The following study concentrates on two-phase flow boiling of refrigerant R134a inside two similar copper multimicrochannel heat sinks, one of which was designed for singlephase water cooling of microprocessors. The two-phase heat sink was composed of 100 parallel microchannels, 100 μm wide, 680 μm high, 15 mm long with 72 μm-thick fins, and 63 parallel microchannels. Base heat fluxes and channel-based mass fluxes were varied from 2.57 to 190 W/cm 2 and from 205 to 1000 kg/m2s, all at a nominal saturation temperature of 63°C. Local heat transfer coefficients were measured at 35 locations using localized heaters and temperature sensors. The main trend identified was that the heat transfer coefficient increased with heat flux at all vapor qualities and mass fluxes tested. Heat transfer coefficients as high as 250'000 W/m2K (relative to the base area) were reached, keeping the chip under 85°C. Backflow and flow instabilities were the main issues with the single-phase heat sink when used in flow boiling, leading to flow mal-distribution and jet impingement effects. © 2010 EDA Publishing/THERMINIC.