In this study, flow boiling of refrigerant R245fa is investigated in a microgap of height 200 mu m populated with a staggered pin fin array of diameter 150 mu m and spacing 200 mu m. For heat fluxes up to 498 W/cm(2), mass flux values up to 7896 kg/m(2) s, and inlet temperatures of 13 degrees C and 18 degrees C, average two-phase heat transfer coefficient up to 60 kW/m(2) K are measured. High speed flow visualizations at frame rate of 2229 fps elucidate the flow boiling patterns inside the microgaps, including bubbly and foggy that are generated in the pin finned area. Surface temperatures are measured for heat fluxes up to 498 W/cm(2) which enable determination of heat transfer characteristics. (C) 2018 Elsevier Ltd. All rights reserved.