Copper thickness is an important design variable in IMS and Metal Core PCBs, influencing not only current-carrying capability but also lateral heat spreading. This article explains how 1 oz, 2 oz, 3 oz and heavier copper constructions affect thermal performance, and why thicker copper does not automatically result in lower overall thermal resistance. Based on ChengYue’s IMS engineering approach, it examines the relationship between copper thickness, copper spreading area, dielectric thermal conductivity, dielectric thickness, metal-base material and heat-source geometry. The article provides practical engineering guidance for selecting copper thickness in high-power LED, automotive electronics, EV power systems and industrial applications.