I'm trying to simulate a simple PCB (3x3" PCB with one IC for now).

I'm having some issues with understanding how user-defined Two-Resistor IC model works.

The results I'm getting show temperatures much higher than what we've observed / tested.

The input parameters when creating a user-defined 2R model are junction-case and junction-board thermal resistance, units are K/W.

The problem I'm seeing is that the simulation results are dependent on the size of the IC, though Theata_jc and Theta_jb are not (no length units).

So if I create two ICs with different sizes but same 2R model, they behave differently.

Does the simulation somehow convert the 2R model into actual thermal conductivity?

I would assume if I define Theta_junction-board as 10K/W, power input of 1W, I should get a temperature difference between the board and the IC of 10 degrees. Simulation results end up much higher.

What is a proper way to define a 2R component, do I make the size the full size of the package or just the center pad (QFN w/ center pad) where most of the heat flows?