Power Module Substrate Thermal Stress Balance
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Solution Overview
Problem
Existing heat sink-attached power module substrate boards face warping issues due to thermal expansion differences between materials, leading to reduced bonding reliability and increased thermal resistance, which cannot be effectively addressed when the circuit layer and heat sink are made from different materials.
Innovation Solution
A heat sink-attached power module substrate board design featuring a ceramic substrate with a laminated circuit layer and heat sink, where the first layer is made of high-purity aluminum, the second layer is copper, and the heat sink is lower-purity aluminum, using a copper-based heat radiation-side bonding material, with specific ratios and areas to ensure symmetry and minimize thermal stress.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a heat sink is joined on a metal layer and circuit layer to form a symmetric structure centering the ceramic substrate board, then the rigidity difference between upper and lower surfaces is reduced, but warping still occurs due to thermal expansion differences when materials are different
Solution Approach 1:
The patent applies parameter changes by precisely controlling the thickness parameters of different layers. The circuit layer thickness is set to 0.5-2.0 mm and the metal layer thickness to 0.3-1.5 mm, creating a specific thickness ratio that compensates for thermal expansion differences between aluminum and copper materials, thereby reducing warping while maintaining structural symmetry
Solution Approach 2:
The patent uses composite materials by combining aluminum for the heat sink and metal layer with copper for the circuit layer. This multi-material composite structure leverages the high thermal conductivity of aluminum and the high electrical conductivity of copper, while the specific thickness design ensures thermal stress balance between different materials
2Reliability
If the circuit layer and metal layer are made of different materials (aluminum and copper), then electrical conductivity is improved, but thermal stress imbalance causes warping
Solution Approach 1:
The patent resolves the thermal stress imbalance by changing the thickness parameters of different layers. By setting the circuit layer thickness to 0.5-2.0 mm and metal layer thickness to 0.3-1.5 mm, the design creates a thickness ratio that balances the thermal expansion forces between aluminum and copper, allowing different materials to coexist without warping
Solution Approach 2:
The patent applies local quality by assigning different materials to different locations based on functional requirements: copper is used in the circuit layer where high electrical conductivity is needed, while aluminum is used in the heat sink and metal layer where high thermal conductivity is prioritized. The specific thickness distribution locally balances the thermal stresses
3Loss of energy
If the heat sink is fixed on a cooler with thermal conductive grease, then heat radiation is improved, but warping causes grease outflow and reduced adhesiveness
Solution Approach 1:
The patent inverts the conventional approach by not relying solely on the heat sink's rigidity to prevent warping, but instead designing the entire layered structure (circuit layer, ceramic substrate, metal layer) with balanced thicknesses that collectively compensate for thermal expansion. This structural inversion distributes the stress balance across multiple layers rather than depending on a single component
4Ease of manufacture
If the circuit layer and metal layer have substantially the same thickness, then manufacturing is simplified, but rigidity difference causes warping under thermal load
Solution Approach 1:
The patent moves away from equal thickness parameters to optimized thickness parameters. The circuit layer is designed with thickness of 0.5-2.0 mm while the metal layer has thickness of 0.3-1.5 mm. This parameter optimization creates a thickness ratio that balances thermal stresses, improving warp resistance while remaining manufacturable
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This design reduces warping and improves bonding reliability by stabilizing thermal stress and maintaining effective heat radiation, even under varying temperature conditions.
Implementation Method 1
a heat sink joined on the metal layer with heat radiation-side bonding material therebetween
Implementation Method 2
a warp may be generated when it is heated in a mounting step of the semiconductors, or by temperature variation in usage environment
Implementation Method 3
the heat sink-attached power module substrate board... reduces warping and improves bonding reliability by stabilizing thermal stress
Data Source
Figure 1
Figure 2
Figure 3A~3B
AI summary
A heat sink-attached power module substrate board has a ratio (A1×t1×σ1×α1)/{(A2×t2×σ2×α2)+(A3×t3×σ3×α3)} at 25°C is not less than 0.70 and not more than 1.30, where A1 (mm2) is a bonding area of a second layer and a first layer composing a circuit layer; t1 (mm) is an equivalent board thickness of the second layer; σ1 (N/mm2) is an yield strength of the second layer; and α1 (/K) is a linear expansion coefficient of the second layer, where A2 (mm2) is a bonding area of the heat radiation-side bonding material and the metal layer; t2 (mm) is an equivalent board thickness of the heat radiation-side bonding material; σ2 (N/mm2) is an yield strength of the heat radiation-side bonding material; and α2 (/K) is a linear expansion coefficient of the heat radiation-side bonding material, and where A3 (mm2) is a bonding area of the heat sink and the heat radiation-side bonding material; t3 (mm) is an equivalent board thickness of the heat sink; σ3 (N/mm2) is an yield strength of the heat sink; and α3 (/K) is a linear expansion coefficient of the heat sink.