The Viscoelastic Continuum Damage (VECD) model has been implemented into a finite element package (FEP++) to predict the fatigue performance of asphalt concrete (AC) mixtures tested at the Federal Highway Administration Accelerated Load Facility (FHWA ALF) and the Korea Expressway Corporation (KEC) test road project sites. Both the VECD model and the FEP++ were developed at North Carolina State University. The conceptual approach taken for this research is to separate the characteristics of the pavement system that are related to the material from those related to the boundary conditions. It is believed that this study is the first application of an integrated structural/material mechanistic model for the fatigue performance prediction of AC pavements where damage in the asphalt layers is considered for the full time history and where the change in stiffness due to damage evolution is captured in the subsequent calculation of damage. The VECD model accounts for the viscoelastic nature of AC mixtures with growing damage, whereas the finite element model accounts for other important characteristics, such as temperature, layer thickness, stiffness gradient, etc. The controlled FHWA ALF experiment allows a direct comparison between observed and modeled fatigue performance. Because the KEC test road experiment is subjected to real and variable traffic and environmental factors, the finite element simulation results are used to examine the effects of specific parameters in the pavement system on fatigue performance. In this regard, the model is found to effectively capture the effects of changes in layer thickness, layer material, and layer type. The need to develop transfer functions for true field performance prediction is also shown, and a simple example function is developed as proof of this concept.

This dissertation presents a new type of uniaxial viscoplastic rate model based on viscoelastic convolution integrals for explaining the behavior of asphalt concrete in compression under repeated loading. Triaxial compression cyclic tests carried out for long rest periods, with different loading times and two different pulse shapes, square and haversine by researchers at North Carolina State University were used in developing the model. These tests demonstrate that the evolution of permanent deformation depends on load history. This history-dependent behavior is not captured accurately by some of the existing Perzyna-type viscoplastic models in which permanent deformation evolution depends on the current values of stress and viscoplastic strain. Therefore, in this study, viscoelastic-like integrals were used in the rate model to capture the effect of history.As a simplification, and to better understand some aspects of the rate model, an incremental model was derived from the rate model by making assumptions of steady state conditions and slow hardening growth. This incremental model is capable of capturing material behavior observed from the cyclic tests mentioned above thereby verifying the rate model.The proposed uniaxial viscoplasticity rate model is applicable to compressive creep and recovery experiments at 54°C with 1) several hundreds of cycles of loading including the secondary creep region, 2) haversine loading shapes at three different peak deviatoric stress levels, 620 kPa, 827 kPa, and 1034 kPa, and square loading shapes at 827 kPa peak deviatoric stress, and 3) long rest periods that allow complete viscoelastic recovery. A notable advantage of the model is the ability to predict the primary creep region behavior without the need for a separate model. Compared to many of the existing viscoplasticity models, the proposed uniaxial viscoplasticity has a relatively simple model form with a physical mechanical analog that can be used to intuitively understand the model behavior.Yet, unlike the existing viscoplasticity models, the model has been successful in capturing the behavior of asphalt concrete in compression under a large number of cycles of creep and recovery loading with long rest period.

