This paper focuses on two-phase flow boiling of refrigerant R134a inside a copper multi-microchannel heat sink for microelectronic central processing unit cooling applications. The heat sink is composed of 100 parallel microchannels, 100 mu m wide, 680 mu m high, and 15 mm long, with 72-mu m-thick fins separating the channels. The base heat flux was varied from 2.57 to 189 W/cm(2) and the mass flux from 205 to 1000 kg/m(2)s, at a nominal saturation temperature of 63 degrees C. Over 40 000 local heat transfer coefficients were measured at 35 locations using local heaters and temperature sensors, for which different heat transfer trends were identified. The main ones were that the heat transfer coefficient increased with heat flux and was independent of mass flow rate. Heat transfer coefficients as high as 270 000 W/m(2)K (relative to the base area) were reached, keeping the chip under 85 degrees C with a maximum of 94 kPa of pressure drop, for no inlet subcooling and a coolant flow rate of 1000 kg/m(2)s.