Microchannel based heat sinks are indicated to have a strong potential to be used in high-power dissipation cooling systems, such as in electronics cooling [1]. The heat sink's geometry must be customized and optimized, as reported in various experimental and numerical studies in the literature [2]. On the other hand, the use of liquid phase change is considered a large advantage in forced convection cooling systems, due to the additional term related with the latent heat of vaporization. However, controlling two-phase flows in microchannels is much more difficult due to the instabilities in the flow and in the heat transfer mechanisms, arising from the presence of the vapour bubbles. Furthermore, the presence of the bubbles is much more difficult to model, increasing the complexity of the system to control. In this context, the present work complements a numerical approach towards the optimization of a microchannel based heat sink and addresses the experimental characterization of the heat transfer processes in well-defined imposed heat fluxes, for flow boiling regimes in a single microchannel. Different imposed working conditions are firstly addressed to identify the boiling regimes, as a function of the imposed heat flux and mass flow rates. Then, under a controlled bubbly flow regime, bubble dynamics is detailed and characterized in terms of the forces locally applied on the bubbles, which in turn may affect the fluid flow and the heat transfer. The results allow identifying several flow regimes, namely liquid, bubbly and slug flow. The latter was observed to cause considerable instability in the pressure and backflow. Less impact was observed in the bubbly flow regime. When using higher heat fluxes, higher mass fluxes can prevent a slug flow from clogging the device. These can be achieved by either having a higher input volumetric flow or a larger section.