Understanding the thermal path is essential when designing high-power electronic systems. This article explains how heat travels from the component junction to the heatsink and how engineers can identify and calculate each major thermal resistance along the path. It covers through-plane heat transfer, copper heat spreading, effective thermal area, dielectric and metal-base contributions, thermal interface resistance, and junction-temperature estimation. The objective is to provide a practical engineering framework for evaluating thermal performance and optimizing the PCB structure based on the actual heat-flow requirements.