Showing papers in "Journal of Electronic Packaging in 2020"

A System to Package Perspective on Transient Thermal Management of Electronics

Abstract: There are many applications throughout the military and commercial industries whose thermal profiles are dominated by intermittent and/or periodic pulsed thermal loads. Typical thermal solutions for transient applications focus on providing sufficient continuous cooling to address the peak thermal loads as if operating under steady-state conditions. Such a conservative approach guarantees satisfying the thermal challenge but can result in significant cooling overdesign, thus increasing the size, weight, and cost of the system. Confluent trends of increasing system complexity, component miniaturization, and increasing power density demands are further exacerbating the divergence of the optimal transient and steady-state solutions. Therefore, there needs to be a fundamental shift in the way thermal and packaging engineers approach design to focus on time domain heat transfer design and solutions. Due to the application-dependent nature of transient thermal solutions, it is essential to use a codesign approach such that the thermal and packaging engineers collaborate during the design phase with application and/or electronics engineers to ensure the solution meets the requirements. This paper will provide an overview of the types of transients to consider—from the transients that occur during switching at the chip surface all the way to the system-level transients which transfer heat to air. The paper will cover numerous ways of managing transient heat including phase change materials (PCMs), heat exchangers, advanced controls, and capacitance-based packaging. Moreover, synergies exist between approaches to include application of PCMs to increase thermal capacitance or active control mechanisms that are adapted and optimized for the time constants and needs of the specific application. It is the intent of this transient thermal management review to describe a wide range of areas in which transient thermal management for electronics is a factor of significance and to illustrate which specific implementations of transient thermal solutions are being explored for each area. The paper focuses on the needs and benefits of fundamentally shifting away from a steady-state thermal design mentality to one focused on transient thermal design through application-specific, codesigned approaches.

Packaging for Laser-Based White Lighting: Status and Perspectives

Abstract: Light-emitting diodes (LEDs) have gained wide adoption in general illumination applications in the last decade. However, the efficiency drop of LEDs with increasing current density limits the luminous flux per wafer area. In contrast, laser diodes (LDs) can achieve higher efficiency at high current density. Likewise, the etendue of LDs is very low due to the small emitting area and divergent angle, facilitating the high-luminance. Hence, LDs may outperform LEDs in future high-luminance solid-state lighting (SSL). However, the rapid development of high-luminance white laser diode (WLD) is still facing some challenges. First, the heat flux of LD chip is extremely high, leading to a higher junction temperature. Second, the laser beam exhibits an elliptical and astigmatic pattern with Gaussian intensity distribution, which may deteriorate the lighting performances. Third, to achieve high-luminance lighting, the laser beam is usually focused onto the phosphor layer, which may easily increase the phosphor temperature to the thermal quenching region. A comprehensive understanding of these problems enables the advancements of packaging designs for WLDs. In this review, we summarized the recent progress in the packaging of WLDs. First, the advantages and applications of LDs were presented. Then, the state-of-the-art methods of generating white light using LDs were reviewed, in terms of packaging structures and performances. Finally, the challenges and corresponding progresses for the packaging of WLDs were overviewed. This review intends to contribute to the development of next-generation high-luminance laser-based white lighting.

Thermal and Manufacturing Design Considerations for Silicon-Based Embedded Microchannel-3D Manifold Coolers (EMMCs): Part 1—Experimental Study of Single-Phase Cooling Performance With R-245fa

Abstract: High performance and economically viable cooling solutions must be developed to reduce weight and volume, allowing for a wide-spread utilization of hybrid electric vehicles. The traditional embedded microchannel cooling heat sinks suffer from high pressure drop due to small channel dimensions and long flow paths in two-dimensional (2D) plane. Utilizing direct “embedded cooling” strategy in combination with top access three-dimensional (3D) manifold strategy reduces the pressure drop by nearly an order of magnitude. In addition, it provides more temperature uniformity across large area chips and it is less prone to flow instability in two-phase boiling heat transfer. This study presents the experimental results for single-phase thermofluidic performance of an embedded silicon microchannel cold plate (CP) bonded to a 3D manifold for heat fluxes up to 300 W/cm2 using single-phase R-245fa. The heat exchanger consists of a 5 × 5 mm2 heated area with 25 parallel 75 × 150 μm2 microchannels, where the fluid is distributed by a 3D-manifold with four microconduits of 700 × 250 μm2. Heat is applied to the silicon heat sink using electrical Joule-heating in a metal serpentine bridge and the heated surface temperature is monitored in real-time by infrared (IR) camera and electrical resistance thermometry. The maximum and average temperatures of the chip, pressure drop, thermal resistance, and average heat transfer coefficient (HTC) are reported for flow rates of 0.1, 0.2. 0.3, and 0.37 L/min and heat fluxes from 25 to 300 W/cm2. The proposed embedded microchannels-3D manifold cooler, or EMMC, device is capable of removing 300 W/cm2 at maximum temperature 80 °C with pressure drop of less than 30 kPa, where the flow rate, inlet temperature, and pressures are 0.37 L/min, 25 °C and 350 kPa, respectively. The experimental uncertainties of the test results are estimated, and the uncertainties are the highest for heat fluxes < 50 W/cm2 due to difficulty in precisely measuring the fluid temperature at the inlet and outlet of the microcooler.

