Xiao Lin (Ph.D.)
Effect of Multiaxial Vibration on Fatigue Durability of Electronic Assemblies Populated With Short and Light Components
Multiaxial vibration can lead to nonlinear cross-axis interactions, which can significantly affect the fatigue durability of electronic printed circuit assemblies.
Depending on the frequency and phase relationship between the in-plane and out-of-plane excitations, there can be significant reduction or increase in fatigue durability.
Previous studies used experiments and nonlinear dynamic finite element simulations to highlight this nonlinear impact in tall, heavy electronic components such as insertion-mount inductors,
wherein the most severe multiaxial interactions resulted in damage that was twice the linear summation of damage caused by corresponding levels of sequential uniaxial vibrations.
The root-cause of the multiaxial effects was determined to be kinematic nonlinear interactions between large deformations in orthogonal axes.
This dissertation builds on this work by conducting two related studies described below.
The first study in this dissertation builds on a prior parametric study, to confirm the findings stated above for tall, heavy components.
Simple beam specimens are designed to represent the kinematic features of tall heavy components with flexible leads mounted on flexible printed circuit boards (PCBs).
Single and double beam tests were conducted with varying height and mass to parametrically examine the influence of geometry on nonlinear behavior.
The purpose of the single beam configuration is to identify the nonlinear role of the flexible leads and the purpose of the double-beam is to investigate the nonlinear interactions between
the flexible leads and the flexible PCBs. The tip response is recorded for various uniaxial and multiaxial excitation profiles to establish the nonlinear interactions.
Dynamic, nonlinear finite element analysis (FEA) is also performed, to validate the experimental results, confirming that kinematic nonlinearity plays a crucial role in the observed multiaxial nonlinear effects.
The second part of this study extends the multiaxial vibration durability investigation to examine the feasibility of nonlinear multiaxial interactions in short, light components, such as surface-mount gull-wing
quad-flat packages (QFPs), since the deformation magnitudes (and hence the kinematic nonlinear interactions) are expected to be significantly smaller in such cases.
Two different types of QFP components are used in printed circuit assemblies (PCAs): QFP100 and an low-profile component LQFP100.
Multiaxial random vibration experiments conducted in this study demonstrate that even these light, low-profile components exhibit strong multiaxial nonlinear effects.
In fact, the lower-profile LQFP100 demonstrates even higher multiaxial interaction than the regular profile QFP100.
These unexpected findings suggest the presence of an additional source of nonlinearity for cross-axis interaction: possibly cross-axis interaction due to material nonlinearity.
The failure mode is revealed to be predominantly lead fatigue in both QFP100 and LQFP100 assemblies.
To further explore these nonlinear effects, multi-scale nonlinear dynamic finite element models of QFP100 and LQFP100
assemblies are developed and vibration response is simulated for both uniaxial and multiaxial vibration.
In the interest of model simplification, the finite element simulations are limited to harmonic analysis, since the underlying physics of the nonlinearity is
expected to be the same for both random and harmonic vibration. These models are used to analyze strain distributions in the leads and to conduct fatigue analysis, providing deeper
insights into the material and geometric interactions that lead to multiaxial nonlinear effects. The simulations confirm the trends of the experimental results, illustrating that multiaxial
vibration can generate complex nonlinear interactions even in short, light electronic components under multiaxial vibration.
This dissertation highlights the importance of considering multiaxial nonlinear effects when designing and qualifying electronic components for dynamic environments, regardless of height or mass.
The findings emphasize the need for comprehensive testing and modeling approaches to accurately assess the reliability and durability of electronic assemblies
that are subjected to multiaxial vibrations, for both heavy, tall components as well as light, short components.
Major Hyaden Richards (Ph.D.)
