Flow patterns and heat transfer characteristics during flow boiling in a microchannel heat sink with a condensing cover plate have been numerically investigated. The phase-field model is used to analyze the effects of different thermal conditions of the condensing cover plate on the bubble dynamics and heat transfer characteristics. Further, the effects of the wettabilities of the cover plate and the heating surface on the bubble evolution have been studied for different values of coolant mass flux. The condensing cover plate maintained at 50 °C significantly improves heat transfer performance with an enhancement of 23.97 % compared to the condenser plate maintained at 80 °C and a 27.35 % improvement compared to an insulated cover plate. The hydrophobicity of the condensing cover plate facilitates vapor bubble evacuation from nucleation sites, producing maximum heat transfer at a contact angle of 130°. Heat transfer performance decreases beyond the 130° contact angle due to the bubble attachment to the condensing cover plate and impeding subsequent nucleation from the cavity. The maximum heat transfer is observed for the heating surface with a contact angle of 60°. The heat transfer in the microchannel is deteriorated below and above this contact angle. At low coolant mass flux, the heat transfer is controlled by bubble nucleation. After reaching a steady state, the combined effects of bubble nucleation and liquid inertia affect the bubble generation frequency and heat transfer in the microchannel.