The Electrification of commercial vehicles is happening at a rapid pace. Most of the major automotive corporations are pursuing this opportunity to include electric vehicles in their portfolios. The commercial trucking industry has also been exploring the use of electric vehicles for goods transport including perishables. The transition to electrically driven vehicles has led to the need for electrically driven HVAC systems. To support this evolving commercial market, we developed an electrically driven variable speed compressor platform comprising of compressor and variable frequency drive (VFD). The product platform addresses two categories of vehicles: 1) Hybrid vehicle – the vehicle in this category uses conventional IC engine and has traditional batteries that output 48VDC. 2) Electric vehicle – the vehicle in this category is electrically driven using battery bank or traction drive that gives 650VDC as output. Hence, 650V DC is input to VFD. Both these applications were addressed with two drive designs. In this paper, we discuss the thermal design aspects of both 48V and 650V variable frequency drives. In this publication, the product development process is described from product conception, to final product. The mechanical / environmental design considerations while designing these drives were, (1) The drive was expected to be mounted under the vehicle bed and hence should be strong enough to withstand shock and vibration, (2) the drive was decided to be air cooled (4) the drive was designed to be IP67 so that it can withstand harsh road conditions, (5) the desired operating temperature range was between −40°C to 85°C for 48V and −40°C to 65°C for 650V and (6) the estimated time of service was expected to be 10 years. We were able to achieve an operating margin of −40°C to 70°C at full load for hybrid vehicle drive (48VDC) and −40°C to 65°C for electric vehicle drive (650VDC) using air cooling.

Electronic parts may be subjected to continuous activity at high temperatures as well as high strain-rate loads in the oil exploration industry, military, automotive, avionics, and space applications, and parts may be stored in non-climate-controlled enclosures prior to deployment. Material properties evolve at even moderate temperatures after a long period of storage, according to previous studies on undoped SAC alloys. To reduce the aging effects, a number of alloy formulations have been proposed. Data on the mechanical properties of lead-free solder alloys used for interconnection in electronic packaging at high strain rates and high storage temperatures is very important for design optimization of electronic package sustainability at extreme temperatures, since SAC soldiers have shown degradation of mechanical properties after prolonged exposure to storage temperature. The use of dopants in SAC solder has been proposed as a solution to minimize degradation. In this study, After keeping the samples in storage at 50°C for 1–8 months, a doped SAC solder called SAC-R (Ecolloy) was subjected to high strain rate testing. Uniaxial impact hammer tensile tests were conducted on samples with no aging and samples that had been aged for up to 8 months to assess the mechanical properties of SAC-R at high and low operating temperatures ranging from −65°C to 200°C and the Mechanical properties has been compared with an undoped solder SAC 105. The constants for the Anand Visco-Plasticity model were calculated using the material data for SAC-R. By comparing model predictions of the uniaxial tensile test with experimental results, the model’s ability to reflect material constitutive behavior has been quantified.

Flip-Chip Ball Grid Arrays (FCBGAs) are finding applications in automotive underhood electronics for enablement of safety-critical functions. Underfills needed to reinforce flip-chip interconnects in FCBGAs need to operate reliably under sustained high temperature operation. Underfill-to-substrate interface is one of the primary failure locations under wide thermal excursions and usually a precursor to flip-chip joint failure. In order to assess the reliability in the end application, there is need for understanding the damage progression of the underfill-to-substrate interface as a function of operating time and operating temperature. In this study, the Substrate-UF interface was exposed to high temperature and the interfacial fracture toughness quantified. A three-point composite beam specimen of PCB/Underfill was fabricated to study the interface and thermally aged for periods of 10 days, 30 days, 60 days at temperatures ranging from 100°C to 150°C. Quasi-static bending was used to observe and determine interfacial delamination of the sample specimen. A 2D-Digital Image Correlation (DIC) method was also employed to understand the Crack tip opening displacement (CTOD), crack initiation and the fracture toughness, CTOD were compared with the aging schedule and temperature.

