This paper proposed a jet-impingement microchannel heat sink to address high-heatflux hot spot cooling issues. Using ammonia as the working fluid, flow boiling experiments were conducted to comparatively study the impact of microchannels and stagnation point structures on the heat sink’s performance. The optimized configuration, comprising peripheral pin fins and central multi-cone, effectively dissipated a heat load of 752 W·cm⁻² within a 1 mm×6 mm hot spot area, maintaining the heating surface temperature below 57.1°C. The corresponding thermal resistance and pressure drop were 0.69 K·W⁻¹ and 5.1 kPa, respectively. Subsequent experimentation on this optimized heat sink revealed that variations in the heat load upstream significantly influenced the inlet quality of the downstream heat sink, consequently impacting its thermal-hydraulic performance.