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 (SWaP) 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 (DC) measurement conditions. However, for switching applications, this steady-state thermal characterization may lose significance since 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 sub-microsecond pulse conditions using both thermoreflectance thermal imaging and Raman thermometry with temporal resolutions down to 15 ns.

Abstract The goal of this work is to develop and model an adaptive thermal management system formed by shape memory alloy (SMA) helical springs and stretchable selective emitters. Emitters are prepared by depositing a metallic layer on an elastomeric film (3M VHB 4910). Strain changes in the film induce alterations of the surface corrugation of the metallic layer, which enables adjustments of its emissivity spectrum. SMAs are materials that undergo moderate recoverable deformations driven by temperature changes. SMA springs are used here as adaptive deformation enablers (both as actuator and thermal sensor). The thermal management system is created by connecting stretchable emitters and SMA springs in series. When the temperature of the system is increased by sunlight irradiation, the SMA springs undergo contractions which elongate the stretchable emitters, flattening their corrugated metallic layer, thereby leading to an increase in their solar reflectivity and allowing radiative cooling. When the system temperature is decreased, the SMA springs relax and allow the emitters to recover their original surface corrugation, leading to an increase in their solar absorptivity and allowing radiative heating. This repeatable process allows the system to exhibit open-loop adaptive regulation of its temperature under varying solar irradiation. A reduced-order model of the system is derived to perform feasibility studies of the concept and results demonstrating the functionality of the system are presented.

Abstract Flexible printed circuits (FPCs) are widely used in electronic devices such as movable part line or wearable sensor. Photolithography is one of the most popular processes for fabricating electric interconnect lines. However, inkjet printing has attracted attention because the method can draw an arbitrary-shape electric lines without any mask. Therefore, nanoparticle metal ink is widely used for printing of conductive electric lines with lowering cost and small-lot production. The physical characteristics such as flexibility or durability of metal nanoparticle ink lines have been evaluated by bending or tensile tests. By contrast, the evaluation method has not been sufficiently established for the electrical characteristics of these lines, and the failure mechanism under high-current density has not been clarified. According to scaling down of electric devices, current density and Joule heating in device lines increase and electromigration (EM) damage becomes a serious problem. EM is a transportation phenomenon of metallic atoms caused by electron wind under high-current density. Reducing EM damage is extremely important to enhance device reliability. In this study, current loading tests of metal nanoparticle ink line were performed to discuss damage mechanism and evaluate electrical reliability under high-current density condition. As the results of current loading tests, the thickness of cathode part of straight-test line was decreased. It is considered that atomic transport from the cathode to the anode occurred by EM phenomenon. The line surface became rough and aggregates of particles generated at middle or anode parts of straight-test line by high-current loading. Both of atomic transport and aggregate generation were closely related the changes of potential drop, their dominances were varied depending the current density value.

Abstract With the advent of GaN as the major material system in the solid-state lighting industry — high power, high brightness LEDs with wavelength ranging from near UV to white are getting fabricated and part of a tremendously large and ever-increasing market. However, device self-heating and environment temperature significantly deteriorates the LED’s optical performance. Hence, it is extremely important to quantify the device self-heating and its impact on optical performance. In this work, three different characterization techniques were used to calculate temperature rise due to self-heating for an InGaN/GaN LED with 5 pairs of multiple quantum wells. The impact of self-heating and increased environment temperature on the device optical performance were also studied. Nanoparticle assisted Raman thermometry was used for the first time to measure the LED mesa surface temperature. The temperature measured using this technique was compared with temperature data obtained by using the forward voltage method and infrared (IR) thermography. The IR and Raman measurement results were in close agreement while the temperature data obtained from forward voltage method underestimated the temperature by 510%. It was also observed that due to environment temperature increase from 25°C to 100°C, LED optical power output drops by 12%.

Abstract Electromigration (EM) is the process of displacing atoms in metals due to current flow leading to interconnect failures in electronic circuits. As electronics feature sizes continue to shrink, EM is becoming an increasingly serious reliability concern. EM, in aluminum (Al) interconnects, has been studied previously, but typically on the device level using Black’s law [1], without emphasis on the localized heating and defect generation around the failure site. To better understand the local EM process, thermoreflectance (TR) thermal imaging technique is used to obtain temperature profiles with submicron resolution [2]. We show that a simple lifetime prediction using Black’s law is not possible for a micro Al wire having a patterned constriction. The wire fails at two distinct failure locations depending on the level of current excitation. Moreover, the lifetime dependence on ambient temperature was studied. Each failure location had its own extracted activation energy. Our findings suggest that Black’s law may be extended to local features. They also show the potential for the design of local features in extending the lifetime of metallic interconnects. In summary, the temperature profile with submicron spatial resolution offers a unique opportunity to better understand the different mechanisms contributing to EM failures which can be used to design highly reliable interconnects.

