Thermal resistance network models are thermal models that represent the complex characteristics of an electrical component in a simple form that can be used to carry out thermal analysis in many ways. We can also use thermal resistance models in CFD to obtain better results without adding excessive complexity to the simulation. Learn what thermal resistance network modelling is, how you can use them in SimScale, and how you can find the resistance values with a detailed thermal model if the manufacturer has not provided them.

1. By Darren Lynch
CFD webinar
Thermal Resistance
Modelling with SimScale

2. About SimScale
Who we are
● Founded in 2012
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● 80+ employees across 6+ time zones
● 150,000 users worldwide
● More than 300k simulation projects
Me

3. We created the world’s first cloud-based
engineering simulation platform.
● Fluid dynamics (CFD)
● Solid mechanics (FEA)
● Thermodynamics
All accessible via a web browser.
About SimScale
What we do

4. 1. Import your CAD model
2. Set up and define your simulation
3. Post-process your results
The SimScale solution
How to use SimScale
3 Results & Post-Processing
2 Simulation Setup
1 CAD Upload

5. Detailed and resistance thermal models
Thermal Resistance Modelling with SimScale
What are they, how can you calculate resistances, and how
can you use them in SimScale.

6. 1. When carrying out analytical thermal calculations in
the design process
2. When simulating electrical components in CFD
When would I need to
know about thermal
models?

7. 1. To reduce complexity
2. To reduce solve times
3. To improve convergence rate
4. To improve mesh quality
5. To maintain highly accurate results
Why use a thermal
resistance model?
Die
Casing
Leads
PCB
Resistance or
Simple Model
PCB
Detailed Model

8. 1. A (thermal) resistance is similar to electrical
resistance where the heat flow is being resisted,
and as a result a temperature gradient is present.
2. A resistance has a unit of K/W (Kelvin per Watt).
3. In the diagram on the left, we could calculate the
resistance from power and temperature difference.
4. Vice versa, if we knew the power and a single
temperature, we could calculate the remaining
temperature.
What is Thermal
Resistance?
T1 T2
P (power)
𝜃
(Resistance)
Temperature
Difference

9. 1. A component is typically complex in physical form,
with many components and materials playing a role
in thermal performance.
2. The resistance network describes how much
resistance there is between the heated element of a
component and the outside (case side, board side
and the sides).
3. The resistances can be used to calculate the junction
temperature in CFD knowing the temperatures at
each face.
4. Each branch in the network has a resistance; 𝛳jb
(junction to board), 𝛳jc (junction to case) and 𝛳js
(junction to side).
What is a resistance
network model?Case
Temperature
(Tc)
Board
Temperature
(Tb)
Side
Temperature
(Ts)
Side
Temperature
(Ts)

10. 1. A component is typically complex in physical form,
with many components and materials playing a role
in thermal performance.
2. The maximum temperature a component can reach
is usually defined by manufacturer, testing and is
given in the data sheet at maximum junction
temperature.
3. The junction is the heating part of the component
usually the die or equivalent.
What is junction
temperature?

11. 1. A thermal resistance network model can be defined
in SimScale as an advance concept.
2. The top face can be selected, and the resistance
networks can be specified.
3. Resistances can come from components tested in a
physical test environment or from a detailed model
virtually tested in CFD.
4. The power is defined exactly like you would in a
detailed or simple model.
Defining a resistance
network in SimScale

12. Results
Thermal Resistance Modelling with SimScale
Key results and visualizations.

13. 1. Junction temperature is within 1% difference in
comparison between the resistance and the detailed
model, showing an almost perfect corroboration.
2. Other temperatures are also similar, meaning that
the airflow surrounding the component is also likely
to show good agreement.
3. Largest difference is in the side temperatures,
where measurement method and complexities due
to the leads cause a difference of just under 20%.
Key results
comparison of models

14. Detailed model
Streamlines and surface temperatures
Results
Temperature(K)

15. Thermal resistance model
Streamlines and surface temperatures
Temperature(K)
Results

16. Thermal Resistance Modelling with
SimScale
CFD webinar
Thank you!