Effects of Mechanical Shock on Reliability of Embedded Component Interconnects in Printed Hybrid Electronics at Elevated Temperatures
The advantages of Printed Hybrid Electronic (PHE) assemblies are of
considerable interest to designers of electro-mechanical systems,
especially for applications in extreme environments. With additional
development, PHEs may offer reliability advantages over traditional
electronic packages in fields like aerospace or applications such as
conformal circuits or integrated sensors. For this work, passive
components were recessed into machined cavities in injection-molded
polysulfone domes and beams by way of a unique ‘mill-and-fill’ method
combining traditional subtractive milling with extrusion-based paste
printing. The components were interconnected to printed silver traces
using printed solder, with circuits then formed from the silver
traces. These assemblies were subject to large strains caused by
mechanical shock at acceleration levels up to 100,000 g and at
temperatures from 25 °C to 125 °C
The populated beam specimens were subjected to drop
testing in a clamped-clamped configuration without secondary impact
using an accelerated-fall drop tower with dual mass shock amplifier,
resulting in substrate strain magnitudes of up to 50,000 µm/m at rates
up to ~1000 /s. Trace degradation characteristics were first
assessed, then the number of drops to failure (as defined by component
separation from the substrate) were documented across four different
component locations on a beam specimen, providing failure data for
four different strain histories. These four strain histories were
compiled across a total of seven different test points ranging from
25,000 g to 100,000 g and 25 °C to 125 °C. Concurrently, a combined
global-local finite element model was used to simulate the physical
response of the sintered silver within the trace adjacent to the
recessed component. This model was matched to experiments by direct
strain measurement in the substrate, supported by digital image
correlation.
Circuit failure occurred due to component separation from
the substrate caused by cracking within the sintered silver beneath
the soldered interconnect – a failure mode common across all
acceleration levels and temperatures. The dependence of rates of
degradation and failure on acceleration level was quantified based on
strain levels expected within the silver trace. Plastic strain
magnitude was used as the basis for damage accumulation in the
sintered silver. Collectively the experimental results and simulation
data were integrated by means of a cumulative damage model to generate
an application-agnostic low-cycle fatigue curve for the sintered
silver from 25-125°C.
Declan Mallamo (Ph.D.)
Sparse Feature Selection and Regime Identification for Advanced Prognostic Methods for Complex Systems
This dissertation introduces two novel methods-Sparse Multivariate Functional Fusion Predictors (SMFFP) and Data-Driven Regime Clustering with Spectral Approximation (DRC-SA)-to advance interpretable
prognostic modeling for complex systems, particularly aircraft engines and Switch-Mode Power Supplies (SMPS).
Traditional black-box approaches, such as deep learning, often achieve high predictive accuracy but lack interpretability, limiting their utility in safety-critical applications.
In contrast, SMFFP and DRC-SA provide a transparent, data-driven framework capable of capturing system degradation patterns with both precision and clarity.
The SMFFP method integrates sparse multivariate functional predictions with Koopman operator theory to create interpretable models.
By mapping nonlinear system dynamics into a linear framework, Koopman theory enables the identification of key observables that characterize system behavior.
Sparse feature selection techniques within SMFFP further enhance model clarity by isolating the most critical predictors while maintaining competitive predictive performance, even in data-constrained environments.
Complementing SMFFP, the DRC-SA method identifies operational regimes critical for accurate degradation predictions. Using spectral clustering with the Nyström approximation, DRC-SA effectively
clusters spatio-temporal patterns under both normal and anomalous conditions. This facilitates detailed classification of operational regimes, supporting the alignment of predictive insights with real-world operational changes and enabling the early detection of system
degradation.
Together, SMFFP and DRC-SA provide a robust and interpretable framework for Prognostics and Health Management (PHM) in complex systems. These methods address the critical need for predictive maintenance solutions that prioritize transparency and reliability in safety-critical applications.
Rudra Bipinbhai Vora (M.S.)
Comparison of Effects of Process Technology-Derived Input Parameters of Die-Level Failure Models.