For improving the functionality and signal speed of electronic devices, electronic components have been miniaturized and an increasing number of elements have been packaged in the device. As a result there has been a steady rise in the amount of heat necessitated to be dissipated from the electronic device. Recently microchannel heat sinks have been emerged as a kind of high performance cooling scheme to meet the heat dissipation requirement of electronics packaging, In the present study an experimental study of subcooled flow boiling in a high-aspect-ratio, one-sided heating rectangular microchannel with gap depth of 0.52 mm and width of 5 mm was conducted with deionized water as the working fluid. In the experimental operations, the mass flux was varied from 200 to 400 kg/m 2 s and imposed heat flux from 3 to 20 W/cm 2 while the fluid inlet temperature was regulated constantly at 90 °C. The boiling curves, flow pattern and onset of nucleate boiling of subcooled flow boiling were investigated through instrumental measurements and a high speed camera. It was found that the slope of the boiling curves increased sharply once the superheat needed to initiate the onset of nucleate boiling was attained, and the slope was greater for lower mass fluxes, with lower superheat required for boiling incipience. As for the visualization images, for relatively lower mass fluxes the bubbles generated were larger and not easy to depart from the vertical upward placed narrow microchannel wall, giving elongated bubbly flow and reverse backflow. The thin film evaporation mechanism dominated the entire test section due to the elongated bubbles and transient local dryout as well as rewetting occurred. Meanwhile the initiative superheat and heat flux of onset of nucleate boiling were compared with existing correlations in the literature with good agreement.

As the automotive industry shifts towards the electrification of drive trains, the efficiency of power electronics becomes more important. The use of silicon carbide (SiC) devices in power electronics has shown several benefits in efficiency, blocking voltage and high temperature operation. In addition, the ability of SiC to operate at higher frequencies due to lower switching losses can result in reduced weight and volume of the system, which also are important factors in vehicles. However, the reliability of packaged SiC devices is not yet fully assessed. Previous work has predicted that the different material properties of SiC compared to Si could have a large influence on the failure mechanisms and reliability. For example, the much higher elastic modulus of SiC compared to Si could increase strain on neighboring materials during power cycling. In this work, the failure mechanisms of packaged Si- and SiC-based power devices have been investigated following power cycling tests. The packaged devices were actively cycled in 4.5 s heating and 20 s cooling at ΔT = 60–80 K. A failure analysis using micro-focus X-ray and scanning acoustic microscopy (SAM) was carried out in order to determine the most important failure mechanisms. The results of the analysis indicate that the dominant failure mechanism is wire bond lift-off at the device chip for all of the SiC-based devices. Further analysis is required to determine the exact failure mechanisms of the analyzed Si-based devices. In addition, the SiC-based devices failed before the Si-based devices, which could be a result of the different properties of the SiC material.

Active power cycling is a standardized and well-established method for reliability assessment and product qualification in power electronics technologies. Repetitive pulses of load current are applied to cause cyclic thermal swings in the p-n junction and in the whole semiconductor device. They induce thermo-mechanical stresses, which ultimately leads to the typical interconnect failure in the ‘devices under test’. However, these tests are insensitive with respect to new automotive system architectures, in which power electronics devices are exposed to additional loads besides the intrinsic thermal swings. The trends in power electronics towards miniaturization, higher power density, heterogeneous system integration, and the deployment of power electronics in harsher environments combined with longer lifetime and higher uptime requirements strongly increase the reliability demands in general and the need for more improved reliability assessment methodologies in particular. The new testing methods shall be more comprehensive and more efficient, i.e., they shall simultaneously cover the real service conditions better and reduce testing time. One promising approach is the combination of loading factors — such as the superposition of active power cycling by passive thermal cycles. Both loading factors are well-known to cause most relevant failure mechanisms in power electronics. In reality, the power electronic devices are exposed to both factors simultaneously. Hence, this load case should also be replicated in the test. The paper will report a systematic investigation of such superimposed test schemes, which cover the case of self-heating and passive heating (from neighboring elements) of the power electronics devices under real service conditions. Typical discrete power electronics components in TO-200 packages are selected as test vehicles as they are likewise relevant for the domains of consumer or automotive electronics. The paper details the test concept and discuss the quantitative and qualitative test results.

