Looking at the datasheet for the TPS54561-Q1, I am trying to figure out why Rjb (12.3°C/W) is so much greater than Rjc_bot (2.2°C/W).
If the junction to case resistance is 2.2 °C/W and we are belly-soldering the exposed thermal pad to the board and using 5 thermal vias through the board, I fail to see why there is so much additional thermal resistance through the solder joint.
I calculate that if the solder joint is .25 mm thick that would add an additional resistance of around .62 °C/W, which would make Rjb equal to 2.2 + .62 = 2.82 °C/W.
Can someone tell me what I am missing? I am trying to determine which number to use for our thermal analysis and the difference in these two numbers determine whether we have positive thermal margins.
