A new technology for onboard liquid cooling of high power density electronic devices is introduced via conformal encapsulation of the devices and direct contact liquid cooling. This research effort addresses size, weight and power constraints of onboard application with a CFD-enabled design that delivers a uniform coolant flow over single-and multi-device layouts through a microgap channel. The paradigm shift is the replacement of inefficient remote air cooling and associated high resistance conduction paths with the use of microgap flow boiling with direct coolant contact at the device level. The coolant used in all measurements is Novec™ 7200, and the electronics are emulated with resistance heaters on a 1â¶1 scale. Thermal performance is demonstrated at power densities on the order of 1 KW/cm3. Parameters investigated include average device temperature, pressure drop, flow field characterization, and overall heat transfer coefficients. For single chip encapsulation, thermal-fluid performance with microgaps of 0.25, 0.5 and 0.75 mm is determined. With low coolant inlet subcooling, two-phase heat transfer is seen at all coolant mass flows. Device temperatures reach 95 °C for power dissipation of 50-80 W depending on coolant flow for a gap of 0.5 mm. Inlet subcooling of 25 and 51 °C permits higher power dissipation with nucleate flow boiling on the device surface. For multi-device encapsulation comprising two memory chips arranged symmetrically in line with a larger processor, the best thermal performance is obtained for inlet flow over the processor. For all measurements, the gap between the processor and encapsulation is 0.5 mm, and the gap above the memory chips is 1.0 mm. For inlet coolant flow first over the memory chips, the small chips exceed the 95°C limit when processor power is ∼50 W or less. Processor temperature reaches 95 °C at ∼80 W over the range of coolant flows tested. For inlet flow first over the processor, memory device temperatures are approximately the same over all levels of processor and memory chip powers. For processor power 40 W, two-phase heat transfer dominates, and a processor power of 120 W is reached within the 95 °C threshold.