AlGaN/GaN high electron mobility transistors (HEMTs) are widely used in high frequency and power applications of the space and military industries due to their high RF power densities. When operated in full capacity, reliability of GaN HEMTs drop significantly due to device degradation, electron collapse phenomena, and concentrated heating effects. Although significant research has been done to investigate the effects of passivation, field-plates on the device degradation and the electron collapse separately, combined electrothermal analysis of the field-plates and the SiO 2 passivation on GaN HEMTs has not been performed from the perspective of device reliability. For this purpose, electrothermal simulations of the field-plated and non-field-plated devices with different SiO 2 passivation thicknesses are performed using Sentaurus TCAD to obtain the electrical field distribution and Joule heating caused temperature distribution in operating devices. Using these results, electrical and thermal effects of the field-plates on the devices with different SiO 2 passivation thicknesses are analyzed to obtain the most effective and reliable operating conditions.

Metal lines used in integrated circuits (ICs) become narrow for raising the device performance. Due to scaling down of the ICs, current density and Joule heating are increased, which induces electromigration (EM) damage. EM is transportation phenomena of metallic atoms caused by electron wind under high current density. EM leads to hillock and void formation in the metal line, thus EM should be considered to evaluate the performances of the device safe. It is known that a value of threshold current density which is critical current density of the EM damage exists in via-connected and passivated lines. In this study, the effect of line geometry on the threshold current density is discussed in the case of taper-shaped line. The evaluation method of threshold current density is conducted based on numerical simulation technique with building-up processes of atomic density distribution in the metal line by using a governing parameter of EM damage. As the simulation results, threshold current density increased in the cases of shorter line length, lower temperature, and wider width in cathode side. Furthermore, a new parameter was proposed for simplified evaluation of the threshold current density in taper-shaped lines. The evaluation method is able to apply various line shapes and conditions and it is expected to use for confirmation of the reliability of the lines in circuit design processes.

This paper provides an overview of a thermoelectric heat pump clothes dryer which was developed with the aim of reducing the significant primary energy consumption attributed to residential electric clothes drying in the United States (623 TBtu/yr). The use of thermoelectric modules in place of the conventional electric resistance heater resulted in a 40% reduction in the energy consumption of the system, compared to the minimum energy efficiency standard. This was achieved for the first time for a standard test load of 8.45 lb, using a clothes dryer prototype with a thermoelectric heat pump module as the sole heating mechanism. The current experimental prototype was developed after extensive modeling, system design and control optimization, and experimental system-level evaluation of control parameters. The demonstration of improved energy consumption has laid the foundation for future development of this technology.

Flexible electronics provide new design options not afforded by rigid electronics in a variety of applications including wearable electronics, robotics and automotive systems. However, the processes for the manufacturing of complex electronic assemblies using fine-pitch components are not as well developed as those for rigid electronics. The lack of structural rigidity of flexible printed circuit cards requires attention to assembly configuration for double-sided flexible assemblies. In addition, mechanisms are needed to compensate for the deformation and warpage of the flexible substrate and components during assembly. In this paper, the stresses in solder joints of double-sided flexible assemblies have been measured during thermal excursions using x-ray micro-computed tomography in conjunction with digital volume correlation. The method allows for non-invasive measurement and does not require cross-sectioning of the part for the purpose of deformation and strain measurement. In addition, the measurements are not limited to the joints in the line of sight. The three-dimensional measurements of deformation and strain have been visualized on the geometry of the solder joints in the package. Digital volume correlation (DVC) method has been used to find the displacements and strains in interconnects of operational electronics. The x-ray microscopic computed tomography (μCT) system has been used to generate the 16 bit digital volume data. The x-ray detector has ability to image the x-ray attenuation of x-rays through the object. Reliability testing of SAC 305 solder interconnects has been performed on double-sided flexible circuit board using x-ray μCT by heating the package to 100°C. The flexible circuit board used in this experiment is of BGA 256-144 combination, two packages, A-PBGA256-1.0mm-17mm and A-CABGA144-1.0mm-13mm. A 3D printed fixture has also been used to support the flexible board and keep it flat while in the CT scan machine. The reference and deformed scans are then re-constructed 3D using Volume Graphics, and Digital Volume Correlation performed using MATLAB modules. Reliability of double-sided flexible printed circuit boards will be discussed and any crack, defects, or deformation in the solder interconnectivity which might occur while heating the package on flexible board is presented. The solder joint strains during thermal excursions are also compared between the flexible and rigid printed circuit assemblies.