FCBGAs are finding applications in automotive underhood environments where they may be subjected to sustained temperatures of 125–200°C for sustained periods during operation. While, FCGBAs have been previously used in consumer applications where operating temperatures typically range in 55–85°C, relatively little is known on methods to design damage-tolerant packages in automotive underhood environments. There is insufficient information on plastic encapsulated electronic components capable of surviving high temperatures for long periods (> 100,000 hours). In this paper, four different types of underfills has been cured and aged. Mechanical tests have been performed on all the four types of underfills too understand the degradation in properties under extended high temperature operation. Uniaxial tensile tests are conducted to study the elastic modulus, ultimate tensile strength and percentage elongation of the underfills. After uniaxial tensile tests, Optical Microscope, SEM and EDS are applied to study the microstructure behaviors of the cross-section area of the underfills. The experimental results are compared before and after aging tests, including pristine, 30 days, 60 days, 90days, 120 days and 240 days. The research focuses on microstructure-property-processing-performance relationships, building the relation between the microstructure evolution and macro-mechanical properties. Reliability physics of high temperature degradation of packaging material is studied.

Electronic equipment in automotive, agricultural and avionics applications may be subjected to temperatures in the range of −55 to 200°C during storage, operation and handling in addition to high strain-rates. Strain rates in owing to vibration and shock may range from 1–100 per sec. Temperature in electronic assemblies depends typically on location, energy dissipation and thermal architecture. Some investigators have indicated that the required operating temperature is between −40 to 200°C for automotive electronics located underhood, on engine, on transmission. Prior data indicates the evolution of mechanical properties under extended exposures to high temperatures. However, the constitutive models are often only available for pristine materials only. In this paper, effect of low operating temperatures (−65°C to 0°C) on Anand-model parameters at high strain rates (10–75 per sec) for aged SAC (SAC105 and SAC-Q) solder alloys has been studied. Stress-Strain curves have been obtained at low operating temperatures using tensile tests. The SAC leadfree solder samples were subjected to isothermal-aged up to 4-months at 50°C before testing. Anand Viscoplastic model has been used to describe the material constitutive behavior. Evolution of Anand Model parameters for SAC solder has been investigated. The computed parameters of the experimental data were used to simulate the tensile test and verified the accuracy of the model. A good correlation was found between experimental data and Anand predicted data.

Electronic parts in military, automotive, avionics and space applications may be subjected to sustained operation at high temperature in addition to high strain-rate loads. Parts may be stored in non-climate-controlled enclosures prior to deployment. Earlier studies on undoped SAC alloys have shown that the material properties evolve after prolonged period of storage at even modest temperatures. In order to mitigate the aging effects, a number of alloy formulation have been proposed. Data of the mechanical properties of lead-free solder alloys which is used for interconnection in the electronic packaging at high strain rates and at high storage temperature is very essential for design optimization of electronic package sustainability at extreme temperature environment because the SAC solders have shown to have degradation in mechanical properties at prolonged exposure to storage temperature. Industries have come up with a solution to reduce the degradation using dopants in SAC solder. In this study, a doped SAC solder called SAC-R has been subjected to high strain rate testing after extended storage at temperature of 50°C for 1 month, 2 months and 3 months. Samples with no aging and aged samples for up to 3-months have been subjected to uniaxial tensile tests to measure the mechanical properties of SAC-R for High and Low operating temperature ranging from −65°C to +200°C. The material data has been used to compute the constants for the Anand Visco-Plasticity model. The ability of the model to represent the material constitutive behavior has been quantified by comparing the model predictions of the uniaxial tensile test with the experimental data.

