A transient numerical analysis for fluid flow and heat transfer from a planar jet impingement on a finite thickness substrate is performed. A discrete heating boundary condition is applied to the substrate’s under side; by including the effect of buoyancy, some assisting or opposing mixed convection configuration can be modelled and regions of momentum dominated, buoyancy dominated and unstable flows can be monitored. For low volumetric flows and small temperature differences, different competitive heat transfer modes can be detected, as conduction may affect heat transfer away from the impact site in the initial times, and flow pattern is driven by the ruling convective mechanism, whether forced or natural. The related flow field and local heat transfer rate are investigated as a function of a variety of geometry configurations, material coupling and thermal-fluid driving factors, for the unitary value of the mixed convection parameter Ri (transitional mixed convection).Normalized heat transfer coefficients along the impinged substrate are reported by a parametric evaluation and may be employed to control the distribution of heat transfer at the given configuration. The inclusion of the conduction mechanism in the analysis confirms that the conjugate effect (heat transfer reversal) cannot be neglected during the initial exposition when an opposing cooling jet configuration is realized, for the largest investigated Re.