Junction Temperature Vs Ambient Temperature

Dear FPMKh,

FPMKh said:

I just want to make sure that the temperature we set in corner definitions are actually "junction" temperatures and not "ambient" temperatures. Is that correct?

Let me give you my understanding and the motivation for my answer. Setting the temperature to, say, T_value, for a set of subcircuits in a simulation sets the ambient temperature for all the specified subcircuits to T_value degrees C. A conventional transient, AC, or DC spectre simulation does not know about the theta [1]. Therefore, the values of theta_ja or theta_jc are set to 0. This essentially means that the junction temperature is the ambient temperature. 

If you were to use the Cadence electrically aware flow for a design, it will provide a set of values for the change in temperature of a specific net  from the ambient with its knowledge of the power conducted by a trace and the thermal parameters. 

Therefore, to guarantee the reliability of a design in light of its performance at your defined extreme junction temperature, choose the extreme junction temperature that a design will see for your spectre performance based simulations. Subsequently, through a set of electrically aware simulations and the actual maximum ambient temperature specification for your design (note - not the junction temperature), verify that the resulting set of temperature rises combined with the maximum ambient temperature do not exceed the extreme junction temperature you used in your performance simulations. Of course, the electrically aware design flow will also monitor the current densities of features to allow validation of its EM performance.

I hope this helps explain both my understanding and the methodology I have used to validate performance in view of a circuits reliability requirements.

Shawn

[1] Theta is a thermal parameter relating power to junction to ambient temperature (theta_ja) or junction to case temperature (theta_jc) nor about the physical environment for the devices . Theta_ja is defined by: TJ = Ta + (PD x theta_ja) where TJ is the junction temperature and Ta is the ambient temperature.

Dear FPMKh,

FPMKh said:

I just want to make sure that the temperature we set in corner definitions are actually "junction" temperatures and not "ambient" temperatures. Is that correct?

Let me give you my understanding and the motivation for my answer. Setting the temperature to, say, T_value, for a set of subcircuits in a simulation sets the ambient temperature for all the specified subcircuits to T_value degrees C. A conventional transient, AC, or DC spectre simulation does not know about the theta [1]. Therefore, the values of theta_ja or theta_jc are set to 0. This essentially means that the junction temperature is the ambient temperature. 

If you were to use the Cadence electrically aware flow for a design, it will provide a set of values for the change in temperature of a specific net  from the ambient with its knowledge of the power conducted by a trace and the thermal parameters. 

Therefore, to guarantee the reliability of a design in light of its performance at your defined extreme junction temperature, choose the extreme junction temperature that a design will see for your spectre performance based simulations. Subsequently, through a set of electrically aware simulations and the actual maximum ambient temperature specification for your design (note - not the junction temperature), verify that the resulting set of temperature rises combined with the maximum ambient temperature do not exceed the extreme junction temperature you used in your performance simulations. Of course, the electrically aware design flow will also monitor the current densities of features to allow validation of its EM performance.

I hope this helps explain both my understanding and the methodology I have used to validate performance in view of a circuits reliability requirements.

Shawn

[1] Theta is a thermal parameter relating power to junction to ambient temperature (theta_ja) or junction to case temperature (theta_jc) nor about the physical environment for the devices . Theta_ja is defined by: TJ = Ta + (PD x theta_ja) where TJ is the junction temperature and Ta is the ambient temperature.