González López, Luis E.

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    Enhancement of capillary performance in grooved silicon substrates for thermal management
    (2024-07-09) González López, Luis E.; Quintero Aguiló, Pedro; College of Engineering; Valentín Rullán, Ricky; Torres Nieves, Sheilla; Department of Mechanical Engineering; Silva Araya, Walter F.
    Capillary wicking in microgrooves is pivotal to a wide variety of applications like microfluidics and heat dissipation devices. In electronic packaging, capillary wicking rate is a critical parameter that governs Si based micro heat pipes (MHP) for the effective dissipation of large heat fluxes from power devices. Higher wicking rates, associated with the wick geometry, structure, surface chemistry, and fluid properties, are of paramount importance for the control of fluid replenishment into the MHP’s evaporator to avoid dry-out conditions. Bulk and surface micro-machining have been used for the fabrication of microgrooves on Si substrates, however a combination of very expensive processes and facilities are required. Recently, the fabrication of microgrooves with custom geometries and surface structures by commercially available laser ablation technologies have been investigated. The passive pumping capacity of microgrooves is fundamentally driven by the capillary force and pressure. In general, grooves with the smaller characteristic length scales usually have the larger capillarity. However, the surface texture of the grooves, affected by the fabrication process, and the wetting behavior at the liquid-solid interface (as given by the contact angle, θ) are also critical factors affecting capillarity. For electronics thermal management the capillarity of the wicking structures is crucial as measured by the capillary rate. With the advent of advanced manufacturing processes, such as laser ablation machining and additive technologies, fabrication of microgrooves for enhanced capillarity may be more accessible for the practitioner’s community. In this study, the geometrical parameters of rectangular microgrooves were first assessed theoretically with the governing analytical models. Test coupons with aspect ratios of 0.25, 0.50, and 0.75, with varying widths and depths, have been fabricated together with simulations showing maximum capillary penetration and wicking dynamics. A capillary rate-of-displacement testbed was designed and built for experimental trials using DI water and a camera set-up, where maximum penetration distance and wicking dynamics were studied. Texturing of the laser ablated grooves, measured by average surface roughness (Ra), was found to increase with the number of lasers passes during the laser process. The roughness factor effect on wetting angle is being proposed as the driving mechanism behind the hydrophilic enhancement of the silicon by the laser ablation process. This work has demonstrated a direct write method to fabricate hydrophilic structures on Si for a wide range of thermal management applications in electronic packaging.