This article investigates the effect of inlet orifices on saturated critical heat flux (CHF) in multi-microchannel heat sinks. Two different multi-microchannel heat sinks made in copper were tested with three low pressure refrigerants (R134a, R236fa, R245fa). One had 20 parallel rectangular microchannels of 467 x 4052 mu m (width x depth) while the other had 29 channels of 199 x 756 pm (width x depth). Flow visualization has been conducted with and without an orifice insert at the inlet of microchannels. Visualization confirmed the existence of flow instability, back flow and non-uniform distribution of flow among the channels when the insert with orifices was removed. Flow patterns in the microchannels and their evolution with increasing heat flux were observed. The flow boiling curves suggest that in the case of omitting the orifice, the boiling incipient occurred at a higher heat flux resulting in a higher overshoot of wall temperature. Moreover, the flow was easily subjected to instability and caused CHF to occur at much lower values than occurred with the orifices in place.