In many industries, such as automotive, oil and gas, aerospace, medical technologies, electronic parts can often be exposed to high strain loads during shocks, vibrations and drop-impact conditions. Such electronic parts can often be subjected to extreme low and high temperatures ranging from −65°C to 200°C. Also, these electronic devices can be subjected to strain rates of 1 to 100 per second in the critical environment. Recently, many doped SAC solder alloys are being introduced in the electronic component including SAC-Q, SAC-R, Innolot. SAC-Q is made with addition of Bi in Sn-Ag-Cu composition. Mechanical characteristic results and data for lead-free solder alloys are extremely important for optimizing electronic package reliability, at high temperature storage and elevated strain rates. Furthermore, the mechanical properties of solder alloys can be changed significantly due to a thermal aging, which is causing modification of microstructure. Data for the SAC-Q solder alloy with a high temp aging and testing at extreme low to high operating temperatures are not available. SAC-Q material was tested and analyzed for this study at range of operating temperatures of −65°C to 200°C and at a strain rate up to 75 per second. After the specimens were manufactured and reflowed, specimens were stored at 100°C for the isothermal aging for up to 90 days, before tensile tests were carried out at different operating temperatures. For the wide range of strain rates and test temperatures, stress-strain curves are established. In addition, the measured experimental results and data were fitted to the Anand viscoplasticity model and the Anand constants were calculated by estimating the stress-strain behavior measured in the wide range of operating temperatures and strain rates.

Automotive underhood electronics are subjected to high operating temperatures in the neighborhood of 150 to 200°C for prolonged periods in the neighborhood of 10-years. Consumer grade off-the shelf electronics are designed to operate at 55 to 85 °C with a lower use-life of 3 to 5 years. Underfill materials are used to provide supplemental restraint to fine-pitch area array electronics and meet the reliability requirements. In this paper, a number of different underfill materials are subjected to automotive underhood temperatures to study the effect of long time isothermal exposure on microstructure and dynamic-mechanical properties. It has been shown that isothermal aging oxidizes the underfill, which can change the mechanical properties of the material significantly. The oxidation of underfill was studied experimentally by measuring oxidation layer thickness using polarized optical microscope. The effect on the mechanical properties was studied using the dynamic mechanical properties of underfill with DMA (Dynamic Mechanical Analyzer). Two different underfill materials were subjected to three different isothermal exposure, which are below, near and above the glass transition temperature of the underfills. The dynamic mechanical viscoelastic properties like storage modulus, loss modulus, tan delta and their respective glass transition temperatures were investigated. Three point bending mode was used in the DMA with a frequency of 1 Hz operating at 3 °C/min.

Leadfree electronics in harsh environments may often be exposed to elevated temperature for the duration of storage, process 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 per seconds and operating temperature up to 200°C in the critical surroundings. SAC solder alloys (e.g. SAC-Q (CYCLOMAX), and Innolot) are being considered for use in fine-pitch electronic components. SAC-Q consists of Sn-Ag-Cu alloy in addition to Bi (SAC+Bi). The data presented till date for lead-free solders like SAC-Q alloy at high aging temperature and at high strain rate are for 50°C sustained exposure. In this paper, effect of sustained exposure 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 per second and operating temperatures starting 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 fit to the Anand Viscoplasticity model for SAC-Q for comparison.

Fully immersion of servers in electrically nonconductive (dielectric) fluid has recently become a promising technique for minimizing cooling energy consumption in data centers. The improved thermal properties of these dielectric fluids facilitate considerable savings in both upfront and operating cost over traditional air-cooling. This technology provides an opportunity for accommodating increased power densities. It also minimizes the common operational issues of air cooling technique like overheating and temperature swing in the system, fan failures, dust, air quality, and corrosion. This paper presents various data about the thermal performance of a fully single-phase dielectric fluid immersed server over wide temperature ranges (environment temperatures) from 25°C to 55°C for prolonged periods in an environmental chamber. This work explores the effects of high temperatures on the performance of a server and other components like pump, along with potential issues associated with extreme climatic conditions. The experimental data serves as a means to determine failure criteria for the server and pump by subjecting the system to accelerated thermal aging conditions i.e. around 55°C, consequently simulating the most extreme environmental condition that the server may encounter. Connector seals are inspected for expected degradation upon temperature cycling typically at such extreme conditions. Throttling limit for the server and pump power draw for different temperatures was determined to assess pump performance. Determining the relations between component behavior and operating temperature provides an accurate measure of lifetime of a server. The scope of this paper can be expanded by reviewing the effects of low temperatures on server and component performance. Changes to various performance parameters like power draw of pump and server at the higher and the lower operating temperatures and an understanding of issues like condensation can be used to quantify upper and lower limits for pump and server performance.