Device-Level Multidimensional Thermal Dynamics With Implications for Current and Future Wide Bandgap Electronics

Abstract: Researchers have been extensively studying wide-bandgap (WBG) semiconductor materials such as gallium nitride (GaN) with an aim to accomplish an improvement in size, weight, and power of power electronics beyond current devices based on silicon (Si). However, the increased operating power densities and reduced areal footprints of WBG device technologies result in significant levels of self-heating that can ultimately restrict device operation through performance degradation, reliability issues, and failure. Typically, self-heating in WBG devices is studied using a single measurement technique while operating the device under steady-state direct current measurement conditions. However, for switching applications, this steady-state thermal characterization may lose significance since the high power dissipation occurs during fast transient switching events. Therefore, it can be useful to probe the WBG devices under transient measurement conditions in order to better understand the thermal dynamics of these systems in practical applications. In this work, the transient thermal dynamics of an AlGaN/GaN high electron mobility transistor (HEMT) were studied using thermoreflectance thermal imaging and Raman thermometry. Also, the proper use of iterative pulsed measurement schemes such as thermoreflectance thermal imaging to determine the steady-state operating temperature of devices is discussed. These studies are followed with subsequent transient thermal characterization to accurately probe the self-heating from steady-state down to submicrosecond pulse conditions using both thermoreflectance thermal imaging and Raman thermometry with temporal resolutions down to 15 ns.

Guidelines for Reduced-Order Thermal Modeling of Multifinger GaN HEMTs

Abstract: The increasing demand for tightly integrated gallium nitride high electron mobility transistors (HEMT) into electronics systems requires accurate thermal evaluation. While these devices exhibit favorable electrical characteristics, the performance and reliability suffer from elevated operating temperatures. Localized device self-heating, with peak channel and die level heat fluxes of the order of 1 MW cm−2 and 1 kW cm−2, respectively, presents a need for thermal management that is reliant on accurate channel temperature predictions. In this publication, a high-fidelity multiphysics modeling approach employing one-way electrothermal coupling is validated against experimental results from Raman thermometry of a 60-finger gallium nitride (GaN) HEMT power amplifier under a set of direct current (DC)-bias conditions. A survey of commonly assumed reduced-order approximations, in the form of numerical and analytical models, are systematically evaluated with comparisons to the peak channel temperature rise of the coupled multiphysics model. Recommendations of modeling assumptions are made relating to heat generation, material properties, and composite layer discretization for numerical and analytical models. The importance of electrothermal coupling is emphasized given the structural and bias condition effect on the heat generation profile. Discretization of the composite layers, with temperature-dependent thermal properties that are physically representative, are also recommended.

The Doping Dependence of the Thermal Conductivity of Bulk Gallium Nitride Substrates

Abstract: Gallium nitride (GaN) has emerged as one of the most attractive base materials for next-generation high-power and high-frequency electronic devices. Recent efforts have focused on realizing vertical power device structures such as in situ oxide, GaN interlayer based vertical trench metal–oxide–semiconductor field-effect transistors (OG-FETs). Unfortunately, the higher-power density of GaN electronics inevitably leads to considerable device self-heating which impacts device performance and reliability. Halide vapor-phase epitaxy (HVPE) is currently the most common approach for manufacturing commercial GaN substrates used to build vertical GaN transistors. Vertical device structures consist of GaN layers of diverse doping levels. Hence, it is of crucial importance to measure and understand how the dopant type (Si, Fe, and Mg), doping level, and crystal quality alter the thermal conductivity of HVPE-grown bulk GaN. In this work, a steady-state thermoreflectance (SSTR) technique was used to measure the thermal conductivity of HVPE-grown GaN substrates employing different doping schemes and levels. Structural and electrical characterization methods including X-ray diffraction (XRD), secondary-ion mass spectrometry (SIMS), Raman spectroscopy, and Hall-effect measurements were used to determine and compare the GaN crystal quality, dislocation density, doping level, and carrier concentration. Using this comprehensive suite of characterization methods, the interrelation among structural/electrical parameters and the thermal conductivity of bulk GaN substrates was investigated. While doping is evidenced to reduce the GaN thermal conductivity, the highest thermal conductivity (201 W/mK) is observed in a heavily Si-doped (1–5.00 × 1018 cm−3) substrate with the highest crystalline quality. This suggests that phonon-dislocation scattering dominates over phonon-impurity scattering in the tested HVPE-grown bulk GaN substrates. The results provide useful information for designing thermal management solutions for vertical GaN power electronic devices.

