Heat transfer with phase change in microchannels may support applications with very high heat fluxes such as the thermal management of high-performance electronic devices. For electronics cooling application, refrigerating fluid HFE-7100 is one of the suitable candidates with appropriate properties and also considered as an environmental-friendly coolant. The objective of this study is to investigate the flow boiling heat transfer of dielectric liquid HFE-7100 in a copper-based microchannel heat sink. In particular, the effect of aspect ratio of a channel with approximately the same hydraulic diameter (about 1.12 mm) on flow boiling heat transfer and critical heat flux (CHF) are thoroughly studied. Flow boiling in a diverging microchannel heat sink with the mean aspect ratio, depth-to-width ratio, of 0.83, 0.99, 1.65, 2.47, 4.23, and 6.06 are investigated experimentally. The experimental results show that at a given wall superheat the base and wall heat fluxes increase with increasing the mass flux. Moreover, the critical heat flux increases linearly with increasing the mass flux. The results further demonstrate that the aspect ratio of microchannels affects boiling heat transfer significantly. The wall heat flux at a given wall superheat and wall critical heat flux demonstrate the peak value at the aspect ratio of 0.99 possibly due to copious existence of the liquid film around the corner. The experimental CHF results are further compared with existing CHF correlations of flow boiling in a microchannel, and a new empirical CHF correlation incorporated the aspect ratio is proposed. The predictions of the proposed correlation demonstrate a consistent trend and excellent agreement with present experimental results. The overall mean absolute error of the proposed correlation is about 3.02%, and all of the experimental CHF data are predicted within a ±10% error band. Under the condition of the same mass flux, the base critical heat flux increases monotonously with increasing the aspect ratio mainly due to the bigger heat transfer area. In the present study, the highest base critical heat flux reached is 1140 kW/m2 for the mass flux of 180 kg/m2s in the present diverging microchannel. From these results, we conclude that the diverging design of the microchannel with a large aspect ratio and high mass flux is recommended to develop an electronic cooling system for the need of very high heat dissipation rate.