The rapid growth in cloud computing, the Internet of Things (IoT), and data processing via Machine Learning (ML), have greatly increased our need for computing resources. Given this rapid growth, it is expected that data centers will consume more and more of our global energy supply. Improving their energy efficiency is therefore crucial. One of the biggest sources of energy consumption is the energy required to cool the data centers, and ensure that the servers stay within their intended operating temperature range. Indeed, about 40% of a data center’s total power consumption is for air conditioning[1]. Here, we study how the server air inlet and outlet, as well as the CPU, temperatures depend upon server loads typical of real Internet Protocol (IP) traces. The trace data used here are from Google clusters and include the times, job and task ID, as well as the number and usage of CPU cores. The resulting IT loads are distributed using standard load-balancing methods such as Round Robin (RR) and the CPU utilization method. Experiments are conducted in the Data Center Laboratory (DCL) at the Georgia Institute of Technology to monitor the server outlet air temperature, as well as real-time CPU temperatures for servers at different heights within the rack. Server temperatures were measured by on-line temperature monitoring with Xbee, Raspberry PI, Arduino, and hot-wire anemometers. Given that the temperature response varies with server position, in part due to spatial variations in the cooling airflow over the rack inlet and the server fan speeds, a new load-balancing approach that accounts for spatially varying temperature response within a rack is tested and validated in this paper.

Resistor reliability rules are defined by certain electrical parameters that depend on operating temperature. As shown by Black’s Equation, a temperature change of 5°C can lead to a 30% reduction in expected lifetime [1]. The temperature rise of a resistor depends on its dimensions and thermal properties as well as those of surrounding materials that separate the resistor from the heat sink. In this study, a modified approximate model based on Schafft’s equation is developed to estimate the operating temperature and assess the reliability of BEOL microresistor structures. The additional contribution of the substrate thermal resistance is added to the model and validated through a combined approach of experimental thermal mapping and numerical modeling. The developed analytical and numerical model are validated with the experimental temperature maps and could then be used for parametrization to aid the BEOL design process.

Lead free solders are renowned as interconnects in electronic packaging due to their relatively high melting point, attractive mechanical properties, thermal cycling reliability, and environment friendly chemical properties. The mechanical behavior of lead free solders is highly dependent on the operating temperature. Previous investigations on mechanical characterization of lead free solders have mainly emphasized stress-strain and creep testing at temperatures up to 125 °C. However, electronic devices, sometimes, experience harsh environment applications including well drilling, geothermal energy, automotive power electronics, and aerospace engines where solders are exposed to very high temperatures from 125–200 °C. Mechanical properties of lead free solders at elevated temperatures are limited. In this work, we have investigated the mechanical behavior SAC305 (96.5Sn-3.0Ag-0.5Cu) and SAC_Q (SAC+Bi) lead free solders at extreme high temperatures up to 200 °C. Stress-strain tests were performed on reflowed uniaxial specimens at four elevated temperatures (T = 125, 150, 175, and 200 °C). In addition, changes of the mechanical behavior of these alloys due to isothermal aging at T = 125 °C have been studied. Extreme care has been taken during specimen preparation so that the fabricated solder uniaxial test specimens accurately reflect the solder material microstructures present in actual lead free solder joints. High temperature tensile properties of the solders including initial modulus, yield stress, and ultimate tensile strength have been compared. As expected, our results show substantial degradations of the mechanical properties of lead-free solders at higher temperatures. With prior aging, these degradations become even more significant. Comparison of the results has shown that the addition of Bi to traditional SAC alloys improves their high temperature properties and significantly reduces their aging induced degradations.

As power densities in advanced electronics continue to rise, the need for high performance thermal solutions becomes increasingly important. Liquid jet impingement has been applied to cooling high power-density electronics due to its ability to dissipate large heat fluxes while maintaining an acceptable operating temperature in the device. Recently, microjets have been embedded within the device substrate, forming a compact solution that is highly scalable. Many practical questions remain, however, on whether microjet technology is ready for actual implementation. In this work, we address several important questions that impede adoption of the technology. Numerical analysis and experimental data are provided to demonstrate the tradeoff between thermal performance and driving pressure requirements through pumping analysis. Additional mechanical concerns regarding robustness to clogging and resistance to erosion are addressed through a 1000-hour extended lifetime test.