Design, Analysis, Comparison, and Experimental Validation of Insulated Metal Substrates for High-Power Wide-Bandgap Power Modules

Abstract: Direct bonded copper (DBC) substrates used in power modules have limited heat spreading and manufacturing capability due to ceramic properties and manufacturing technology. The ceramic and copper bonding is also subject to high mechanical stress due to coefficient of thermal expansion mismatch between the copper and the ceramic. For wide-bandgap (WBG) devices, it is of interest exploring new substrate technologies that can overcome some of the challenges of direct bonded copper substrates. In this technical paper, the design, analysis, and comparison of insulated metal substrates (IMSs) for high-power wide-bandgap semiconductor-based power modules are discussed. This paper starts with technical description and discussion of state-of-the-art DBC substrates with different ceramic insulators such as aluminum nitride (AlN), Al2O3, and Si3N4. Next, an introduction of IMSs and their material properties, and a design approach for SiC (silicon carbide) metal-oxide-semiconductor field-effect transistor (MOSFET)-based power modules for high-power applications is provided. The influence of dielectric thickness on the power handling capability of the substrate are also discussed. The designed IMS and DBC substrates were characterized in terms of steady-state and transient thermal performance using finite element simulation. Finally, the performance of the IMS and DBC are validated in an experimental setup under different loading and cooling temperature conditions. The simulation and experimental results showed that the IMS can provide high steady-state thermal performance for high-power modules based on SiC MOSFETs. Furthermore, the IMS provided enhanced transient thermal performance, which provided a reduced junction temperature when the module is operated at low fundamental output frequencies in traction drive systems.

Warpage Characterization of Molded Wafer for Fan-Out Wafer-Level Packaging

Abstract: This study presents a comprehensive assessment of the process-induced warpage of molded wafer for chip-first, face-down fan-out wafer-level packaging (FOWLP) during the fan-out fabrication process. A process-dependent simulation methodology is introduced, which integrates nonlinear finite element (FE) analysis and element death-birth technique. The effects of the cure-dependent volumetric shrinkage, geometric nonlinearity, and gravity loading on the process-induced warpage are examined. The study starts from experimental characterization of the temperature-dependent material properties of the applied liquid type epoxy molding compound (EMC) system through dynamic mechanical analysis (DMA) and thermal mechanical analysis. Furthermore, its cure state (heat of reaction and degree of cure (DOC)) during the compression molding process (CMP) is measured by differential scanning calorimetry (DSC) tests. Besides, the cure dependent-volumetric (chemical) shrinkages of the EMC system after the in-mold cure (IMC) and postmold cure (PMC) are experimentally determined by which the volumetric shrinkage at the gelation point is predicted through a linear extrapolation approach. To demonstrate the effectiveness of the proposed theoretical model, the prediction results are compared against the inline warpage measurement data. One possible cause of the asymmetric/nonaxisymmetric warpage is also addressed. Finally, the influences of some geometric dimensions on the warpage of the molded wafer are identified through parametric analysis.

Effect of Gas Flow Rates on Quality of Aerosol Jet Printed Traces With Nanoparticle Conducting Ink

Abstract: This paper focuses on the influence of carrier gas flow rate (CGFR) and sheath gas flow rate (SGFR) on the quality of conductive traces printed with nanoparticle inks using aerosol jet printing (AJP). This investigation was motivated by previous results of two AJP specimens that were printed at different gas flow rates and yielded significantly different thermal cycling durability lifetimes. A parametric sensitivity study was executed by printing and examining serpentine trace structures at 15 different combinations of CGFRs and SGFRs. The analysis included quantifying the trace's macroscale geometry, electrical properties, and micromorphological features. Interesting macroscale results include an increase in effective conductivity with increasing CGFR. At the microscale, image processing of high magnification scanning electron microscope (SEM) images of the printed traces revealed that agglomerations of silver clusters on the surface of traces became coarser at higher CGFR and also that agglomerates in the bulk were finer than those on the surface. Crystalline silver deposits were observed at all flow rates. In addition, cross sectioning of the printed traces showed higher incidences of buried cohesive cracking at higher gas flow rates. These cohesive cracks reduce the robustness of the traces but may not always be visible from the surface. The degree of cohesive cracking was seen to be broadly correlated with the coarseness of the surface agglomerates, thus suggesting that the coarseness of surface agglomerates may provide a visible surrogate measure of the print quality. The results of this study suggest that print quality may degrade as gas flow rates increase.

Experimental Study of Flexible Electrohydrodynamic Conduction Pumping for Electronics Cooling

Abstract: As modern-day electronics develop, electronic devices become smaller, more powerful, and are expected to operate in more diverse configurations. However, the thermal control systems that help these devices maintain stable operation must advance as well to meet the demands. One such demand is the advent of flexible electronics for wearable technology, medical applications, and biology-inspired mechanisms. This paper presents the design and performance characteristics of flexible electrohydrodynamic (EHD) pumps, based on EHD conduction pumping technology in macro- and mesoscales. Unlike mechanical pumps, EHD conduction pumps have no moving parts, can be easily adjusted to the microscale, and have been shown to generate and control the flow of refrigerants for electronics cooling applications. However, these pumping devices have only been previously tested in rigid configurations unsuitable for use with flexible electronics. In this work, for the first time, the net flow generated by flexible EHD conduction pumps is measured on a flat plane in various configurations. In this study, the results show that the flexible EHD conduction pumps are capable of generating significant flow velocities in all size scales considered in this study, with and without bending. This study also proves the viability of screen printing as a manufacturing method for these pumps. The selection of working fluid for EHD conduction pumping is also a topic of discussion. Novec Engineered Fluids have been a popular choice for EHD pumping; however, long-term testing has shown that some Novec fluids degrade over time.

