Thermostatic applied on electronic chip

[schnuber]

Hi everybody
I am an electronic engineer and I have a question concerning thermostatic behaviour of an electronic component.

I have a printed circuit board (PCB) with electronic chips on it. At room temperature 25°C I measure with the infrared themometer the case temperature of one component which is 40°C (It is an ethernet PHY).
Now I made the following assumption: If I put the PCB in a climatic chamber and adjust the chamber temperature to 85°C degree then the case temperature of the component should be 100°C = (40+(85-25)). But that is not the case, it is lower, only 90°C.
So what is going on there I don't understand this.
Please help

 

[Mordred]

I don't believe its a linera rise to ambient temperature as I would have to study all the calculations involved in PCB I'll instead direct you to a claculator that I've used in the past.

http://home.comcast.net/~pcb.george/trace.html

 

[schnuber]

This is a calculator that computes the temperature and resistance of a copper trace on the PCB in relation to its mechanical dimensions and electrical current.
I don't see how that gives me an answer to my question.