Thermal conductivity and thermal resistance are two fundamental parameters in IMS PCB thermal management, but they describe different aspects of heat transfer. Thermal conductivity indicates how efficiently a material conducts heat, while thermal resistance describes the resistance of a specific thermal path. This article explains the relationship between W/m·K and °C/W, and examines how dielectric thickness, heat-transfer area, copper thickness, metal-base material, TIM, and heatsink performance affect overall thermal performance. Using practical examples of 2W/m·K, 3W/m·K, 5W/m·K, and 8W/m·K+ IMS materials, ChengYue provides an engineering-focused guide to selecting the appropriate IMS construction for high-power LED, automotive electronics, EV power electronics, and industrial applications.