Start with a power and temperature budget
Identify heat sources, dissipation under the relevant operating modes and allowable component or material temperatures. Distinguish electrical input power from the heat dissipated in a particular device. State ambient conditions, airflow and duty cycle.
Map the paths through the package, solder connection, copper, dielectric, mechanical interfaces and surrounding air. A PCB does not have one universal thermal conductivity that captures every layer and direction. Copper distribution and construction can make the effective behavior strongly directional.
Separate conductivity from thermal resistance
Thermal conductivity is a material property expressed in W/(m·K). Thermal resistance expresses a temperature rise per unit heat flow for a defined path, commonly K/W. Geometry and interfaces determine how a material property contributes to the complete path.
A simple uniform one-dimensional layer has thermal resistance approximately thickness divided by conductivity times area. Real boards include spreading, parallel paths and contact effects. A high-conductivity material number by itself does not establish the junction temperature of a populated product.
Review construction and interface options
Copper spreading, thermal vias, metal-core construction, ceramic substrates, heatsinks and airflow can address different limitations. Metal-core and insulated-metal-substrate terminology may overlap; define the actual dielectric and metal construction. DBC ceramic is another construction and should be evaluated on its own requirements.
For a heatsink interface, select the pad, paste or other material using gap, pressure, electrical isolation and environmental requirements. Application thickness and assembly condition matter. Do not substitute generic paste instructions for the supplier process and mechanical design.
Verify the system under representative conditions
Package thermal metrics are tied to defined test or characterization conditions and can be misused when applied directly to another board. Review the device supplier guidance before converting a datasheet metric into a predicted junction temperature.
Measure a representative assembly in the intended enclosure and relevant operating modes. Record ambient conditions, load, airflow and measurement method. Use the results to validate the thermal model, then control the material, interface and assembly details needed to reproduce the accepted behavior.