The constant increase in power and heat flux densities encountered in electronic devices fuels a rising demand for lightweight heat sink materials with suitable thermal properties. In this study, discontinuous pitch-based carbon fiber reinforced aluminum matrix (Al-CF) composites with aluminum-silicon alloy (Al-Si) were fabricated through hot pressing. The small amount of Al-Si contributed to enhance the sintering process in order to achieve fully dense Al-CF composites. A thermal conductivity and CTE of 258 W/(m K) and 7.0 9 10 -6 /K in the in-plane direction of the carbon fibers were obtained for a (Al 95 vol% ? Al-Si 5 vol% )-CF 50 vol% composite. Carbon fiber provides the reducing of CTE while the conservation of thermal conductivity and weight of Al. The achieved CTEs satisfy the standard requirements for a heat sink material, which furthermore possess a specific thermal conductivity of 109 W cm 3 /(m K g). This simple process allows the low-cost fabrication of Al-CF composite, which is applicable for a lightweight heat sink material.
Transient Liquid Phase (TPL) bounding of Sn foil sandwiched between two Cu foils involves, in the temperature range above the melting point of Sn (232°C) and below 350°C, the formation and the growth of two intermetallic compounds (IMCs) Cu6Sn5 and Cu3Sn and mostly unintended micro-pores. The present study aims to analyze the mechanism of void development during the soldering process through an experimental and modeling approach of diffusion controlled IMC transformation. This modeling couples the diffusion process and the interface motion with the volume shrinkage induced by the difference of partial molar volumes of atoms between each phase. We also consider two types of interdiffusion transports: i) inter-diffusion based on the exchange of Cu and Sn-atoms and ii) inter-diffusion of Sn atoms with vacancies allowing Kirkendall void formation. The simulations of IMC growth performed corresponds to a sequence of planar phase layers, where the distinctive scallop morphology of the Cu6Sn5 layer is described through an analytical function allowing to quantify the grain boundary diffusion pathway. We take into account of the volume diffusion mechanism for Cu3Sn intermetallic.For Cu6Sn5 intermetallic two mechanisms are considered, volume diffusion and grain boundary diffusion, limited by grain growth. The simulations of IMC growth kinetics, for different transport scenarios, are compared to the experimental evolving morphologies to determine the most likely mechanism of micro-void formation.
Developing solder joints capable of withstanding high-power density, hightemperature, and significant thermomechanical stress is essential to further develop electronic devices performances. This study demonstrates an effective route of producing dense, robust, and reliable high-temperature Cu-Sn soldering by modifying the interfacial exchange during a transient liquid phase bonding (TLP) process. Our approach relies thus on altering internal phenomena (diffusion and transport of reactive species) rather than classical external TLP bonding parameters (e.g., time, temperature, and pressure). By adding a Cu3Sn coated layer between Cu and Sn before the TLP process, a fast dissolution of Cu in liquid Sn is achieved, altering undesired Cu6Sn5 scallop grains impingement and promoting their uniform growth within the liquid. A bonding and pore formation mechanism of the solder with or not the Cu3Sn coated layer is proposed based on experimental and theoretical analysis. The developed TLP joint possesses a shear stress resistance of more than 80 MPa with a thermal cycle endurance 2 superior to 1200 (-45 to 180 ºC), making it highly reliable compared to a classical solder joint with shear and thermal cycling resistance of 45 MPa and 500, respectively. The developed approaches provide, thus, an easy, affordable, and scalable method of producing hightemperature and durable Cu-Sn joint for high-power module applications.
International audienceDiamond coatings are investigated for thermal management, wear protection and corrosion resistance in harsh environments. In power electronic industries, copper (Cu), which shows high thermal conductivity, is considered as a promising substrate for diamond based heat-spread materials. However, the coefficient of thermal expansion (CTE) mismatch between diamond and Cu induces thermo-mechanical stresses that affect the integrity of the diamond-Cu assembly. In fact, diamond films deposited on Cu substrates tend to peel-off upon cooling due to the compressive stresses present at the diamond-Cu interface. This investigation is focused on the growth of polycrystalline diamond thin films onto Cu/CF (CF) composite materials, using combustion flame chemical vapor deposition (CVD). It has been found that increased CF content in the Cu/CF materials leads to a reduced CTE improving, hence, the adhesion between the diamond film and the Cu/CF substrate and reduces Cu/CF-diamond interfacial residual thermal stresses. At a CF content of 40% in volume, the residual thermal stress of the diamond film deposited on the Cu/CF composite is lower than that on bare Cu and adapted with CVD diamond growth. Naturally engineered composite surfaces have enhanced the adhesion of the diamond film on the composite substrate via mechanical interlocking
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