In this work, we investigate the thermal response of GaN PIN diodes grown on a sapphire substrate and compare the results to GaN PIN diodes grown on a free standing GaN substrate (FS-GaN). Until now, thermal characterization techniques have been developed to assess the temperature distribution across lateral devices. Raman thermometry has shown to accurately measure the temperature rise across the depth of the GaN layer. Implementing this technique to assess the temperature distribution across the depth of a vertical GaN device is more challenging since a volumetric depth average is measured. The use of TiO 2 nanoparticles is shown to overcome this issue and reduce the uncertainty in the peak temperature by probing a surface temperature on top of the device. For the sapphire substrate, an additional temperature rise of about 15 K was seen on the surface of the PIN diode as compared to the average in the bulk. While the steady state thermal measurements show an accurate estimation of the device’s peak temperature, the PIN diodes are normally operated under pulsed conditions and the thermal response of these devices under periodic joule heating must be assessed. A recently developed transient thermoreflectance imaging technique (TTI) is used in this study to monitor transient temperature rise and decay of top metal contact. Under the same biasing conditions, the FS-GaN PIN diode is found to result in less than half the temperature rise obtained by the sapphire substrate diode. Extracting time constants, a longer rise and decay is also observed in the sapphire substrate diode.

It has been reported that tablet computer surface temperatures can rise from room temperature up to 47°C. Holding a warm or hot computer surface might cause user’s thermal discomfort and possibly skin burns. The use of a tablet often requires holding the device for prolonged time with multiple fingers and palm areas in contact with the tablet lower surface. Previous research has not tested whole finger/palm thermal sensation at a specific surface temperature in a moderate environmental heat range. The current research investigates user’s thermal sensations on the palm and fingers, in response to warm/heat stimuli in a tablet size device with a longer contact duration than used in previous studies, to provide ergonomic design guidelines for electronic device designers and manufacturers. A tablet-size heating surface was developed comprising of nine rectangular aluminum heating pads connected with computer-controlled heaters and thermal sensors. Participants were asked to report their finger/palm thermal sensation and comfort every 45 seconds when they held the prototype for 90 seconds. Results showed a positive linear relationship between surface temperature and user’s thermal sensation and thermal discomfort. Duration of holding the prototype had no significant effect on user’s thermal comfort, but it did significantly affect thermal sensation ratings.

The spectral components of the phonon transport in the locally thermally excited graphene samples were studied by molecular dynamics (MD) method. In order to be able to select and analyze separate phonon modes in the time of propagation, the transient Green-Kubo approach to the definitions of density of states (DOS) and thermal conductivity was tested in quasi-equilibrium regimes for limited region of the graphene sample studied. Propagation of single modes at the background of diffusional phonon distribution and energy decay of such modes are studied by calculation of the DOS and dispersion relations, their dependence on the heating condition and temperature is studied. Similar conditions can be generated at localized heating of small areas of graphene structures in electronic devices. In transient regime, many issues of thermal transport evaluation still remain not sufficiently tested, especially phonon dynamics. Thermal conductivity of graphene samples related to transport of separate phonon modes is still not completely investigated, however, recent result give indication on the difference in the contribution of phonon modes. In the study, we consider mostly high temperature transport modes that are generated at the heated spot in order to be able to define their velocities and lifetimes in the limit of transient MD sampling. The single-layer graphene nanoribbon of 150 nm to 40 nm was relaxed and prepared in equilibrium in zigzag and armchair orientations. REBO potential for graphene was utilized. Our calculation has shown that at the heating to high temperatures of 1000K and higher, the G mode of graphene remains stationary and has a minimal contribution into thermal transport by coherent modes. The coherent phonon mode or modes that contribute the most into thermal transport were confined in the vicinity of 30 THz and can possibly be attributed to the D modes of graphene.

Forests comprised of nominally vertically aligned carbon nanotubes (CNTs) are excellent candidates for thermal interface materials (TIMs) due to their theoretically predicted outstanding thermal and mechanical properties. Unfortunately, due to challenges in the synthesis and characterization of these materials reports of the thermal conductivity and thermal contact resistance of CNT forests have varied widely and typically fallen far short of theoretical predictions. In particular, the micro- and nano-length scales characteristic of the heat transfer in CNT forests pose significant challenges and may lead to misreported results. Here we examine the ability of a popular and well-established thermal metrology technique, time-domain thermoreflectance (TDTR), to resolve the properties of CNT forest TIMs. The characteristic heating frequencies of TDTR (1–10 MHz) are used to probe heat transfer at length scales spanning ∼0.1–1 μm, applicable for measuring the contact resistance between the CNT forest free tips and an opposing substrate. We identify the range of CNT forest-opposing substrate interface resistances that can be resolved with TDTR, and simultaneously demonstrate the effectiveness of several processes developed to reduce the resistance of these interfaces. The limitations of characterizing CNT forests with TDTR are discussed in terms of uncertainty and sensitivity to parameters of interest.