Comparative Study on Power Module Architectures for Modularity and Scalability

Abstract: Silicon carbide (SiC) wide bandgap power electronics are being applied in hybrid electric vehicle (HEV) and electrical vehicles (EV). The Department of Energy (DOE) has set target performance goals for 2025 to promote EV and HEV as a means of carbon emission reduction and long-term sustainability. Challenges include higher expectations on power density, performance, efficiency, thermal management, compactness, cost, and reliability. This study will benchmark state of the art silicon and SiC technologies. Power modules used in commercial traction inverters are analyzed for their within-package first-level interconnect methods, module architecture, and integration with cooling structure. A few power module package architectures from both industry-adopted standards and proposed patented technologies are compared in modularity and scalability for integration into inverters. The current trends of power module architectures and their integration into inverter are also discussed. The development of an eco-system to support the wide bandgap semiconductors-based power electronics is highlighted as an ongoing challenge.

Novel Transient Liquid Phase Bonding for High-Temperature Automotive Power Electronics Systems

Abstract: This paper demonstrates the concept and fabrication process for a novel (copper, nickel)–tin, (Cu, Ni)–Sn, transient liquid phase (TLP) high-temperature bond that exploits the addition of a pure aluminum (Al) core layer for die attachment in a power electronics power card structure. The bond quality and composition of the bond layer are characterized using confocal scanning acoustic microscopy (CSAM), scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX). The experimental results indicate that the material composition and established fabrication process enable a crack-free bond that exhibits a void fraction of 2% or less. Compared to a traditional foil-based single-layer TLP bonding system, the proposed foil-based TLP/metal laminated structure displays ∼60% reduction in thermally induced mechanical stress built-in after fabrication, as verified through steady-state thermal stress simulations.

Additive Manufactured Impinging Coolant, Low Electromagnetic Interference, and Nonmetallic Heat Spreader: Design and Optimization

Abstract: With the increase of electronic device power density, thermal management and reliability are increasingly critical in the design of power electronic systems. First, increased density challenges the capability of conventional heat sinks to adequately dissipate heat. Second, higher frequency switching in high voltage, high current, wide bandgap power modules is creating intensified electromagnetic interference (EMI) challenges, in which metallic heat removal systems will couple and create damaging current ringing. Furthermore, mobile power systems require lightweight heat removal methods that satisfy the heat loads dissipated during operation. In this effort, we introduce an additive manufacturing (AM) pathway to produce custom heat removal systems using nonmetallic materials, which take advantage of impinging fluid heat transfer to enable efficient thermal management. Herein, we leverage the precision of additive manufacturing techniques in the development of three-dimensional optimized flow channels for achieving enhanced effective convective heat transfer coefficients. The experimental performance of convective heat removal due to liquid impingement is compared with conventional heat sinks, with the requirement of simulating the heat transfer needed by a high voltage inverter. The implementation of nonmetallic materials manufacturing is aimed to reduce electromagnetic interference in a low weight and reduced cost package, making it useful for mobile power electronics.

Thermal and Manufacturing Design Considerations for Silicon-Based Embedded Microchannel-Three-Dimensional Manifold Coolers—Part 2: Parametric Study of EMMCs for High Heat Flux (∼1 kW/cm2) Power Electronics Cooling

Abstract: Thermal management of power electronics modules is one of the limiting factors in the peak power capability of the traction inverter system and overall efficiency of the e-drive. Liquid cooling using embedded microchannels with a three-dimensional (3D)-manifold cooler (EMMC) is a promising technology capable of removing heat fluxes of >1 kW/cm at tens of kPa pressure drop. In this work, we utilize computational fluid dynamics (CFD) simulations to conduct a parametric study of selected EMMC designs to improve the thermofluidic performance for a 5 mm 5 mm heated area with the applied heat flux of 800 W/cm using single-phase water as working fluid at inlet temperature of 25 C. We implemented strategies such as: (i) symmetric distribution of manifold inlet/outlet conduits, (ii) reducing the thickness of cold-plate (CP) substrate, and (iii) increasing fluid–solid interfacial area in CP microchannels, which resulted in a reduction in thermal resistance from 0.1 for baseline design to 0.04 cm K/W, while the pressure drop increased from 8 to 37 kPa. [DOI: 10.1115/1.4047883]

Mechanical Characterization of Embedded Serpentine Conductors in Wearable Electronics