Predominant high melting point solders for high temperature and harsh environment electronics (operating temperatures from 200 to 250°C) are Pb-based systems, which are being subjected to RoHS regulations because of their toxic nature. In this study, high bismuth (Bi) alloy compositions with Bi-XSb-10Cu (X from 10 wt.% to 20 wt.%) were designed and developed to evaluate their potential as high-temperature, Pb-free replacements. Reflow processes were developed to make die-attach samples made out of the cast Bi alloys. In particular, die-attach joints made out of Bi-15Sb-10Cu alloy exhibited an average shear strength of 24 MPa, which is comparable to that of commercially available high Pb solders. These alloy compositions also retained original shear strength even after thermal shock between −55°C and +200°C and high temperature storage at 200°C. Brittle interfacial fracture sometimes occurred along the interfacial NiSb layer formed between Bi(Sb) matrix and Ni metallized surface. In addition, heat dissipation capabilities, using flash diffusivity, were measured on the die-attach assembly, compared to the corresponding bulk alloys. The thermal conductivity of all the Bi-Sb alloys was higher than that of pure Bi. By creating high volume fraction of precipitates in a die-attach joint microstructure, it was feasible to further increase thermal conductivity of this joint to 24 W/m·K, which is three times higher than that of pure Bi (8 W/m·K). Bi-15Sb-10Cu alloy has so far shown the most promising performance as a die-attach material for high temperature applications (operated over 200°C). Hence, this alloy was further studied to evaluate its potential for plastic deformation. Bi-15Sb-10Cu alloy has shown limited plastic deformation in room temperature tensile testing, in which premature fracture occurred via the cracks propagated on the (111) cleavage planes of rhombohedral crystal structure of the Bi(Sb) matrix. The same alloy has, however, shown up to 7% plastic strain under tension when tested at 175°C. The cleavage planes, which became oriented at smaller angles to the tensile stress, contributed to improved plasticity in the high temperature test.

Phase Change Materials (PCMs) have been widely investigated as a cooling solution due to their significant latent heat capacity. However, the current PCMs generally suffer a low thermal conductivity, thus hindering the application of PCMs. Composite Phase Change Materials (CPCMs) filling with high thermal conductivity materials have been proposed to solve this issue. Nevertheless, the latent heat of the CPCMs decreases with the mass fraction of fillings, thus leading to a lower allowable working time under safe operating temperature. Therefore, an optimal filling mass fraction of CPCMs is in urgent needed to improve the application of CPCMs. In this study, we developed a one-dimensional conduction heat transfer model of CPCMs to predict the optimal filling mass fraction of CPCMs to realize the maximum allowable working time. The filling mass fraction was introduced into the model and the relationship between the thermal conductivity and latent heat was built. We adopted paraffin as the matrix material and Expanded Graphite (EG) as the thermal conductivity enhancer. The allowable working time of the CPCMs as the function of filling mass fraction was obtained. Based on the principle of the maximum allowable working time, the optimal filling mass fraction was calculated. Comparative experiments were also conducted to validate the accuracy of the prediction model. The parameters which affect the maximum allowable working temperature were also investigated, including input heat flux, safe temperature, and height of CPCMs. The results show that a higher heat flux and height requires a larger filling mass fraction, and it’s opposite for the safe temperature.

A series of experiments was conducted to investigate the performance characteristics of a heat pipe with a hybrid wick that combined grooves and a wire screen. The heat pipe in this study was designed primarily for the cooling of high-density power electronic elements such as IGBTs, and it had tiny triangular grooves along its entire length. The container was a copper tube which had an outer diameter of 19 mm and length of 0.8 m, and the working fluid was water. To lower the thermal resistance against increased thermal loads, a higher performance was desired for the heat pipe, without changing the external dimensions. A fine mesh wire screen was partially applied to the evaporator to enhance the heat transfer performance. The hybrid wick heat pipe was tested and analyzed from the viewpoints of thermal resistance, effective thermal conductance, and operating temperature. For a 1.6 kW effective thermal load, as a typical result, the heat pipe with the hybrid wick exhibited a 70 % decrease in thermal resistance compared to that with a groove wick only. The paper includes results for various thermal loads and fluid charges. The results herein can be utilized in applications that require an intensive enhancement in heat pipe performance.

