Hot Disk Addressing the Gap Between Thermal Simulation and Reality: The Critical Role of Material Characterization
For thermal management professionals, the accuracy of a simulation is only as reliable as the input data. A recurring challenge in the industry is the discrepancy between "catalog values" found in simulation software databases and the actual physical properties of the materials used in production. Using generic parameters for materials like copper or thermal interface materials (TIMs) is often the primary reason why simulation results fail to align with real-world hardware performance.
The "Pure Copper" Fallacy: Why Grade and Source Matter
Source:Simcenter Flotherm
In many CFD (Computational Fluid Dynamics) tools—such as Simcenter Flotherm—the material library offers limited choices for metals. However, "pure copper" is not a singular entity. The thermal conductivity (W/m.k) of copper varies significantly based on its oxygen content, impurities, and manufacturing process.
- Oxygen-Free Copper (OFC, e.g., C1020): Contains less than 10ppm oxygen. It can reach thermal conductivities as high as 420 W/m·K.
- Electrolytic Tough Pitch (ETP, e.g., C1100): Contains 100ppm to 500ppm oxygen. Thermal conductivity typically drops to around 380 W/m·K.
- Copper Alloys: The introduction of elements like Co, Mn, Ni, or Zn can cause thermal conductivity to plummet to as low as 30 W/m·K—a 10-fold difference compared to pure copper.
Even for the exact same metal grade, different suppliers or batches can exhibit high variance in thermal performance due to distinct purification processes.
Empirical Evidence: Testing 99.9% Pure Copper Samples
To illustrate this variance, five samples of 99.9% pure copper were tested using the Hot Disk TPS 3500 thermal constants analyzer, following ISO 22007-2 and ASTM E3088 standards. The results demonstrate that even high-purity samples are not uniform:
| Sample ID | Type | Thermal Conductivity (W/m·K) | Thermal Diffusivity (mm²/s) |
|---|---|---|---|
| C1020-1 | Oxygen-Free Copper | 422.4 | 125.0 |
| C1020-2 | Oxygen-Free Copper | 385.2 | 115.6 |
| C1100-1 | Electrolytic Copper | 352.7 | 106.0 |
| C1100-2 | Electrolytic Copper | 359.7 | 105.8 |
| C1100-3 | Electrolytic Copper | 377.3 | 109.5 |
Beyond Metals: The High Stakes of TIM and Component Parameters
The discrepancy is even more pronounced in non-metallic materials. Thermal Interface Materials (TIMs), molding compounds, and PCBs are notorious for having "marketing" specifications that differ from their end-use performance.
- TIM Variance: A generic TIM might be rated at 1 W/m·K in a database, while a high-performance Japanese-sourced TIM could reach 15 W/m·K—a 15x difference.
- Complex Assemblies: Parameters like specific heat, thermal diffusivity, and conductivity must be accurately defined for every layer, from the semiconductor package to the heat sink, to achieve a predictive model.
Conclusion: Precision Testing is the Solution
To bridge the gap between simulation and reality, professionals should not rely solely on software libraries or supplier datasheets. Direct material characterization of the specific components used in the BOM (Bill of Materials) is essential. By inputting measured values for thermal conductivity, diffusivity, and specific heat, engineers can ensure that their thermal models are a true "Digital Twin" of the physical product.