Hot spots and temperature non-uniformities are critical thermal characteristics of current high power electronics and future three dimensional (3D) integrated circuits (ICs). Experimental investigation to understand flow boiling heat transfer on hot spots is required for any two-phase cooling configuration targeting these applications. This work investigates hot spot cooling utilizing novel radial microchannels with embedded pin arrays representing through-silicon-via (TSV) interconnects. Inlet orifices were designed to distribute flow in radial channels in a manner that supplies appropriate amounts of coolant to high-power-density cores. Specially designed test vehicles and systems were used to produce non-uniform heat flux profiles with nominally 20 W/cm 2 background heating, 200 W/cm 2 core heating and up to 21 W/mm 2 hot spot (0.2 mm × 0.2 mm) heating to mimic a stackable eight core processor die (20 mm × 20 mm) with two hot spots on each core. The temperatures associated with flow boiling heat transfer at the hot spots were locally measured by resistance temperature detectors (RTDs) integrated between the heat source and sink. At nominal pressure and flow conditions, use of R1234ze in these devices resulted in a maximum hot spot temperature ( T hs ) of under 63 °C and average T hs of 57 °C at a hot spot power density of 21 W/mm 2 . A semi-empirical model was used to calculate the equivalent heat transfer rate around the hot spots which can provide a baseline for future studies on local thermal management of hot spots.

In this study, development of a novel system for combined water heating, dehumidification, and space cooling is discussed. The system absorbs water vapor from an air stream into an absorbent. The latent heat of absorption, released into the absorbent, is transferred into the process water that cools the absorbent. The solution is regenerated in the desorber, where it is heated by a heating fluid. The water vapor generated in the desorber is condensed and its heat of phase change is also transferred to the process water. The condensed water is then used in an evaporative cooling process to cool the dehumidified air exiting the absorber. Essentially, this open-absorption cycle collects space sensible heat and transfers it to hot water. Another novel feature of the cycle is recovery of the heat energy from the solution exiting the desorber by heat exchange with process water rather than with the solution exiting the absorber. This approach has enabled heating the process water from an inlet temperature of 15°C to 57°C (conforming to the required DOE building hot water standard) and compact fabrication of the absorber, solution heat exchanger, and desorber in plate and frame configuration. The system under development currently has a water heating capacity of 1.5 kW and a thermal coefficient of performance (COP) of 1.45.

Thermal performance for embedded two phase cooling using dielectric coolant (R1234ze) is evaluated on a ∼20 mm × 20 mm large die. The test vehicles incorporate radial expanding channels with embedded pin fields suitable for through-silicon-via (TSV) interconnects of multi-die stacks. Power generating features mimicking those anticipated in future generations of processor chips with 8 cores are included. Initial results show that for the types of power maps anticipated, critical heat fluxes in “core” areas of at least 350 W/cm 2 with at least 20 W/cm 2 “background” heating in rest of the chip area can be achieved with less than 30 °C temperature rise over the inlet coolant temperature. These heat fluxes are significantly higher than those seen for relatively long parallel channel devices of similar base channel dimensions. Experimental results of flow rate, pressure drop, “device,” and coolant temperature are also provided for these test vehicles along with details of the test facility developed to properly characterize the test vehicles.

Modeling and simulation of two-phase phenomena, as well as their impact on electrical performance and physical integrity are critical to the success of embedded cooling strategies. In DARPA’s Intrachip/Interchip Embedded Cooling (ICECool) program, thermal/electrical/mechanical co-simulation and modeling tools are being applied to the analysis and design of RF GaN MMIC ( Monolithic Microwave Integrated Circuit) Power Amplifiers (PA) and digital ICs, with the ultimate goal of achieving greater than 3X electronic performance improvement. This paper addresses various simulation strategies and numerical techniques adopted by the DARPA ICECool performers, with attention devoted to co-simulation through coupled iterations of thermal, mechanical and electrical behavior for capturing device characteristics and predicting reliability and “best in class” simulations that can provide an understanding of device behavior during rugged operating conditions impacted by multi-physics environments. The effect of CTE (Coefficient of Thermal Expansion) mismatch on bond and structural integrity, the impact of cooling fluid choice on performance, the factors affecting erosion/corrosion in the microchannels, as well as electro-migration limits and joule heating effects, will also be addressed. A separate discussion of various two-phase issues, including interface tracking, system pressure drops, conjugate heat transfer, estimating near wall heat transfer coefficients, and predicting CHF (Critical Heat Flux) and dryout is also provided.