Discover the details of flip-chip theta-JA thermal resistance characterization using Ansys Fluent.
As electronic devices become more powerful and compact, managing heat effectively is essential to maintain their reliability and extend their lifespan. Besides theta-JB and theta-JC thermal resistances, it is also important to understand the thermal dissipation of the flip-chips during natural convection conditions. Understanding and accurately predicting thermal dissipation during these conditions helps engineers to optimize cooling strategies for their final application.
Flip-chip designs are transforming high-performance electronics, but their intricate architecture poses challenges for accurate heat dissipation analysis. As power densities rise and layouts become more compact, estimating theta-JA thermal resistance early in the design phase is essential, even before the first physical prototype is built. High-power density AI packages frequently employ flip-chip package technologies, making precise thermal analysis and characterization essential for modern electronic system design. Many high-power density AI packages adopt flip-chip package technologies, which highlights the importance of precise thermal analysis and characterization for this type of package in modern electronic system design.
Key factors like chip geometry, material layering, interconnects, and underfill properties all impact thermal behavior. With Ansys Fluent, engineers can virtually model these parameters, simulate heat flow, airflow interactions, and predict temperature distributions. This enables early validation of thermal strategies, identification of critical hotspots, and optimization of cooling solutions
In this blog, I will go through the different steps to set up the simulation for characterizing the theta-JA thermal resistance in Ansys Fluent.
A custom-made flip chip and a 2S2P PCB were used for this example. An outer air domain is located around the assembly to model the air circulation. The computational domain is shown in Figure 1.
Figure 1. Computational Domain Flip Chip Package Setup.
Ansys Fluent allows you to set different material properties for each of the flip chip parts. It is important to create a body that represents each of the components so we can add the required thermal properties to them. For this demonstration, a simplified version was used and is shown in Figure 2. You can add any additional details that are important for your analysis.
Figure 2. Flip Chip Package Common Material Distribution.
To solve natural convection, we need to enable the energy equation, add gravity in the correct direction, and set the operating density. The operating density is a key parameter for this kind of application. An incorrect setup of this value could lead to unrealistic and inaccurate results. We recommend that, for this kind of application, you calculate the operating density based on the maximum density in all the openings. This setup will help with the convergence and also with the accuracy of the results. For this demonstration, it is only natural convection, but Fluent allows for the inclusion of radiation modeling if required.
For the viscous model, keep the default k-omega SST model configuration.
For the fluid phase, we assumed air at 25 °C for all the properties except for density. For the density, we need to calculate based on the incompressible ideal gas to capture the changes in density due to the temperature changes. You can go to the material, edit the air properties, and select it from the density drop-down list.
For non-isotropic solid materials, Ansys Fluent allows the configuration of orthotropic thermal conductivity. You can create new material, go to thermal conductivity, and select orthotropic. Then you can define a different coordinate system. Fluent will ask for two principal directions to form a plane, and the third direction is calculated by Fluent; then, you can assign the corresponding thermal conductivity in each direction.
To account for the thermal dissipation, a volumetric source term is applied in the cells of the die. In this demo, we used an expression that can be used to input the power in Watts, and Fluent calculates the corresponding W/m3 for the die. This configuration facilitates doing a parametric study, but you can also add a constant value in W/m^3.
Another approach is to add the thermal dissipation in a thin layer at the top of the die. This approach will be discussed in a different blog.
Boundary conditions are an important part of setting up the thermal cases. For theta-JA thermal resistance, the setup requires that all the heat generated be removed by the air through natural convection. The boundary conditions for this case are shown in Figure 3.
Figure 3. Boundary Condition Theta-JA Thermal Resistance Case.
After the simulation finishes, you can use the built-in postprocessing tools of Ansys Fluent to explore and calculate final values; some results are shown below.
To obtain the maximum temperature in the die, it is possible to use the report tool on the Results tab. For this demo, the results obtained are shown below:
Some contour air patterns, density, and temperature contours are possible to report within Fluent as shown below:
Figure 4. Velocity Vectors and Velocity Distribution on mid-plane YZ.
Figure 5. Density Distribution on Domain Mid-Plane YZ.
Figure 6. Temperature Distribution on Domain Mid-Plane YZ.
Figure 7. Temperature Distribution on the Package and Board Surface Theta-JA.
You can download the case file (2025R1)
Ansys provides powerful thermal simulation capabilities for semiconductor design, enabling engineers to analyze geometry configurations, material distributions, interconnect types, underfill properties, and substrate thickness—all without building physical prototypes. Beyond Ansys Fluent, Ansys has specialized tools like Icepak for electronics cooling, SiWave for signal and power integrity, Maxwell and HFSS for electromagnetic analysis, Mechanical for structural and thermal simulations, and DesignXplorer and OptiSLang for design optimization and parametric evaluation, making it a comprehensive suite for multi-physics modeling and performance refinement.
Ozen Engineering Inc. leverages its extensive consulting expertise in CFD, FEA, thermal, optics, photonics, and electromagnetic simulations to achieve exceptional results across various engineering projects, addressing complex challenges like multiphase flows, erosion modeling, and channel flows using Ansys software.
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