Electronics in automotive underhood environments may be subjected to high temperatures in the neighborhood of 175°C while subjected to high strain rate mechanical loads of vibration. Portable products such as smartphones and tablets stay in the powered on condition for a majority of their operational life during which time the device internals are maintained at higher than ambient temperature. Thus, it would be expected for interconnects in portable products to be at a temperature high than room temperature when subjected to accidental drop or shock. Furthermore, electronics in missile-applications may be subjected to high strain rates after prolonged period of storage often at high temperature. Electronics systems including interconnects may experience high strain rates in the neighborhood of 1–100 per sec during operation at high temperature. However, the material properties of SAC305 leadfree solders at high strain rates and high operating temperatures are scarce after long-term storage. Furthermore, the solder interconnects in simulation of product drop are often modeled using elastic-plastic properties or linear elastic properties, neither of which accommodate the effect of operating temperature on the solder interconnect deformation at high operating temperature. SAC305 solders have been shown to demonstrate the significant degradation of mechanical properties including the tensile strength and the elastic modulus after exposure to high temperature storage for moderate periods of time. Previously, Anand’s viscoplastic constitutive model has been widely used to describe the inelastic deformation behavior of solders in electronic components under thermo-mechanical deformation. Uniaxial stress-strain curves have been plotted over a wide range of strain rates ( ε . = 10, 35, 50, 75 /sec) and temperatures (T = 25, 50, 75, 100, 125, 150, 175, 200°C). Anand viscoplasticity constants have been calculated by non-linear fitting procedures. In addition, the accuracy of the extracted Anand constants has been evaluated by comparing the model prediction and experimental data.

Thermal management for electric machines (motors/generators) is important as the automotive industry continues to transition to more electrically dominant vehicle propulsion systems. Cooling of the electric machine(s) in some electric vehicle traction drive applications is accomplished by impinging automatic transmission fluid (ATF) jets onto the machine’s copper windings. In this study, we provide the results of experiments characterizing the thermal performance of ATF jets on surfaces representative of windings, using Ford’s Mercon LV ATF. Experiments were carried out at various ATF temperatures and jet velocities to quantify the influence of these parameters on heat transfer coefficients. Fluid temperatures were varied from 50°C to 90°C to encompass potential operating temperatures within an automotive transaxle environment. The jet nozzle velocities were varied from 0.5 to 10 m/s. The experimental ATF heat transfer coefficient results provided in this report are a useful resource for understanding factors that influence the performance of ATF-based cooling systems for electric machines.

Vapor chamber technologies offer an attractive approach for passive cooling in portable electronic devices. Due to the market trends in device power consumption and thickness, vapor chamber effectiveness must be compared with alternative heat spreading materials at ultra-thin form factors and low heat dissipation rates. A test facility is developed to experimentally characterize performance and analyze the behavior of ultra-thin vapor chambers that must reject heat to the ambient via natural convection. The evaporator-side and ambient temperatures are measured directly; the condenser-side surface temperature distribution, which has critical ergonomics implications, is measured using an infrared camera calibrated pixel-by-pixel over the field of view and operating temperature range. The high thermal resistance imposed by natural convection in the vapor chamber heat dissipation pathway requires accurate prediction of the parasitic heat losses from the test facility using a combined experimental and numerical calibration procedure. Solid Metal heat spreaders of known thermal conductivity are first tested, and the temperature distribution is reproduced using a numerical model for conduction in the heat spreader and thermal insulation by iteratively adjusting the external boundary conditions. A regression expression for the heat loss is developed as a function of measured operating conditions using the numerical model. A sample vapor chamber is tested for heat inputs below 2.5 W. Performance metrics are developed to characterize heat spreader performance in terms of the effective thermal resistance and the condenser-side temperature uniformity. The study offers a rigorous approach for testing and analysis of new vapor chamber designs, with accurate characterization of their performance relative to other heat spreaders.