Abstract: Wearable electronics undergo stretching, flexing, bending, and twisting during the process of being put on and while being worn. In addition, wearable textile electronics also need to survive under cyclic washing. During such processes, it is necessary to ensure that the electronics as well as the conductors and various other supporting materials remain reliable. In this work, mechanical characterization of various materials in a commercially available smart shirt is presented. The serpentine conductor used in the smart shirt has been carefully examined to understand the strain distribution at various locations under stretching. Both analytical formulations and numerical simulations have been carried out to determine the strain distribution in the serpentine structure, and the results from the simulations have been compared against experimental data obtained through two-dimensional digital image correlation (2D DIC). Various design configurations of the semicircular serpentine structure have been studied in this work, and a relationship between width and the neutral line radius of the semicircular serpentine structure has been obtained to reduce maximum strains in the serpentine structure under stretching.

Stacking Yield Prediction of Package-on-Package Assembly Using Advanced Uncertainty Propagation Analysis: Part I Stochastic Model Development

Abstract: A comprehensive stochastic model is proposed to predict Package-on-Package (PoP) stacking yield loss. The model takes into account all pad locations at the stacking interface while considering the statistical variations of the warpages and the solder ball heights of both top and bottom packages. The goal is achieved by employing three statistical methods: (1) an advanced approximate integration-based method called eigenvector dimension reduction (EDR) method to conduct uncertainty propagation (UP) analyses, (2) the stress-strength interference (SSI) model to determine the noncontact probability at a single pad, and (3) the union of events considering the statistical dependence to calculate the final yield loss. In this first part, theoretical development of the proposed stochastic model is presented. Implementation of the proposed model is presented in a companion paper.

Measurement of the Thermal Performance of a Custom-Build Single-Phase Immersion Cooled Server at Various High and Low Temperatures for Prolonged Time

Abstract: The next radical change in the thermal management of data centers is to shift from conventional cooling methods like air-cooling to direct liquid cooling to enable high thermal mass and corresponding superior cooling. There has been in the past few years a limited adoption of direct liquid cooling in data centers because of its simplicity and high heat dissipation capacity. Single-phase engineered fluid immersion cooling has several other benefits like better server performance, even temperature profile, and higher rack densities and the ability to cool all components in a server without the need for electrical isolation. The reliability aspect of such cooling technology has not been well addressed in the open literature. This paper presents the performance of a fully single-phase dielectric fluid immersed server over wide temperature ranges in an environmental chamber. The server was placed in an environmental chamber and applied extreme temperatures ranging from −20 °C to 10 °C at 100% relative humidity and from 20 to 55 °C at constant 50% relative humidity for extended durations. This work is a first attempt of measuring the performance of a server and other components like pump including flow rate drop, starting trouble, and other potential issues under extreme climatic conditions for a completely liquid-submerged system. Pumping power consumption is directly proportional to the operating cost of a data center. The experiment was carried out until the core temperature reached the maximum junction temperature. This experiment helps to determine the threshold capacity and the robustness of the server for its applications in extreme climatic conditions.

Predictive Model Development and Validation for Raised Floor Plenum Data Center

Abstract: With the explosion in digital traffic, the number of data centers as well as demands on each data center, continue to increase. Concomitantly, the cost (and environmental impact) of energy expended in the thermal management of these data centers is of concern to operators in particular, and society in general. In the absence of physics-based control algorithms, computer room air conditioning (CRAC) units are typically operated through conservatively predetermined set points, resulting in suboptimal energy consumption. For a more optimal control algorithm, predictive capabilities are needed. In this paper, we develop a data-informed, experimentally validated and computationally inexpensive system level predictive tool that can forecast data center behavior for a broad range of operating conditions. We have tested this model on experiments as well as on (experimentally) validated transient computational fluid dynamics (CFD) simulations for two different data center design configurations. The validated model can accurately forecast temperatures and air flows in a data center (including the rack air temperatures) for 10–15 min into the future. Once integrated with control aspects, we expect that this model can form an important building block in a future intelligent, increasingly automated data center environment management systems.

High Temperature Performance Evaluation and Life Prediction for Titanium Modified Silicone Used in Light-Emitting Diodes Chip Scale Packages

Abstract: Light-emitting diodes (LED) chip scale packages (CSPs) have been promoted as a new light source with many advantages in smaller package size, lower material and process cost, and better heat dissipation effect. However, as it is exposed in harsh environments such as high temperature, high humidity, and high blue light irradiation, silicone material used in LED CSPs always suffers deterioration, which will seriously affect the LED's reliability and working life. Thus, the preparation of high reliable silicone has practical significance to promote the application of LED CSPs in lighting. In this research, titanium was introduced into the molecular chain of phenyl silicone by using the hydrolysis condensation method. A high temperature aging test was then performed to the prepared silicone before and after modification, and their optical, thermomechanical, and dielectric properties were characterized to evaluate their reliabilities. The results show that: (1) the Arrhenius function with the dielectric property as an aging characterization can be used as a temperature accelerated life model to predict the service life of the prepared silicone and (2) the titanium modified silicone can advance the high temperature stability on optical properties, thermomechanical, and dielectric properties and enhance the life expectancy. The major contributions of this study are to support the improvement of the novel LED CSP packaging materials and processes, and also to provide the technical guidance on the fast, accurate, and cost-effective reliability assessment for high-quality LED light sources.