In integrated circuits, time-dependent dielectric breakdown, hot carrier injection, and electromigration are primary die-level wear-out failure mechanisms. Failure models for these mechanisms can be used to assess and compare the parts based on their ability to withstand these failure mechanisms. The failure models require electrical input parameters: gate voltage, drain current, and current through interconnect, as well as dimensional input parameters: gate oxide thickness, gate length, and die metallization dimensions. These input parameters are unavailable in traditional documentation, such as datasheets and application notes. As a result, part users face difficulty using the failure model for part assessment. This thesis presents methodologies to obtain die-level electrical and dimensional input parameters for individual parts. The approach developed to find the input parameters uses the process technology information of a part and literature on process technology. The electrical input parameters for the time-dependent dielectric breakdown and hot carrier injection failure model are determined from transistor-level voltage-current characteristic curves provided in the literature on process technologies. The methodologies to determine the electrical input parameters are developed by utilizing transistor circuit information and the associated characteristic curves.
Part manufacturers use different technologies and design rules, leading to differences in input parameters such as die-level dimensions and electrical and environmental loads. These variations affect the ability of parts to withstand die-level failure mechanisms. Therefore, a die-level comparative assessment should be performed to compare and select the parts. Comparative assessment refers to quantifying and comparing the influence of die-level input parameters on time-to-failure of parts for individual die-level failure mechanisms using simulation-based design of experiments. Identifying the parameters that affect the part’s time-to-failure using a simulation-based design of experiments supports decision-making for derating considerations, acceptable manufacturing variations, and part selection. This thesis provides guidelines for extracting input parameters for die-level failure mechanisms and a methodology to perform comparative part assessment based on the application load condition of the system.
Jonathan Hower (M.S.)
Drop Durability Assessment of Electronic Assemblies Under Off-Axis Loading with Skewed Fixtures
This thesis studies drop durability of electronic assemblies when the acceleration vector is oriented at 45° to the out-of-plane direction of the circuit card. The off-axis drop tests are accomplished with a skewed fixture and are conducted as a proxy for multiaxial drop testing. Advanced shock testing and vibration test methods have been developed over the last few decades to better represent real-world field environments during ground-based laboratory testing. However, many of these test methods require expensive and specialized equipment not available in most laboratories. An alternative approach for approximating simultaneous loading along multiple axes on conventional equipment utilizes skewed fixtures which have seen use in off-axis random vibration and drop impact testing. These methods generally rely on the conversion of a uniaxial input load from the test equipment (using a uniaxial drop tower or shaker) into a multiaxial load when resolved in the reference frame of the test article (mounted on a skewed fixture).
Skewed fixture design is presented and recommendations for conducting skewed angle drop testing are introduced based on local measurements along the skewed face of the fixture to accurately monitor the impact event. Characterization tests were performed with a skewed fixture, at simultaneous acceleration loads from 500 to 3,000 g in two (in-plane and out-of-plane) directions, while meeting standard time domain tolerances. Upon experimental characterization, drop shock durability tests were conducted on a printed circuit assembly (PCA). Mean drops-to-failure were measured and quantified with Weibull statistics. Dominant solder joint failure modes were identified via failure analysis.
Prior work on inclined angle impact testing is limited, and the majority of interconnect-level solder fatigue studies are conducted considering perpendicular loading normal the circuit card. Low-cycle fatigue curves are generated based on plastic strain and plastic work density within the solder joint. A multi-scale nonlinear finite element model is used to relate board-level flexure to interconnect-level plastic strain. A high strain rate solder constitutive model allows for accurate modeling of solder plasticity resulting from high-impact drop shock. Fatigue parameters are computed from the Coffin-Manson relation and Palmgren-Miner damage accumulation. This work serves to apply established low-cycle fatigue methods for conventional drop shock loading (impact normal to circuit card) to non-perpendicular loading with a skewed fixture.
Welch, Jacob (M.S.)
Spectral Methods for Modeling and Estimating Vibration Fatigue Damage in Electronic Interconnects.
