Heat sinks are the easiest component to overlook in machinery and electrical circuits. But this is not the case when designing hardware because heat sinks serve a very crucial function. Almost all technology including CPUs, diodes, and transistors generates thermal energy which can decrease thermal performance and make operations inefficient.
In order to overcome the challenges of heat dissipation, different types of heat sinks are designed to serve different elements and keep them operating at their best-suited temperatures.
With the rapid development in technology (especially nano-tech) and making things as compact and efficient as possible, the right heat sink design has become a check and balance while manufacturing electrical products. So, this article will discuss the detail and design considerations of the heat sinks.
What is a Heat Sink?
A heat sink is a device that removes excess heat away from another component. In an integrated circuit, a heat sink is made of a material with high thermal conductivity to dissipate unwanted heat. It works through expanding surface area and allowing cooler air or fluid to pass across its surface area.
According to Fourier’s law heat transfers from a hotter to a cooler area. A Heat sink based on this law operates through a temperature gradient. It takes unwanted heat generated in an electrical component and transfers it to colder air or fluid through heat transfer methods like conduction or natural convection.
Types of Heat Sinks
The first step in identifying the design of heat sinks is to determine the configuration type. There are two popular heat sink types.
Passive Heat sinks
Passive heat sinks are the conventional option, they use natural airflow. Natural convection sets up across the heat sink that moves heat away from the component. They are cheaper to operate and require no extra electrical power to function.
Active Heat Sinks
Active heat sinks are more efficient and powerful, but they require forced convection that uses air or fluid to make heat transfer possible. They can be easily identified because they use electrically powered devices like a fan near the heat sink.
Choosing either heat sink leads to a variation in design. For instance, active heat sinks relying on fans need a smaller surface area and lighter material to dissipate the same amount of heat as a passive conductor.
Factors and Considerations in Heat Sink Design
Any heat sink works on the same general principle. First, heat is produced in a component and the heat sink aids in the dissipation of said heat. A heat sink attached to the component or circuit receives heat via conduction or through heat pipes. The thermal conductivity of the heat sink material affects this process significantly.
Next, heat transfer within the entire heat sink occurs. The passive heat exchanger or heat sink is hotter near the source of the heat and colder towards the other end, as such natural conduction transfers the heat unevenly across the surface.
After this, heat dissipates away through natural and forced convection. Air or fluid passing directly above the heat sink’s surface removes the generated heat using diffusion and transfers it to the ambient air.
By now, it is evident a lot of factors impact heat sink performance including material, geometry, and coolant type. Heat sink design prior to making one should consider these parameters.
1. Thermal Resistance
Thermal resistance can simply be defined as the combined thermal resistance the generated heat faces as it moves through a temperature gradient. This includes thermal resistance within the heat sink assembly, any friction, and thermal energy wastes because of resistance between the coolant and heat exchanger surface.
Thermal resistance can be calculated using the thermal resistance value. It is extremely helpful in finding out the most effective thermal resistance for components and ICs.
2. Types of Materials
Heat sink design is largely affected by the material of choice. Popular materials for heat sinks include aluminum alloys like AA 6063-T6 and copper tungsten or copper-molybdenum. Copper heat sinks offer excellent thermal conductivity and corrosion resistance but are heavier and more expensive than aluminum. Moreover, Chemical Vapor Diamonds (CVDs), lab-grown diamonds are used owing to their thermal conductivity coming from vibrations within their lattice structure.
Other heat-generating devices affected by thermal resistance include lithium-ion batteries used in cars and transferring heat away from these batteries is crucial, otherwise, thermal runaway can lead to a chain reaction destroying the battery. A carbon fiber heat sink with higher conductivity offers a thermal management solution to the car industry.
3. Heat Sink Fins
Fins are the component of a heat sink that allows heat transfer away from the heat sink toward ambient temperature. The shape and location can largely affect how the heat sink transfers energy while the size and a sheer number of these fins determine its efficiency.
3.1 Arrangement, Shape, Size, and Location of Fins
The fins are where the cooling fluid like air passes over the heat sink allowing heat dissipation. Thus, arrangement and size are important in heat sink construction. Enhancing these factors can lead to better heat dissipation and easier heat flow. In a study on Science Direct, the findings of a report discussed how shape can affect the thermal management of heat sinks.
“The results showed that the circular fins enhanced the thermal performance of the heat sink and lower the thermal resistance by 25% and 12% than the other two shapes” – Case Studies in Thermal Engineering
3.2 Fin Efficiency
Electrical components in extensive technology units like servers produce excessive heat. Slow heat transfer can reduce performance and lifetime. A heat pipe and liquid cooling have great potential, but a finned heat exchanger is essential for reliably and continuously dissipating heat.
Fins enlarge the surface for power dissipation in electrical components, and factors like thickness, height, and thermal resistance impact the fin efficiency. To improve efficiency, the thermal boundary layer is made thinner, and the direction of the air is set at the optimum angle with the heat sink itself made from a material with high thermal conductivity.
4. Thermal Interface Material
The heat sink has to be attached to electrical components and circuits. When doing so, the interface between the heat exchangers and typical electrical components is called thermal interface material.
Thermal interface resistance can reduce the efficiency of heat sinks and surface roughness, and trapped air and voids can increase thermal contact resistance. Naturally, with these flaws in the interface, the heat transfer is poor and to lower thermal resistance various thermal interface materials like phase-changing ones are used. The component’s thermal resistance is decreased as air is removed and the voids are filled, ensuring complete contact.
