This article explores the importance of accurately defining thermal properties as a key part of the thermal analysis process for PCBs. We will focus on how these definitions affect two primary types of analysis:
- Steady-State Thermal Analysis – Assumes the system has reached thermal equilibrium, with no changes in temperatures or heat flow over time.
- Transient Thermal Analysis – Considers how temperatures change over time.
Each of these approaches reflects a different perspective on material and system thermal behavior. Choosing the appropriate method—aligned with component type, operational conditions, and time scale—directly impacts result reliability, thermal design efficiency, and engineering decisions.
Before diving into the differences between analysis types and thermal properties, it’s important to understand two basic material behavior concepts:
- Isotropic Materials: Materials that exhibit identical physical properties in all directions. For example, when considering thermal conductivity, an isotropic material has the same value along all three axes (X, Y, Z), so a single value is sufficient.
- Orthotropic Materials: Materials whose physical properties vary by direction. For instance, in the case of thermal conductivity, orthotropic materials, such as composites or multi-material systems like PCBs, require separate values for each principal axis (X, Y, Z).
Key Thermal Properties in Analysis
We’ll now explore three core properties critical to meaningful thermal analysis—specific heat, density, and thermal conductivity—their significance, effects, and how they relate to steady-state vs. transient analysis.
Specific Heat Capacity [J/kg·K]
Represents the energy needed to raise the temperature of 1 kg of a material by 1 Kelvin.
- In Steady-State analysis, where temperatures don’t change over time, specific heat doesn’t directly influence results.
- In Transient analysis, specific heat is crucial, it determines how quickly a component heats or cools, i.e., how the material absorbs thermal energy over time.
In composite or orthotropic materials, specific heat should be calculated effectively based on the composition, such as PCBs made of FR4 and copper.
Density [kg/m³]
Density expresses a material’s mass relative to its volume.
- In Steady-State analysis, like specific heat, density doesn’t directly influence results, but still must be defined.
- In Transient analysis, it directly affects the system’s behavior—defining mass, thermal storage capacity, and thermal inertia.
For complex parts like PCBs, effective density must account for the contribution of different layers (e.g., FR4, copper).
Thermal Conductivity [W/m·K]
This defines a material’s ability to transfer heat and it’s one of the most influential properties in heat dispersion.
- In Steady-State analysis, thermal conductivity is the most important factor, dictating how heat is distributed and where hot spots form.
- In Transient analysis, conductivity affects not just heat direction and distribution but also how fast heat propagates over time.
Unlike specific heat and density, which matter mostly in Transient analysis, thermal conductivity is critical in both.
Why is a PCB an Orthotropic Material?
PCBs are composed of layered structures:
- Conductive copper layers
- Insulating FR4 substrates
This layering creates directional thermal behavior:
- In the XY plane (board surface): copper allows efficient heat spreading, so conductivity is relatively high.
- In the Z direction (through thickness): most of the material is FR4, which conducts heat poorly (This can be improved using vias or embedded copper, depending on design.
Thus, the PCB behaves as an orthotropic material, requiring directional thermal property definitions.
Conclusion
A deep understanding of thermal properties—specific heat, density, and thermal conductivity—is essential for conducting accurate and reliable thermal analysis of electronic circuits. While specific heat and density mainly affect transient analysis, thermal conductivity is critical in both steady-state and transient evaluations.
Moreover, when dealing with orthotropic materials such as PCBs, it is not enough to simply input general property values. Material properties especially thermal conductivity must be defined per axis (X, Y, and Z) to reflect the directional behavior of layered structures. Ignoring this anisotropy can lead to significant errors in heat flow prediction and thermal performance.
Choosing the appropriate analysis type and aligning it with precise, axis-specific material definitions enables smarter, data-driven thermal design, leading to stable systems and optimal long-term performance.
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