The purpose of this thesis is to explore the accuracy of fatigue damage estimation in printed wiring assembly (PWA) interconnects, using purely frequency-domain (also known as spectral) information such as the power spectral density (PSD) of the input excitation. The test case used in this study is the estimation of fatigue damage accumulation rate in the critical interconnects of low profile quad flat-pack (LQFP) components on a PWA, under broad-band random vibration excitation. This study examines whether the fatigue predictions made with this frequency-domain approach are consistent with those obtained from a direct time-domain approach. The frequency-domain response modeling is achieved using a two-stage global-local modeling process using a finite element model (in ABAQUS©), where the dominant modal participation factors for the dynamic response is obtained using a dynamic global simplified dynamic finite element model consisting of shell elements to represent the entire PWA. The PSD of the input excitation is applied as a boundary condition and the PSD of the PWA strain response is recorded at the base of critical components. The corresponding PSD for the dynamic strain response at critical interconnects is estimated with strain-transfer functions (STFs) for each dominant mode, obtained from detailed 3D quasi-static nonlinear local models of the component, adjacent PWB, and the interconnects. The global-local STF provides a relationship between the level of equivalent strain in the critical interconnects and the flexural strain at the adjacent surface of the PWB. The STF for each of the dominant vibration modes is obtained by imposing the corresponding mode-shape predicted by the dynamic global model on the PWB, in the quasi-static local model, using multi-point constraint equations. The PSD of the equivalent strain in the critical interconnect is then estimated via linear modal superposition. A deterministic estimate of the cyclic fatigue damage accumulation rate in the critical interconnect is then conducted with the Basquin high cycle fatigue (HCF) model and linear damage superposition approach, by using three different spectral approaches for representing the strain severity with estimated probability density functions (PDFs). The three approaches include: (i) Raleigh method; (ii) Dirlik method and (iii) Range distribution function created with the Rainflow cycle counting method. Methods (ii) and (iii) are derived from a pseudo time-history created with an inverse Fourier transform. These frequency-domain results are compared to corresponding fatigue damage estimates from a multi-modal time-domain analysis method, to assess the consistency of the two approaches.
Harsha Walvekar (M.S.)
Component Selection for Use Beyond Manufacturer's Specifications.
Products in many industries need electronic parts that can operate over wide temperature ranges.
Electronic parts may not always be rated to meet the application requirements. The uprating
process was developed in the late 1990s as a possible method to address this problem. Uprating
is a process to assess the ability of a part to meet the functionality and performance requirements
of the applications in which the part is used outside the manufacturers’ specification range.
Uprating is built into the performance assessment step as part of the part selection and
management process and requires validation through electrical testing. This process can be
resource-intensive in terms of time and money, and the difficulties are higher with an increase in
part complexity. Therefore, there is a need to preselect parts that have the potential to be uprated.
This thesis developed an uprateability assessment process to facilitate this pre-selection. This
process utilizes the thermal ratings information for the parts, including absolute maximum
ratings and recommended operating conditions.
In this thesis, a quantitative analysis of 140 datasheets (representing over 500 part numbers) is
performed for absolute maximum ratings, recommended operating conditions, assembly
information, thermal resistances, and temperature dependence of electrical parameters. The
analysis identifies the best practices of information sharing, along with inconsistencies and
incompleteness in the information provided by the manufacturers. This thesis provides a
methodology to establish recommended operating conditions and absolute maximum ratings
when unavailable in the datasheets. It outlines the criteria necessary to verify a comprehensive
rating section, assigning an information availability level to the thermal ratings to assess the
quality of the information provided by the manufacturer, and recommends sources to obtain the
missing information.
This thesis also provides an uprateability assessment methodology to identify the components
that have the potential to be uprated out of an initial pool of components. This process evaluates
and eliminates the parts that cannot be uprated, narrowing the potential candidates for uprating
and reducing the cost of the selection process.