Heat Propagation
Rate at which a material transfers thermal energy relative to its ability to store that energy defines its response to temperature changes. For a printed circuit board, thermal diffusivity determines how quickly heat from a high power component spreads across the laminate to the copper planes. A higher value indicates that the material reaches thermal equilibrium faster.
It is calculated by dividing the thermal conductivity by the product of density and specific heat capacity. This property is essential for predicting the transient thermal behavior of an electronic device during power up cycles.
Design Factor
Selection of the substrate material influences the longevity of the assembly. When a component enters a high power state, thermal diffusivity governs the initial temperature spike at the solder joint. Materials with high diffusivity prevent localized hot spots by conducting heat away from the source before the temperature can rise to a critical level.
Ceramic substrates often outperform standard FR4 in this regard.
Test Requirement
Laser flash analysis measures the time it takes for a heat pulse to travel through a sample. During the evaluation of new laminate materials, thermal diffusivity is measured to ensure the board can handle the heat of lead free reflow. If the value is too low, the internal layers may experience excessive stress during heating.
This test confirms the material is fit for use.
