Heat Sink Design: 6 Factors to Consider
Electronic components such as diodes, transistors, and integrated circuits generate considerable amounts of heat during operation. Extreme heating can damage or significantly affect the performance of semiconductor devices, and supplemental cooling is therefore required to maintain the temperature within the limits specified by a manufacturer. Whereas some electronic components can dissipate heat on their own, most optoelectronic devices — like lasers and power transistors such as MOSFETs and IGBTs — cannot sufficiently dissipate heat without a heat management solution. This is where a well-thought-out heat sink design can make a big difference.
Heat sinks are used in electronic devices and assemblies to provide supplemental cooling that is required to prevent overheating of components. These elements are designed and optimized to ensure that electronic devices are operated within the temperature ranges provided by manufacturers. Heat sinks are designed using thermal conductive materials such as copper and aluminum, and they work by dissipating heat through liquid cooling, natural convection, forced convection, or radiation. The thermal management needs vary from one application to another and it is, therefore, essential to look beyond the heat sink when designing a thermal solution for a particular application. Some of the important factors that should be considered when designing a thermal solution include heat sink level requirements, component level requirements, system level requirements, and chassis level requirements.
Key Considerations in Heat Sink Design
A heat sink transfers the thermal energy generated by an electronic assembly or component into a cooling medium. The heat is transferred from a higher temperature region (electronic component) to a lower temperature region (fluid medium) by conduction, convection, radiation or by a combination of these heat transfer methods. The performance of this passive heat exchanger is determined by many factors including the velocity of the coolant fluid, the thermal conductivity of the material, thermal interface material, and attachment method. For a specific application, the parameters of a heat sink can be precisely determined through modeling and analysis. To illustrate the key factors affecting the heat sink design performance we used one of the public projects from the SimScale Library — heat sink-electronics cooling using CHT. In the following image, SimScale was used to simulate the heat flow in a heat sink design.
1. Thermal Resistance
Thermal resistance refers to the sum of resistances to heat flow between the die and the coolant fluid. These heat flow resistances include the resistance between the die and the component casing, the resistance between the casing and the heat sink (thermal interface resistance), and the resistance between the heat sink and the fluid in motion. Thermal resistance does not factor non-uniform heat distribution and it is unsuitable for modeling systems that are not in thermal equilibrium.
Although the thermal resistance value is an approximation, it enables modeling and analysis of thermal characteristics of semiconductor devices and heat sinks. Analyses of different heat sink designs are used to determine heat sink geometries and parameters that enable maximum heat dissipation. Complex modeling of thermal characteristics can be achieved by meshing heat sinks using 3D thermal resistances. In the following image, the SimScale simulation software was used to mesh a heat sink model. The Hex-dominant Parametric (only CFD) mesh was used to generate the mesh for the 4 volumes (3 solids and 1 fluid). This is used to create refinements and maintain the volumes as different regions to later define interfaces.
Heat sinks are designed using materials that have high thermal conductivity such as aluminum alloys and copper. Copper offers excellent thermal conductivity, antimicrobial resistance, biofouling resistance, corrosion resistance, and heat absorption. Its properties make it an excellent material for heat sinks but it is more expensive and denser than aluminum.
Diamond offers a high thermal conductivity that makes it a suitable material for thermal applications. Its lattice vibrations account for its outstanding thermal conductivity. Composite materials such as AlSiC, Dymalloy, and copper-tungsten pseudo-alloy are also commonly used in thermal applications.
3. Arrangement, Shape, Size, and Location of Fins
The flow of the coolant medium is greatly impacted by the arrangement of fins on a heat sink. Optimizing the configuration helps to reduce fluid flow resistance thus allowing more air to go through a heat sink. The performance of a heatsink is also determined by the shape and design of fins. Optimizing the shape and size of the fins helps to maximize the heat transfer density. Through modeling, the performance of different fin shapes and configurations can be evaluated.
4. Fin Efficiency
A heat sink fin receives heat from an electronic device and dissipates it into the surrounding coolant fluid. The heat transferred by a fin to the coolant medium decreases as the distance from the base of the heat sink increases. Using a material that has a higher thermal conductivity and decreasing the aspect ratio of the fins help boost the overall efficiency of the fins. In the following image, the SimScale platform was used to analyze the temperature characteristics of a heat sink design.
5. Thermal Interface Material
Surface defects, roughness, and gaps increase thermal contact resistance thereby reducing the effectiveness of a thermal solution. These defects increase heat flow resistance by reducing the thermal contact area between an electronic component and its heat sink. Thermal resistance is reduced by increasing interface pressure and decreasing surface roughness. In most cases, there are limits to these resistance reduction methods. To overcome them, thermal interface materials are used. The electrical resistivity of a material, contact pressure, and size of the surface gaps should be considered when selecting a thermal interface material for a given thermal application.
6. Heat Sink Attachment Methods
The thermal performance of a heat sink can be enhanced by selecting an appropriate method of attaching a heat sink to an electronic device or component. The selection process should factor in both thermal and mechanical requirements of the thermal management solution. Common heat sink attachment methods include standoff spacers, flat spring clips, epoxy, and thermal tape.
Heat sinks are essential parts of most electronic assemblies, power electronic devices, and optoelectronic components. These passive heat exchangers dissipate heat generated by electronic devices to ensure that they are operating within the limits specified by manufacturers. Some of the key factors that should be considered when designing a heat sink include thermal resistance, material, fin configuration, fin size and shape, fin efficiency, heat sink attachment method, and thermal interface material. Geometries and parameters that provide maximum heat dissipation are obtained by analyzing different heat sink models.
To learn more about how engineering simulation can be used in electronics design, download this case study for free: Thermostructural Analysis of a Chip.