Thermal interface materials (TIMs) play a vital role in the performance of electronic packages by enabling improved heat dissipation. These materials typically have high thermal conductivity and are designed to offer a lower thermal resistance path for efficient heat transfer. For some semiconductor components, thermal solutions are attached directly to the bare silicon die using TIM materials, while other components use an integrated heat spreader (IHS) attached on top of the die(s) and the thermal solution attached on top of the IHS. For cases with an IHS, two TIM materials are used—TIM1 is applied between the silicon die and IHS and TIM2 is used between IHS and thermal solution. TIM materials are usually comprised of a polymer matrix with thermally conductive fillers such as silica, aluminum, alumina, boron nitride, zinc oxide, etc. The polymer matrix wets the contact surface to lower the contact resistance, while the fillers help reduce the bulk resistance by increasing the bulk thermal conductivity. TIM thickness varies by application but is typically between 25 μm and around 250 μm. Selection of appropriate TIM1 and TIM2 materials is necessary for the reliable thermal performance of a product over its life and end-use conditions. It has been observed that during reliability testing, TIM materials are prone to degradation which in turn leads to a reduction in the thermal performance of the product. Typical material degradation is in the form of hardening, compression set, interfacial delamination, voiding, or excessive bleed-out. Therefore, in order to identify viable TIM materials, characterization of the thermomechanical behavior of these materials becomes important. However, developing effective metrologies for TIM characterization is difficult for two reasons: TIM materials are very soft, and the sample thickness is very small. Therefore, a well-designed test setup and a repeatable sample preparation and test procedure are needed to overcome these challenges and to obtain reliable data. In this paper, we will share some of the TIM characterization techniques developed for TIM material down-selection. The focus will be on mechanical characterization of TIM materials—including modulus, compression set, coefficient of thermal expansion (CTE), adhesion strength, and pump-out/bleed-out measurement techniques. Also, results from several TIM formulations, such as polymer TIMs and thermal gap pads, will be shared.

Permanent deformation (rutting) is one of the major distresses in asphalt pavement. To predict permanent deformation of asphalt concrete, repeated creep and recovery (or flow number) tests are typically used in the laboratory. However, models for the prediction of permanent deformation that incorporate flow number testing cannot represent the primary region because they concentrate on the secondary region. A new simple permanent deformation model called the incremental model is proposed. The proposed model is derived from the rate model, which is a rigorous mechanical model based on viscoplasticity. Four parameters of the new model provide an understanding of the permanent deformation. Parameter A is related to the initial permanent strain level, and Parameter C provides information about where the secondary region starts. That is, Parameters A and C govern the primary region, where α (alpha) is the slope of the secondary region, and B represents the permanent deformation level of the secondary region. Two mixtures are selected to investigate the deformation characteristics, and repeated creep and recovery tests are performed in compression. The incremental model is verified by applying it to various loading conditions for two mixtures. Furthermore, it is found that α is the material constant and the time-temperature superposition principle is applicable to each parameter. All parameters, except a, depend on both deviatoric stress and reduced load time, which is the product of load time and temperature. The incremental model describes ways to apply the time-temperature supposition principle to permanent deformation.

This paper describes the use of the double cantilever beam (DCB) method for characterizing the adhesion strength of interfaces in advanced microelectronic packages at room and high temperatures. Those interfaces include silicon–epoxy underfill, solder resist–epoxy underfill and epoxy mold compounds (EMCs), and die passivation materials–epoxy underfill materials. A unique sample preparation technique was developed for DCB testing of each interface in order to avoid the testing challenges specific to that interface—for example, silicon cracking and voiding in silicon–underfill samples and cracking of solder resist films in solder resist–underfill samples. An asymmetric DCB configuration (i.e., different cantilever beam thickness on top compared to the bottom) was found to be more effective in maintaining the crack at the interface of interest and in reducing the occurrence of cohesive cracking when compared to symmetric DCB samples. Furthermore, in order to characterize the adhesion strength of those interfaces at elevated temperatures seen during package assembly and end-user testing, an environmental chamber was designed and fabricated to rapidly and uniformly heat the DCB samples for testing at high temperatures. This chamber was used to successfully measure the adhesion strength of silicon–epoxy underfill samples at temperatures up to 260 °C, which is the typical maximum temperature experienced by electronic packages during solder reflow. For the epoxy underfills tested in this study, the DCB samples failed cohesively within the underfill at room temperature but started failing adhesively at temperatures near 150 °C. Adhesion strength measurements also showed a clear degradation with temperature. Several other case studies using DCB for material selection and assembly process optimization are also discussed. Finally, fractography results of the fractured surfaces are presented for better understanding of the failure mode.