TL;DR:
Thermal stress in ceramic substrates happens when materials expand or contract unevenly due to high temperatures, common in power electronics. Major consequences include cracking of the ceramic and delamination at ceramic–metal interfaces, which can reduce reliability and performance. Main causes are elevated operating temperatures, mismatched thermal expansion between materials, and rapid temperature changes. To prevent or reduce thermal stress, focus on improving thermal management through effective cooling, using materials with compatible thermal expansion, and designing substrates to distribute heat more evenly.
Thermal stress in ceramic substrates: causes and prevention
What Is Thermal Stress in Ceramic Substrates?
Uniformity thermal environment causing thermal stress, which because the inadequate adaption of internal and external constrain. The reliability of thermal stress magnitude estimates depends directly on the precision of quantitative data related to changes in thermal conditions. Since thermal environment generate amount thermal stress. Combining heat expansion and contraction cycle intensify, microcracks propagate in brittle ceramics, accelerating degradation. High degree of brittleness result in low resistive power and high sensitive to mechanical, and catastrophic failure that occurs without the deformation of plastic typically observed in metals. Thus, identifying the causes of thermal stress and implementing preventive measures are crucial.
Main Causes of Thermal Stress in Ceramic Substrates
coefficients of thermal expansion
In electronic packaging, differences in CTE between the insulated base layer, which is commonly a particulate composite, and the substrate may generate thermal stresses that increase the risk of failure. Aluminum composite material with high thermal conductivity and low coefficients of thermal expansion are considered promising materials for aerospace and electronic application. However, these composites inherently experience thermal mismatch stresses due to the large difference in coefficients of thermal expansion between ceramic reinforcements and the aluminum matrix when subjected to elevated temperatures. This thermal expansion mismatch induces compressive stresses in the aluminum matrix and tensile stress in the reinforcement phase.
| Material | Thermal Conductivity (W/m·K) | Coefficient of Thermal Expansion (×10⁻⁶/K) | Fracture Toughness (MPa·m½) | Typical Thermal Shock Resistance |
|---|---|---|---|---|
| Alumina (Al₂O₃) | 20 – 30 | 6.5 – 8 | 3 – 4 | 170-300 |
| Silicon Nitride (Si₃N₄) | 70 – 90 | 2.8 – 3.2 | 6 – 7 | 900 |
| Silicon Carbide (SiC) | 120 – 200 | 3.8 – 4.5 | 3 – 4 | 400 |
In certain composite systems, such as SiC-reinforced metal matrices, the presence of residual compression stresses can lead to pore shrinkage[1], thereby making low porosity of the material. As a result, the overall coefficient of thermal expansion of the composite is further decreased.
Rapid Temperature Changes(rtc)
The mechanical performance of materials under transient power dissipated is strongly influenced by multiple intrinsic properties, such as thermal coefficient of expansion, thermal conductivity, thermal diffusivity, fracture toughness, tensile strength, and elastic parameters. For functionally graded materials, additional factors related to material nonuniformity, particularly the gradient index, have a pronounced impact on transient thermal stresses and the likelihood of thermal fracture stress[3].A functionally graded material(FGM) is made up two types of materials, which the composition and corresponding properties of physical change gradually from one surface to the other. When ceramic substrate suffer from temperature change, transient temperature reveals, leading to the thermal stress. When contacting with temperature difference at the same time, resulting in thermal shock. Thermal shock severity is controlled by the process of thermal energy exchange by temperature difference between the material and its environment. Compared with ascending thermal shock, descending thermal shock is more damaging to materials that are brittle because it causes surface tensile stresses. On the other hand, thermal conductivity of materials is influenced by high temperature of ceramic structure, it increases the specific heat and specific heat capacity of the composite. Therefore, thermal stress become essential. Which can produce crack, making ceramic fracture or damage[2].
Elevated Temperature in Power Electronics
High operating temperatures are a defining characteristic of power packaging, particularly IGBTs, where the temperature at junction can increase dramatically under transient fault conditions such as overloads and short circuit. Even within short time durations, the temperature of device may reach several hundred degrees Celsius[4]. In this temperature range, the thermophysical properties of key materials (e.g., Si, Al, and Cu) exhibit pronounced temperature dependence, especially thermal conductivity and specific heat. As a result, the thermal response of power supply module is strongly governed by local temperature, and neglecting temperature-dependent parameters in conventional thermal model can lead to significant inaccuracies in junction temperature and thermal properties of materials.
