A system in package (SiP) is a number of integrated circuits integrated into a single module. SiP can perform various functions and are often used in small systems such as mobile phones and wearable devices. The fabrication of SiP can be challenging due to their small size and high complexity. For example, during the reflow soldering process, non-uniform temperature distribution can occur, affecting the reliability of the SiP. In this study, numerical simulation is used to investigate the thermal behavior of a SiP during the reflow process and the model is validated via experimental measurements. A forced convection reflow oven was modeled using computational fluid dynamics software where the heating of the SiP assembly was performed using a conjugate heat transfer model. A complex flow field in the reflow oven was observed from the simulation results, showing a free jet region, a stagnation flow region, a wall jet region, a recirculation region, and vortices. The simulation results agreed well with experimental data. The method developed here can accurately predict the temperature distribution in a reflow oven and allow the design of temperature profiles for the reflow process that result in minimal temperature variations across the SiP assembly. (C) 2016 Elsevier Ltd. All rights reserved.