Impact of Aging on Mechanical Properties of Thermally Conductive Gap Fillers

Abstract: Thermal interface materials (TIMs) are an important component in electronic packaging, and there is a concerted effort to understand their reliability when used under various environmental load conditions. Previous researchers have investigated gap fillers and other types of TIMs to understand their performance degradation under loading conditions such as thermal cycling and thermal aging. Most of the study in the literature focuses on studying the changes in thermal properties, and there is a lack of understanding when it comes to studying the mechanical behavior of TIMs. Degradation of mechanical properties is the cause for the loss in thermal performance and is critical during TIM selection process. Moreover, mechanical properties such as modulus and coefficient of thermal expansion (CTE) are critical to assess performance of TIMs using finite element analysis (FEA) and potentially save time and money in the evaluation and selection process. Due to the very soft nature of TIMs, sample preparation is a challenging part of material characterization. In this paper, commercially available TIMs are studied using testing methods such as thermomechanical analyzer (TMA), dynamic mechanical analyzer (DMA), and Fourier infrared spectroscopy (FTIR). These methods are used to characterize the material properties and study the changes in properties due to aging. In this work, the followings are presented: impact of filler content on the mechanical properties, sample preparation method for curable TIM materials with specified thicknesses, and impact of thermal aging on mechanical properties.

Thermomechanical Degradation of Thermal Interface Materials: Accelerated Test Development and Reliability Analysis

Abstract: Due to the inherently low adhesive strength and structural integrity of polymer thermal interface materials (TIMs), they present a likely point of failure when succumbed to thermomechanical stresses in electronics packaging. Herein, we present a methodology to quantify TIM degradation through an accelerated and repeatable mechanical cycling technique. The testing apparatus incorporated a steady-state thermal conductivity measurement system, consistent with ASTM 5470-06, with added displacement actuation and force sensing to provide controlled cyclic loading between −20 N and 20 N. Additionally, a novel optical technique was utilized to observe void formation, pump-out, and dry-out behavior during cycling, in order to correlate the thermal performance with physical behaviors of different TIMs under cyclic stress. Of the two different pastes analyzed, cyclic testing was found to degrade the thermal performance of the less viscous TIM by increasing its interfacial resistance. Optical qualitative measurements revealed the breakdown of the TIM structure at the interface, which indicated the formation of voids due to TIM degradation. Applying this testing method for future TIM development could help in optimizing TIM structure for particular package applications.

Concurrent Thermal and Electrical Property Effects of Nano-Enhanced Phase Change Material for High-Voltage Electronics Applications

Abstract: Rapid temperature transients sustained during the operation of high-voltage electronics can be difficult to manage by relying solely on uniform heat removal mechanisms. Phase-change materials (PCMs) can be useful as a buffer against these intermittent temperature spikes when integrated into electronic packages. However, their integration poses challenges of both physical and electrical interactions within the package, particularly in high-voltage systems. This study aims to evaluate electrical and thermal properties of nano-enhanced PCMs to inform their integration in high-voltage systems. The nanocomposites are obtained by seeding 3 × 10−5 and 3 × 10−4 wt % of gold and iron oxide particles to sorbitol. Improvements in thermal properties including thermal conductivity as high as 8% are observed; however, this comes at the expense of the dielectric strength of the PCM. Additionally, an implementation scheme for the nano-enhanced PCMs in a high-voltage-capable power module is proposed with accompanying computational and experimental performance data.

High Strain Rate Mechanical Properties of SAC-Q With Sustained Elevated Temperature Storage at 100 °C

Abstract: Leadfree electronics in harsh environments often may be exposed to elevated temperature for the duration of storage, transport, and usage in addition to high strain rate triggering loads during drop-impact, vibration, and shock. These electronic components may get exposed to high strain rates of 1 to 100 s−1 and operating temperatures up to 200 °C in critical surroundings. Doped SAC solder alloys such as SAC-Q are being considered for use in fine-pitch electronic components. SAC-Q consists of Sn-Ag-Cu alloy in addition to Bi (SAC+Bi). Prior data presented to date for lead-free solders, such as SAC-Q alloy, at high aging temperature and high strain rate are for 50 °C sustained exposure. In this paper, the effect of sustained exposure to temperature of 100 °C on high strain rate properties of SAC-Q is studied. Thermally aged SAC-Q samples at 100 °C have been tested at a range of strain rates including 10, 35, 50, and 75 s−1 and operating temperatures ranging from 25 °C up to 200 °C. Stress–strain curves are established for the given range of strain rates and operating temperatures. Also, the computed experimental results and data have been fitted to the Anand viscoplasticity model for SAC-Q for comparison.

