Traditionally, analytical models are developed to predict the thermal behavior of Printed Circuit Boards (PCBs). The predicted results are then verified with other analytical or experimental results prior to using the models in design. This is a time intensive process and the results predicted using these models may include limitation due to idealization. In order to overcome this disadvantage, a semi-analytical model is proposed in this study. The semi-analytical model uses analytic theory in conjunction with results obtained from an Infrared-Radiation Thermal Imaging System (IRTIS). The model is verified and updated, allowing the results to be immediately incorporated into PCB thermal design. The parameters in the semi-analytical model are determined by employing a least squares temperature curve fit for the first upstream heating component in conjunction with a simple conduction model for the PCB wall. The model was tested with a natural convection cooled PCB with three inline heating components. Temperature data for the first upstream component was measured with an IRTIS and fit to the semi-analytical model. The model predicted temperature results for all three heating components with good agreement. The PCB thermal design technique enables the IRTIS to be employed as a preliminary design tool that significantly reduces design iterations through the use of the semi-analytical model.