Understanding two-phase convective heat transfer under extreme conditions of high heat and mass fluxes and confined geometry is of fundamental interest and practical significance. In particular, next generation electronics are becoming thermally limited in performance, as integration levels increase due to the emergence of 'hotspots' featuring up to ten-fold increase in local heat fluxes, resulting from non-uniform power distribution. An ultra-small clearance, 10 mu m microgap, was investigated to gain insight into physics of high mass flux refrigerant R134a flow boiling, and to assess its utility as a practical solution for hotspot thermal management. Two configurations - a bare microgap, and inline micro-pin fin populated microgap - were tested in terms of their ability to dissipate heat fluxes approaching 1.5 kW/cm(2). Extreme flow conditions were investigated, including mass fluxes up to 3000 kg/m(2) s at inlet pressures up to 1.5 MPa and exit vapor qualities approaching unity. Dominant flow regimes were identified and correlated to two phase heat transfer coefficients which were obtained using model based data reduction for both device configurations, The results obtained were compared to predictions using correlations from literature, with the maximum heat transfer coefficient reaching 1.5 MW/m(2) K in the vapor plume regime in the case of the finned microgap. (C ) 2016 Elsevier Ltd. All rights reserved.