Thermal Assessment and In-Situ Monitoring of Insulated Gate Bipolar Transistors in Power Electronic Modules

Insulated gate bipolar transistor (IGBT) power modules are devices commonly used for high-power applications. Operation and environmental stresses can cause these power modules to progressively degrade over time, potentially leading to catastrophic failure of the device. This degradation process may cause some early performance symptoms related to the state of health of the power module, making it possible to detect reliability degradation of the IGBT module. Testing can be used to accelerate this process, permitting a rapid determination of whether specific declines in device reliability can be characterized. In this study, thermal cycling was conducted on multiple power modules simultaneously in order to assess the effect of thermal cycling on the degradation of the power module. In-situ monitoring of temperature was performed from inside each power module using high temperature thermocouples. Device imaging and characterization were performed along with temperature data analysis, to assess failure modes and mechanisms within the power modules. While the experiment aimed to assess the potential damage effects of thermal cycling on the die attach, results indicated that wire bond degradation was the life-limiting failure mechanism.

Demonstration of a Compliant Micro-Spring Array As a Thermal Interface Material for Pluggable Optoelectronic Transceiver Modules

Pluggable optoelectronic transceiver modules are widely used in the fiber-optic communication infrastructure. It is essential to mitigate thermal contact resistance between the high-power optical module and its riding heat sink in order to maintain the required operation temperature. The pluggable nature of the modules requires dry contact thermal interfaces that permit repeated insertion–disconnect cycles under low compression pressures (∼10–100 kPa). Conventional wet thermal interface materials (TIM), such as greases, or those that require high compression pressures, are not suitable for pluggable operation. Here we demonstrate the use of compliant micro-structured TIM to enhance the thermal contact conductance between an optical module and its riding heat sink under a low compression pressure (20 kPa). The metallized and polymer-coated structures are able to accommodate the surface nonflatness and microscale roughness of the mating surface while maintaining a high effective thermal conductance across the thickness. This dry contact TIM is demonstrated to maintain reliable thermal performance after 100 plug-in and plug-out cycles while under compression.

Electrothermal Modeling and Analysis of Gallium Oxide Power Switching Devices

Gallium oxide is an emerging wide band-gap material that has the potential to penetrate the power electronics market in the near future. In this paper, a finite-element gallium oxide semiconductor model is presented that can predict the electrical and thermal characteristics of the device. The finite element model of the two-dimensional device architecture is developed inside the Sentaurus environment. A vertical FinFET device architecture is employed to assess the device’s behavior and its static and dynamic characteristics. Enhancement-mode device operation is realized with this type of device architecture without the need for any selective area doping. The dynamic thermal behavior of the device is characterized through its short-circuit behavior. Based on the device static and dynamic behavior, it is envisioned that reliable vertical transistors can be fabricated for the power electronics applications.

Folding-Reliability of Flexible Electronics in Wearable Applications

There is a growing need for flexible hybrid electronics solutions for wearable applications, in which the user may often wear electronics on body, on fabric or on skin. Electronics in wearable application may be subjected to stresses of daily motion including bending, twisting and stretching. Thus, there is need for technologies capable of flexibility, robustness and small size while being lightweight. Existing standards for focus on rigid electronics and there is scarcity of guidance for test-levels needed to assure reliability of flexible electronics. There is need for studies focused on the development of accelerated test conditions representative of field applications and the identification of failure mechanisms for test levels. In this study, experimental analysis on fatigue life of the PCB in cyclical folding load is conducted. A folding test-stand capable of replicating the stresses of daily motion in a lab-environment has been developed for the test. For the better understanding of the failure mechanism, analysis of failure modes is carried out. Consequently, it is found that fatigue life of the PCB is related to the several conditions: folding direction, moving distance, folding diameter and strain rate.

Experimental Investigation of Single-Phase Cooling in Embedded Microchannels: 3D Manifold Heat Exchanger With R-245fa

High performance and economically viable thermal 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 2D-plane. Utilizing direct “embedded cooling” strategy in combination with top access 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. Here, we present the experimental results for single-phase thermofluidic performance of an embedded silicon microchannel cold-plate bonded to a 3D manifold for heat fluxes up to 300 W/cm2 using single-phase R-245fa. The heat exchanger consists of a 52 mm2 heated area with 25 parallel 75 × 150 μm2 microchannels, where the fluid is distributed by a 3D-manifold with 4 micro-conduits 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 Infra-red (IR) camera and electrical resistance thermometry. The experimental results for maximum and average temperatures of the chip, pressure drop, thermal resistance, average heat transfer coefficient for flow rates of 0.1, 0.2. 0.3 and 0.37 lit/min and heat fluxes from 25 to 300 W/cm2 are reported. 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 lit/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 micro-cooler.

Stress Evaluation of Flexible Displays With Multiple-Laminations Architecture Enabled by Experimental Measurement and Simulation Based Factorial Design

Fatigue behavior of multiple-stacked film-type flexible displays under flexural load has received considerable research attention, whereas the requirement of a considerably thin and flexible packaging structure with single/multiple neutral axis has not been systemically explored. Consequently, this study evaluated the flexural load induced strain and corresponding resistance change in flexible display architecture by both experimental and simulation works. The relationship between mechanical strains and the relevant resistance change in a touch panel module is estimated by both nonlinear finite element analysis and actual experiments. The aforementioned results revealed that the simulated strain and the resistance change of indium tin oxide (ITO) film were increased as the bending radius becomes narrow. Moreover, the influences of several mechanical parameters within an entire organic light-emitting diode device package with multiple coatings were estimated by a simulation-based parametric study. It should be noted that the structure design would lead the single/multiple neutral axis (N.A.) occurred in the concerned flexible displays. Among all the designed structural and material properties, the Young’s modulus of the adhesive is the most dominant factor to determine the bending strain of ITO film and the phenomenon occurrence of multiple N.A. The analytic results indicated that the multiple N.A. design is contributed to decrease the flexural strain and corresponding resistance change of ITO film. Therefore, the design rules of single/multiple N.A. and its influences on stress-induced electric variation in flexible display are revealed.