Nearly Efficiency-Droop-Free AlGaN-Based Deep-Ultraviolet Light-Emitting Diode Without Electron-Blocking Layer

Abstract: An aluminum-rich AlGaN layer is commonly implemented to act as an electron-blocking layer (EBL) to block electron overflow from the active region in the conventional deep-ultraviolet light-emitting diodes (DUV LEDs). Herein, we propose a DUV LED device architecture with specially designed band-engineered quantum barriers (QBs) to “serve” as an alternative approach to alleviate such overflow effect, suppressing the electron leakage, and facilitating the electron and hole injection into the active region for efficient radiative recombination. Intriguingly, a much smaller efficiency droop with a significant enhancement of light output power (LOP) by nearly 50% can be achieved at the injection current level of 120 mA in such EBL-free device, in comparison with the conventional EBL-incorporated DUV LED structure. Thus, the EBL-free device architecture provides us an alternative path toward the realization of efficient DUV light emitters.

Minimizing Temperature Nonuniformity by Optimal Arrangement of Hotspots in Vertically Stacked Three-Dimensional Integrated Circuits

Abstract: The semiconductor packaging technologies have seen its growth from multichip module (MCM), system in package (SiP), system on chip (SoC) to the heterogeneous integration of the MCM. Thermal management of multichip vertically integrated systems poses additional constraints and limitations beyond those for single chip modules. Three-dimensional-integrated circuits (3D ICs) technology is a potential approach for next-generation semiconductor packaging technologies. A 3D IC is formed by vertical interconnection of multiple substrates containing active devices which offer reduced die footprint and interconnect length. This paper discusses the optimal arrangement of two hotspots on each layer of a two-die stacked 3D IC. An analytical heat transfer model for prediction of three-dimensional temperature field of a 3D IC based on the solution of governing energy equations has been developed and used for this study. The model is subject to adiabatic boundary conditions at the walls except for the bottom wall which is subject to convective boundary condition. A feed-forward back propagation artificial neural network (ANN) is employed for obtaining the functional relationship between the location of the hotspots and the objectives. Genetic algorithm is employed for solving two nonconflicting objective functions subject to set of constraints. The first objective aims to minimize the maximum temperature on both layers, and the second objective aims to achieve temperature uniformity in the layers. The results of the optimization study are expected to provide recommendations on the design guidelines for arranging hotspots on vertically stacked substrates.

Electro-Thermal Codesign Methodology of an On-Board Electric Vehicle Charger

Abstract: Significant advances are needed to optimize the charging speed, reliability, safety, and cost of today's conservatively designed electric vehicle (EV) charging systems. The design and optimization of these novel engineering systems require concurrent consideration of thermal and electrical phenomena, as well as component and system level dynamics and control to guarantee reliable continuous operation, scalability, and minimum footprint. This work addresses the concurrent thermal and electrical design constraints in a high-density, on-board, bidirectional charger with vehicle-to-grid (V2G), grid-to-vehicle (G2V), vehicle-to-house (V2H), and vehicle-to-vehicle (V2V) power transfer capabilities. The electrical design of this charger consists of DC–DC and DC–AC power stages connected in series. The power-stage circuits are implemented on a printed circuit board (PCB) with 16 surface-mount silicon carbide MOSFETs, three inductors, and one transformer. The main goal of this work is to investigate the interplay between the cooling architecture and the PCB layout, and the corresponding impact on the heat dissipation and parasitic inductance. This work compares the performance of three generations of this multifunctional charger that employ different design methodologies and proposes high-level design guidelines derived from multiphysics simulations and experimental tests.

Fatigue Life of Sn3.0Ag0.5Cu Solder Alloy Under Combined Cyclic Shear and Constant Tensile/Compressive Loads

Abstract: Solder joints in electronic assemblies experience damage due to cyclic thermomechanical loading that eventually leads to fatigue fracture and electrical failure. While solder joints in smaller, die-sized area-array packages largely experience shear fatigue due to thermal expansion mismatch between the component and the substrate, larger area-array packages experience a combination of cyclic shear and axial tensile/compressive loads due to flexure of the substrate. Additionally, on larger processor packages, the attachment of heatsinks further exacerbates the imposed axial loads, as does package warpage. With the increase in size of packages due to 2.5D heterogeneous integration, the above additional axial loads can be significant. Thus, there exists a critical need to understand the impact on fatigue life of solder joints with superposed compressive/tensile loads on the cyclic shear loads. In this paper, we describe a carefully constructed multi-axial microprecision mechanical tester as well as fatigue test results on Sn3.0Ag0.5Cu (SAC305) solder joints subjected to controlled cyclic shear and constant compressive/tensile loads. The tester design allows one to apply cyclic shear loads up to 200 N while maintaining a constant axial load of up to 38 N in tension or compression. The tester is capable of maintaining the axial load to within a tolerance of ±0.5 N during the entirety of fatigue experiment. Carefully constructed test specimens of Sn3.0Ag0.5Cu solder joints were isothermally fatigued under systematically increased compressive and tensile loads imposed on the test specimen subject to repeated loading (R = 0) under lap-shear. In general, the imposition of the superposed compressive load increases the fatigue life of the solder joint compared to application of pure cyclic shear, while the imposition of the superposed tensile load decreases the fatigue life. At larger compressive loads, friction between fractured surfaces is responsible for significant energy dissipation during the cyclic load–unload cycles.

