This study investigates microchannel heat sinks (MCHS) for mitigating flow boiling instability and enhancing thermal performance in high-end engineering applications. Two MCHS configurations, fabricated from oxygen-free copper substrates, are examined: closed and open designs, each with 44 parallel microchannels (500 µm depth, 200 µm width). Degassed deionized water serves as the working fluid, with mass fluxes of 520 kg/m2 s and wall heat fluxes spanning 22–147 W/cm2. Results show open MCHS designs reduce temperature fluctuations and improve heat transfer, particularly at high heat fluxes. These findings suggest open microchannels can effectively mitigate flow boiling instabilities, enhancing overall system efficiency in thermal management applications for modern electronics.