We have a small PCB on which there are voltage regulators and MOSFET etc.

The bottom of the PCB is characterized to have 10 degC rise in temperature per W dissipation on the voltage regulators. The temperature can be measured by a temperature sensor mounted on the bottom of the PCB.

I am trying to do calculation for 10 W power dissipation with ambient temperature 40 degC.

Theoretically, the temperature at the bottom of the PCB will be 10 deg/W x 10 W + 40 degC = 100 degC. Please let me know if this calculation is correct.

The size of the exposed PCB where we can attach heat sink for a certain height is 2 cm x 2 cm.

 

10 deg/W x 10 W + 40 degC = 100 degC

Do your math!

 

This may not help you but it's an interesting and useful tool.
https://www.heatsinkcalculator.com

 

You might just as well guess. There are too many unknowns to be able to work it out.
What is your airflow? What will it be in six months' time after the dust has accumulated? What happens when a customer puts something on top of the vents? What is your power dissipation? How much does it change as the mains voltage varies?

 

The bottom of the PCB is characterized to have 10 degC rise in temperature per W dissipation on the voltage regulators. The temperature can be measured by a temperature sensor mounted on the bottom of the PCB.

10 degC rise over what? Ambient? Case temp?

Lets be clear about your stated info - does this diagram show what you mean? If not, show what you mean...



This PCB calculator might help, but I think it'll confirm that this looks un-viable @10W

https://www.heatsinkcalculator.com/pcb-temperature-calculator.html