize fatigue was examined by observing the correlation between its readings and the host material's stiffness. Low carbon steel dog bone coupons and welded cruciform specimens were subjected to quasi-static cyclic fatigue testing. Additionally, dynamic flexural tests were performed on low carbon steel cantilever beams and welded cruciform specimens, which underwent random vibration fatigue testing. The results demonstrated that CS sensors consistently track fatigue damage, offering a promising potential for in-situ structural health monitoring and for providing continuous, real-time estimations of the remaining useful life (RUL) of materials.