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Conventional straight microchannel heat sinks often develop progressively thicker hydrodynamic and thermal boundary layers, which elevates wall temperatures and weakens downstream cooling performance under high heat flux. This work investigates a slotted converging–diverging microchannel (S–CD) configuration that combines periodic area variation with transverse slot interruptions to repeatedly re-initiate boundary layers and intensify near-wall mixing. A three-dimensional, single-phase conjugate heat-transfer model is implemented in COMSOL Multiphysics for deionized-water cooling with uniform bottom-wall heat flux. The S–CD design is compared against a straight microchannel (S) and an unslotted converging–diverging baseline (CD) under identical envelope size and operating conditions. The simulations show that slot disturbances located in diverging segments generate localized jetting and multi-scale recirculation, suppressing persistent hot streaks and improving temperature uniformity. For the representative cases reported, the S–CD configuration reduces peak substrate temperature and overall thermal resistance relative to the CD baseline while maintaining a comparable pressure drop. A performance evaluation criterion based on heat-transfer enhancement at equal hydraulic penalty indicates a net thermohydraulic advantage of the proposed slot-assisted CD concept. The presented modeling workflow provides a practical platform for geometry optimization and design studies of compact liquid-cooled heat sinks for high-heat-flux electronics.