Novel hierarchical radially expanding micro-channel networks with pin fins have been proposed recently to enable high-performance embedded two-phase liquid cooling of two-and three-dimensional integrated circuits dissipating extremely high heat fluxes (of the order of 1kW/cm2) . The effective design of such a complex two-phase liquid cooling architecture requires a comprehensive understanding of the various constituent sub-systems. Fundamental experiments were performed as a part of this work to study and model two-phase flow boiling and heat transfer using R-1234ze refrigerant in a two-port micro-scale cavity populated with pin fins which provide structures to accommodate vertical electrical interconnects (TSVs) as well as enhance heat transfer. In this first part of a two-part paper, results from the aforementioned fundamental study are presented. First, experimental procedure, including motivation, test set up, data acquisition and analysis is described. Next, the procedure for data reduction is detailed where an assumption of one-dimensional (1D) heat conduction in silicon is applied to resolve the two-phase flow boiling data. From this reduced data, empirical pressure drop and heat transfer correlations were derived as a function of mass flux, wall heat flux, pin fin angle of attack and the local vapor quality. These correlations were used to simulate and design two-phase cooled microchannels with enhanced heat transfer geometries such as pin fins, using compact low-complexity thermal models called STEAM and RTP. The accuracy and the speed of the models are demonstrated using simulations and validation against the experimental data.