~2 Watts /in^2 is a good place to start with for warming up an IC, the only concern will be how to control the temperature from continuing to rise... Is there a temperature sensor you are using? I can't tell you exactly the temperature that your system will reach equilibrium, so you will likely need to do some testing with the heater and some kind of sensor. If this will be high volume you may want to consider a custom heater design with an integrated sensor.
"A witty saying proves nothing."
-Voltaire