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This paper presents a study on the characterization of density as a function of temperature for phase change materials (PCMs). More specifically, in this study we analyze organic alkane PCMs, often called paraffins. PCMs are materials that have the ability to absorb a substantial amount of heat during phase transition from solid to liquid, and therefore prove to be useful in thermal energy storage. The density of paraffin wax PCMs is largely dependent on temperature, and during the phase change process, the density decreases dramatically as the PCM transitions from solid to liquid. Consequently, the PCM experiences dramatic volumetric expansion during this transition. Besides the thermal energy storage uses of PCMs, this volumetric expansion that they exhibit is also used in thermal actuator applications, often referred to as wax motors. While density of PCMs does affect their thermal and mechanical performance, the property is not well-characterized within the literature. In this paper, we examine ten paraffin wax PCMs with varying meltingtemperatures and characterize their densities as a function of temperature. This characterization was done usinga piston and cylinder dilatometer test setup within a temperature-controlled thermal chamber that we designedand validated to the well-characterized density properties of water. The density and temperature relationships werefurther analyzed using piecewise linear regression analysis to develop mathematical models of density as it relates totemperature, which will be useful to those wishing to analyze designs in which PCMs are used, such as in PCM-filled heat sinks. 

Abstract The data center’s server power density and heat generation have increased exponentially because of the recent, unparalleled rise in the processing and storing of massive amounts of data on a regular basis. One-third of the overall energy used in conventional air-cooled data centers is directed toward cooling information technology equipment (ITE). The traditional air-cooled data centers must have low air supply temperatures and high air flow rates to support high-performance servers, rendering air cooling inefficient and compelling data center operators to use alternative cooling technology. Due to the direct interaction of dielectric fluids with all the components in the server, single-phase liquid immersion cooling (Sp-LIC) addresses mentioned problems by offering a significantly greater thermal mass and a high percentage of heat dissipation. Sp-LIC is a viable option for hyper-scale, edge, and modular data center applications because, unlike direct-to-chip liquid cooling, it does not call for a complex liquid distribution system configuration and the dielectric liquid can make direct contact with all server components. Immersion cooling is superior to conventional air-cooling technology in terms of thermal energy management however, there have been very few studies on the reliability of such cooling technology. A detailed assessment of the material compatibility of different electronic packaging materials for immersion cooling was required to comprehend their failure modes and reliability. For the mechanical design of electronics, the modulus, and thermal expansion are essential material characteristics. The substrate is a crucial element of an electronic package that has a significant impact on the reliability and failure mechanisms of electronics at both the package and the board level. As per Open Compute Project (OCP) design guidelines for immersion-cooled IT equipment, the traditional material compatibility tests from standards like ASTM 3455 can be used with certain appropriate adjustments. The primary focus of this research is to address two challenges: The first part is to understand the impact of thermal aging on the thermo-mechanical properties of the halogen-free substrate core in the single-phase immersion cooling. Another goal of the study is to comprehend how thermal aging affects the thermo-mechanical characteristics of the substrate core in the air. In this research the substrate core is aged in synthetic hydrocarbon fluid (EC100), Polyalphaolefin 6 (PAO 6), and ambient air for 720 hours each at two different temperatures: 85°C and 125°C and the complex modulus before and after aging are calculated and compared. 

Abstract Structural components such as printed circuit boards (PCBs) are critical in the thermomechanical reliability assessment of electronic packages. Previous studies have shown that geometric parameters such as thickness and mechanical properties like elastic modulus of PCBs have direct influence on the reliability of electronic packages. Elastic material properties of PCBs are commonly characterized using equipment such as tensile testers and used in computational studies. However, in certain applications viscoelastic material properties are important. Viscoelastic influence on materials is evident when one exceeds the glass transition temperature of materials. Operating conditions or manufacturing conditions such as lamination and soldering may expose components to temperatures that exceed the glass transition temperatures. Knowing the viscoelastic behavior of the different components of electronic packages is important in order to perform accurate reliability assessment and design components such as printed circuit boards (PCBs) that will remain dimensionally stable after the manufacturing process. Previous researchers have used creep and stress relaxation test data to obtain the Prony series terms that represent the viscoelastic behavior and perform analysis. Others have used dynamic mechanical analysis in order to obtain frequency domain master curves that were converted to time domain before obtaining the Prony series terms. In this paper, nonlinear solvers were used on frequency domain master curve results from dynamic mechanical analysis to obtain Prony series terms and perform finite element analysis on the impact of adding viscoelastic properties when performing reliability assessment. The computational study results were used to perform comparative assessment to understand the impact of including viscoelastic behavior in reliability analysis under thermal cycling and drop testing for Wafer Level Chip Scale Packages. 

Thermal conductive gap filler materials are used as thermal interface materials (TIMs) in electronic devices due their numerous advantages, such as higher thermal conductivity, ease of use, and conformity. Silicone is a class of synthetic materials based on a polymeric siloxane backbone which is widely used in thermal gap filler materials. In electronic packages, silicone-based thermal gap filler materials are widely used in industries, whereas silicone-free thermal gap filler materials are emerging as new alternatives for numerous electronics applications. Certainly, characterization of these TIMs is of immense importance since it plays a critical role in heat dissipation and long-term reliability of the electronic packages. Insubstantial studies on the effects of various chemical compounds on the properties of silicone-based and silicone-free TIMs has led to this study, which focuses on the effect of thermal aging on the mechanical, thermal, and dielectric properties of silicone-based and silicone-free TIMs and the chemical compounds that cause the changes in properties of these materials. Characterization techniques such as dynamic mechanical analysis (DMA), thermomechanical analysis (TMA), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), and broadband dielectric spectroscopy (BbDS) are used to study the mechanical, thermal, and dielectric characteristics of these TIMs, which will guide towards a better understanding of the applicability and reliability of these TIMs. The experiments demonstrate that upon thermal aging at 125 °C, the silicone-free TIM becomes hard, while silicone-based TIM remains viscoelastic, which indicates its wide applicability to higher temperature applications for a long time. Though silicone-based TIM displays better mechanical and thermal properties at elevated temperatures, dielectric properties indicate low conductivity for silicone-free TIM, which makes it a better candidate for silicone-sensitive applications where higher electric insulation is desired.