(theta ja) = (theta jc) (theta cs) (theta sa)
where:
theta jc: junction-to-case thermal resistance
theta cs: case-to-sink thermal resistance
theta sa: sink-to-ambient thermal resistance
Theta ja is the junction-to-ambient thermal resistance. When a heat sink is involved, there are three basic materials that affect thermal resistance. The first is the actual device, whose thermal resistance is theta jc. The second value is the thermal resistance of the material used to stick or glue the heat sink to the semiconductor package (theta cs). The final value is the thermal resistance of the heat sink itself (theta sa). You can find the theta jc value from the Altera Device Package Information Data Sheet. However, you must contact the glue and heat sink third-party vendors for theta cs and theta sa.