Direct Ink Printing of Cavities in DPC Ceramic Substrates With Kaolin Pastes for Hermetic Packaging

Fabrication of three-dimensional cavities containing kaolin pastes to be used as direct plated copper (3DPC) substrates ceramics is a very important advancement for electronic packaging of hermetic and ultraviolet light emitting diodes. This work demonstrates usage of pastes consisting of 32–40 wt% of kaolin clay and polyacrylic acid for direct ink printing (DIP) of 3DPC. Rheological and zeta potential tests were performed to determine printability and stability, respectively, of these kaolin pastes. Kaolin content variation had minimum effect on absolute values of the zeta potentials. All pastes had enough stability with the absolute values larger than 30 mV. 40 wt% kaolin solids mass paste was the optimal for DIP due to its excellent shear thinning and viscoelastic properties. Cured 40 wt% kaolin solids mass paste had superior compressive, flexural and bonding strengths. DIP using pastes containing 40 wt% of kaolin is promising for electronic chip integrated hermetic packaging.

CFD Analysis of Thermal Shadowing and Optimization of Heatsinks in 3rd Generation Open Compute Server for Single-Phase Immersion Cooling

In today’s networking world, utilization of servers and data centers has been increasing significantly. Increasing demand of processing and storage of data causes a corresponding increase in power density of servers. The data center energy efficiency largely depends on thermal management of servers. Currently, air cooling is the most widely used thermal management technology in data centers. However, air cooling has started to reach its limits due to high-powered processors. To overcome these limitations of air cooling in data centers, liquid immersion cooling methods using different dielectric fluids can be a viable option. Thermal shadowing is an effect in which temperature of a cooling medium increases by carrying heat from one source and results in decreasing its heat carrying capacity due to reduction in the temperature difference between the maximum junction temperature of successive heat sink and incoming fluid. Thermal Shadowing is a challenge for both air and low velocity oil flow cooling. In this study, the impact of thermal shadowing in a third-generation open compute server using different dielectric fluids is compared. The heat sink is a critical part for cooling effectiveness at server level. This work also provides an efficient range of heat sinks with computational modelling of third generation open compute server. Optimization of heat sink can allow to cool high-power density servers effectively for single-phase immersion cooling applications. A parametric study is conducted, and significant savings in the volume of a heat sink have been reported.

Enabling Thermal Management of High-Powered Server Processors Using Passive Thermosiphon Heat Sink

In recent years, rapid growth is seen in computer and server processors in terms of thermal design power (TDP) envelope. This is mainly due to increase in processor core count, increase in package thermal resistance, challenges in multi-chip integration and maintaining generational performance CAGR. At the same time, several other platform level components such as PCIe cards, graphics cards, SSDs and high power DIMMs are being added in the same chassis which increases the server level power density. To mitigate cooling challenges of high TDP processors, mainly two cooling technologies are deployed: Liquid cooling and advanced air cooling. To deploy liquid cooling technology for servers in data centers, huge initial capital investment is needed. Hence advanced air-cooling thermal solutions are being sought that can be used to cool higher TDP processors as well as high power non-CPU components using same server level airflow boundary conditions. Current air-cooling solutions like heat pipe heat sinks, vapor chamber heat sinks are limited by the heat transfer area, heat carrying capacity and would need significantly more area to cool higher TDP than they could handle. Passive two-phase thermosiphon (gravity dependent) heat sinks may provide intermediate level cooling between traditional air-cooled heat pipe heat sinks and liquid cooling with higher reliability, lower weight and lower cost of maintenance. This paper illustrates the experimental results of a 2U thermosiphon heat sink used in Intel reference 2U, 2 node system and compare thermal performance using traditional heat sinks solutions. The objective of this study was to showcase the increased cooling capability of the CPU by at least 20% over traditional heat sinks while maintaining cooling capability of high-power non-CPU components such as Intel’s DIMMs. This paper will also describe the methodology that will be used for DIMMs serviceability without removing CPU thermal solution, which is critical requirement from data center use perspective.

Health Monitoring of PCB’s Under Mechanical Shock Loads

Feature vectors for health monitoring of electronic assemblies under repetitive mechanical shock have been developed for assemblies subject to 3,000g acceleration levels. The resistance and strain measurements of the PCB are acquired during each drop to analyze the changes in the values during the experiment. Analysis on the progression of failure was carried out using frequency-based techniques on the strain signals from different locations of the board and failure of the package was identified from the increase in the resistance values of the package during the drop. Feature vectors selected were based on the time-frequency data as well as the logarithmic decrement of the strain signals during the different drops. Different statistical approaches on identifying the changes in the damping characteristics of the package during drop were also carried out. Statistical analysis on the changes in the resistance values were quantified in accordance with the changes in the strain and correlation of the both were attempted. The dependence on position of the strain gauge on the PCB were also studied by comparing the variation of the feature vectors of the corresponding strain signals. The before and after failure strain signals were compared on the frequency components and as well as the changes in the damping characteristics of the strain signals.