5. Heat Sink Attachment Methods
A heat sink is used in a variety of places including mechanical components. The choice of heat sink attachment can significantly improve the max power dissipation and dissipate more heat energy. The widely used methods include thermal tape, epoxy, clips, and push-pins.
Many factors in heat sink design affect the choice of attachment. For instance, large weighted heat sinks cannot be supported by thermal tapes. Epoxy is a great choice in thermal management solutions but it is costly and requires skill to apply.
Heat Sink Design: Thermal Resistance Calculation
The heat sink design completes the geometry and functionality for its intended use. However, to assess the performance of how well heat sinks dissipate heat, some thermal resistance calculations are necessary.
Few terms to know:
Tjunction : Maximum junction operating temperature
Tambient : Ambient air temperature
Q : Heat Generated
Tjunction-case : Junction to case resistance
Tinterface : Interface material resistance
Theatsink : Heat sink thermal resistance
Total thermal resistance is the resistance to heat flow from the junction to the casing, interface material thermal resistance, and resistance between the heat sink and surrounding air.
The calculations above are also used in the design stage to draw comparisons between different materials, and interfaces and to find the proper thermal resistance that matches the maximum operating temperature of the component.
Heat Sink Design: Size Calculation
The first step in sizing a heat sink is to determine its dimensions of the heat sink. Once the dimensions are set, the next step is to follow a few calculations simplified by heat sink calculator.
- Finding heat of convection
Qc = 2hA(Tcompent – Tambient )
A = HL + t(2H + L)
h = 1.42[( Tcompent – Tambient )/H ] ＾0.25
Qc : 2hA( Tcompent – Tambient )
- You will find there is another surface where heat can dissipate, finding the heat of convection at A2
Qc = 2h2A2(Tcompent – Tambient )
A2 = L[2( H – b) + sb] + tL
s = 2.71 [ gβ(Tcompent – Tambient )/Lαv ]
- Finding Heat emitted through radiation.
Just like convection, heat can escape through radiation from the two surfaces.
Qr = 2ϵσA1( T4compent – T4ambient )
Qr2 = 2ϵσA2( T4compent – T4ambient )
A2 = L(t + s) + 2(tH + sb)
- Finally number of fins is given by
Fn = 1 + [(Q – Qr2 – Qc2) / (Qr1 + Qc1)]
- Width of each fin is given by
W = (N – 1) + Nt
Applications of Heat Sinks
Heat sinks are used where something is producing heat and needs to be regulated constantly to keep the operating temperature within the maximum allowable temperatures. In electrical units heat sinks are found almost everywhere you see LEDs and transistors. This is to keep them working in room temperature surroundings without overheating.
Applications of heat sinks are largely characterized by the speed at which they conduct heat. Hence, higher thermal conductivity materials are used for processes where temperature rise is a risk to the operation.
When computers are used and the CPU chips are overclocked, significant heat is produced. A heat sinks thermally coupled to a CPU can not only store this heat, quickly removing it from the chip, but also transfer it across its surface, eventually dissipating it to the ambient air.
In power transistors, the heat produced can lead to leakage in current. This rise in collector current can in turn increase power dissipated, which can further increase the temperature. This can lead to a loop of thermal runaway destroying the transistor. A heat sink can quickly transfer the heat produced through its surface, preventing any damage.
Heat sinks are commonly used with LEDs as the component itself is unable to moderate its operational temperature.
Common Manufacturing Methods For Heat Sink Design
In order to create an effective heat sink design, engineers must consider many different factors, such as material selection, and aluminum is the most common material used to manufacture heat sinks. In this section, we will discuss the 3 most common methods of manufacturing aluminum heat sinks.
Extrusion of Heat Sink
The extrusion process is suitable for the simpler heat sink designs, which are characterized by high production quantities, less investment, short development cycles, and low production costs.
Commonly used aluminum extrusion material is aluminum 6063, which has good thermal conductivity. The working principle is to heat the aluminum ingot to 520-540℃ at high pressure to let the aluminum liquid flow through the extrusion mold with grooves to make the heat sink embryo. And then cut the heat sink embryo, profile groove, and other processing are made of the heat sink fin we commonly see.
CNC Machining Heat Sink
CNC machining involves a variety of processes, such as turning, drilling, milling, and grinding. This process can be flexibly applied according to the structure of the parts, the use of most structural parts, such as plate forming, heat sink grooving, bottom surface dressing, etc. This method is particularly suitable for the design of heat sinks with complex geometry.
Die Casting Heat Sink
The principle of diecasting aluminum heat sink aluminum is similar to injection molding. The raw material of aluminum alloy is dissolved into liquid at high pressure, and then the liquid material is quickly filled into the mold. According to the model, the die casting machine started to work, and one-time die casting molding, after cooling and subsequent treatment, was made into a monomer heat sink. The process can be integrated into a thin, dense, or complex heat sink fin design.
The uses of heat sinks will continue to expand as more technical advancements are made and better solutions are found to make a process more useful. The design of the heat sink will also be more diversified and customized.
If you are looking for a manufacturer to optimize your heat sink design for better performance, Here at WayKen, with rich experience in the processing of heat sinks, we can support your custom design and customization requirements. Our team always pay attention to every details of communication and interface with you so that there will be no surprises for your good customer experiences. Get an quote today!