System-Level Thermal Management and Reliability of Automotive Electronics: Goals and Opportunities Using Phase-Change Materials

Abstract: Electronic packaging for automotive applications are at particular risk of thermomechanical failure due to the naturally harsh conditions it is exposed to. With the rise of electric and hybrid electric vehicles (EVs and HEVs), combined with a desire to miniaturize, the challenge of removing enough heat from electronic devices in automotive vehicles is evolving. This paper closely examines the new challenges in thermal management in various driving environments and aims to classify each existing cooling method in terms of performance. Particular focus is placed upon emerging solutions regarded to hold great potential, such as phase-change materials (PCMs). PCMs have been regarded for some time as a means of transferring heat quickly away from the region with the electronic components and are widely regarded as a possible means of carrying out cooling in large scale from small areas, because of their high latent heat of fusion, high specific heat, temperature stability, and small volume change during phase change, etc. They have already been utilized as a method of passive cooling in electronics in various ways, but their adoption in automotive power electronics, such as in traction inverters, has yet to be fulfilled. A brief discussion is made on some of the potential areas of application and challenges relating to more widespread adoption of PCMs, with reference to a case study using computational model of a commercially available power module used in automotive applications.

Multiscale Evaporation Rate Measurement Using Microlaser-Induced Fluorescence

Abstract: As the heat generation at device footprint continuously increases in modern high-tech electronics, there is an urgent need to develop new cooling devices that balance the increasing power demands. To meet this need, cutting-edge cooling devices often utilize microscale structures that facilitate two-phase heat transfer. However, it has been difficult to understand how microstructures enhance evaporation performances through traditional experimental methods due to low spatial resolution. The previous methods can only provide coarse interpretations on how physical properties such as permeability, thermal conduction, and effective surface areas interact at the microscale to effectively dissipate heat. This motivates researchers to develop new methods to observe and analyze local evaporation phenomena at the microscale. Herein, we present techniques to characterize submicron to macroscale evaporative phenomena of microscale structures by using microlaser-induced fluorescence (lLIF). We corroborate the use of unsealed temperature-sensitive dyes by systematically investigating the effects of temperature, concentration, and liquid thickness on the fluorescence intensity. Considering these factors, we analyze the evaporative performances of microstructures using two approaches. The first approach characterizes the overall and local evaporation rates by measuring the solution drying time. The second approach employs an intensity-to-temperature calibration curve to convert temperature-sensitive fluorescence signals to surface temperatures, which calculates the submicron-level evaporation rates. Using these methods, we reveal that the local evaporation rate between microstructures is high but is balanced with a large capillary-feeding. This study will enable engineers to decompose the key thermofluidic parameters contributing to the evaporative performance of microscale structures. [DOI: 10.1115/1.4046767]

Investigation of Elevated Temperature Mechanical Properties of Intermetallic Compounds in the Cu–Sn System Using Nanoindentation

Abstract: In electronic packaging, most researchers are mainly focused on the mechanical properties of Cu–Sn intermetallic compounds (IMCs) at room temperature; few studies have looked into the relationship between hardness, elastic modulus, and plasticity of IMCs and elevated temperature. The hardness, elastic modulus, and plasticity of Cu6Sn5 and Cu3Sn at 25–200 °C are investigated by the nanoindentation method. The results show that the hardnesses of Cu6Sn5 and Cu3Sn obey linear attenuation law with elevated temperature. The hardness of Cu6Sn5 is more sensitive to temperature than that of Cu3Sn. This is due to the fact that the melting point of Cu6Sn5 (415 °C) is lower than that of Cu3Sn (670 °C), Cu6Sn5 has a lower normalization temperature than that of Cu3Sn. The elastic modulus of Cu6Sn5 and Cu3Sn and temperature have a parabolic law at 25–200 °C. The elastic modulus of Cu6Sn5 is more sensitive to temperature. This is attributed to the fact that the lattice structure of Cu6Sn5 is changed from hexagonal lattice to monoclinic lattice, causing its volume to expand, thereby making it more sensitive to temperature. The plasticity factors of Cu6Sn5 and Cu3Sn meet the polynomial relationship with elevated temperature. The plasticity factors of Cu6Sn5 and Cu3Sn increase with increasing temperature, which will reduce the resistance to plastic deformation. This is attributed to the fact that the vacancy generated into the material is conducive to the dislocation movement, the dislocation movement will be more active so that the plasticity factors of Cu6Sn5 and Cu3Sn gradually increase.