Cracking of Ceramic Substrates
As the interfacial energy is approached, stress relaxation localizes at the film center, and damage initiates there before propagates outward to the edges in homogeneous film/substrate systems. Radial residual strain reduce interfacial strength and shift damage initiates at the interface to stress concentric zone, while axial residual compressive stress increases the interface resistance to failure. Crack propagation is influenced by film–substrate friction and neck growth rate, although continuum in mechanics struggles to explain crack initiation. Discrete element modelling suggests that particle rearrangement plays a key role. Which restricted rearrangement increases local stress and promotes cracking, while easier rearrangement can suppress delamination[5]. Moreover, crack initiation does not necessarily require pre-existing defects, although geometric constraint is essential for crack propagation.
Delamination at Ceramic–Metal Interfaces
The fracture behavior of co-continuous metal–composites and ceramics was investigated and compared with conventional metal matrix composites, with neutron diffraction used to quantify thermal residual stress. Results showed that the ceramic phase was under compressive stress, while the metal phase experienced tensile stress, with significantly higher tensile stresses in the Cu phase than in the Al phase due to differences in volume fraction, melting point, and phase contiguity. In situ three-point bending observations revealed that, in Cu/Al₂O₃composites, high tensile stresses in the Cu phase and at the metal–ceramic interfaces promoted crack propagation within the metal and along the interfaces, whereas in Al/Al₂O₃ systems cracks mainly propagated through the ceramic phase. Finite element simulations further confirmed that thermal residual stresses strongly influence stress distribution and fracture paths. Overall, the study demonstrates that thermal residual stresses and phase contiguity play a decisive role in governing crack propagation and interfacial failure in metal–ceramic composite.
How to Prevent or Reduce Thermal Stress in Ceramic Substrates
Selecting Materials with Compatible Thermal Expansion
Using materials with high thermal conductivity helps reduce temperature gradient and thereby lowers thermal stress levels. Structural design strategies, such as optimizing substrate thickness and introducing compliant or buffer layers, can further alleviate stress concentration at interfaces. Aluminum oxide is a common substrate, popular for cheaper price and insulation properties. However, the thermal conductivity of alumina is not appropriate for high power modules. Aln is widely use for high modules with higher conductivity (180 W·m−1·K−1).
Improving Thermal Management and Cooling
Matching CTE of materials is a method to prevent fracturing and reduce thermal stress. The combination of intrinsic brittleness, low thermal conductivity, and high stiffness renders ceramic materials especially susceptible to catastrophic fracture under thermal loading conditions that produce high-magnitude thermal stresses. Unstable environment like temperature change and sudden heating or cooling arise thermal stress. To solve this, according to characterizes in terms of heat flux characteristics, temperature rate of change, temperature range, and absolute temperatures, as these variables may be functions of both space and time in thermal management. Under the strict control, heat transfer is unstable and uncertain in real states. So taking tests and analysis is recommend. Minimizing heat loss is crucial for fuel-efficient turbine and engine components, favoring low thermal conductivity materials. However, enhanced thermal stress resistance requires higher thermal conductivity, presenting a significant materials design challenge. In order to meet demands of both functional component and improve thermal stress ability[6]. A practical strategy to mitigate this constraint is to build large structures by combining multiple small, interlocking elements that are not rigidly bonded but loosely fitted together. Thermal barrier coating is another way to approach, generating compressive residual stresses in areas of ceramic components that are most prone to tensile thermal stress.
Ceramic substrate is essential for heat dissipation, relating to reliability and a durability. Thick copper–clad ceramic substrates for power electronics are generally known as Thick Printed Copper (TPC) substrates[7]. This technology involves the selective deposition of a specially designed copper paste, which is subsequently sintered in a nitrogen atmosphere at temperatures above 900 °C to improve surface tension.
Through this process, the ceramic substrate is usually metallized on both sides. The upper metallized layer is primarily used for circuit routing and component interconnection, whereas the lower copper layer acts as a heat-spreading interface. By distributing heat over a wider area, the backside metallization enhances thermal dissipation efficiency and allows effective integration with metal baseplates or heat sinks for improved cooling performance in high-power applications.
