As integration levels increase in next generation electronics, high power density devices become more susceptible to hotspot formation, which often imposes a thermal limitation on performance. Flow boiling of R134a in two microgap heat sink configurations was investigated as a solution for hotspot thermal management: a bare microgap and inline micro-pin fin populated microgap, both with 10 μm gap height, were tested in terms of their ability to dissipate heat fluxes approaching 5 kW/cm2 at the heat source. Additional parameters investigated include mass fluxes up to 3000 kg/m2 s at inlet pressures up to 1.5 MPa and exit qualities approaching unity. The microgap testbeds investigated consist of a silicon layer which is heated from the bottom using resistive heaters and capped with glass to enable visual observation of two-phase flow regimes. Wall temperature, device thermal resistance, and pressure drop results are presented and mapped to the dominant flow regimes that were observed in the microgap.