Heatsinks have a main purpose of exhausting heat to the surrounding environment. Technically, it’s a process of moving thermal energy from a defined boundary object to a boundary-less thermal reservoir. They are most commonly applied to a hot part of a machine to dissipate heat to the surrounding air or water to cool the machine.
How Do Heatsinks Work?
Thermal energy is transferred by three different modes:
Heatsinks primarily concern 1 and 2, though all three can be involved to a significant level in some circumstances.
Radiation is the process of transmitting thermal energy through electromagnetic waves – usually in the infrared spectrum. While this always does occur, it is such a minute component as to be negligible for most heatsinks.
Conduction is the transfer of thermal energy between two or more solids in physical contact. Heatsinks make use of conduction to move heat out of the machine and distribute it to where it can be more easily dispersed.
Convection concerns the transfer of thermal energy from a solid to a fluid (liquid or gas). Convection is the primary mode of concern in heatsink design considerations. Convection occurs where atoms or molecules of the fluid come into contact with the solid surface. Convection heat transfer is increased by increasing the quantity or rate of those interactions. This can be done by two ways:
1. Increase the size of the surface contact between the fluid and solid
2. Increase the flow of particles within the fluid
Imagine you’re trying to advertise your new product. You need to maximize the amount of information distributed to as many people as possible to increase potential sales. You need to disperse your message to the world. Your message is hot. It’s the thermal energy we’re talking about.
You can print out a flyer and staple it to a telephone pole along a city street, but that won’t be seen by everyone passing by. Drivers or pedestrians would have to be intentionally looking to that small area on that side of the street to see it. You’re only going to reach a fraction of the passersby.
Instead, change that to a 50ft tall billboard! Now it’s in your face. It’s unavoidable. Everyone going down that street will see the message. By increasing the size, you’re connecting with a much greater proportion of the people traveling there.
But that isn’t enough. You want to reach even more people. So you move the 50ft billboard from the city street to the nearby eight lane interstate highway. Now, nearly everyone that drives past sees your message and a ton more people are driving past it. You’ve just dispersed your message much more effectively.
Likewise, heatsinks aim to maximize the surface area of contact with the ambient fluid, while also increasing the flow rate of the fluid. You can see why heatsinks are designed the way they are when you think about maximizing these factors. Let’s explore more…
The hot machine component transfers heat to the base of the heatsink by conduction. Ideally, the entire surface of the hot machine component is directly in contact with the heat sink base. The surfaces are smooth and there are no gaps or air bubbles of any kind. A maximum amount of thermal energy is transferred in this way.
The surface area is significantly increased by stretching the surfaces away from the base into large flat plates or fins. The surface in contact with the fluid will transfer its thermal energy away. Since it is of interest to ultimately pull as much heat away from the base as possible, the thermal energy must be conducted up to the tips of the fins as well. This is the reason for the slightly thicker base and tapering at the fins. Conduction more evenly distributes the thermal energy to the exposed surfaces.
What is the Best Heatsink Design?
A truly maximized surface area would be an array of tiny pins with gaps only a few atoms wide. While this would be more surface area than the fins shown above, a fluid would not flow very well across that area. We’d be limited to the country road, but we’re interested in the 8-lane highway (to revisit our analogy above). Flat fins support a smooth flow of air or water and allow the heatsink to be a much more effective convection device.
This is directional though. If airflow is known, allowed, or forced in a particular direction, fins are best. If no directional airflow exists, pins may be more effective.
Flow = Fins
Passive = Pins
Convection is enhanced when the flow is forced to a higher rate. This is called “forced convection” and the improvements are significant.
Even a small amount of flow, equivalent to a gentle exhale, can improve convective cooling by a factor of 3x! The differences are so dramatic that the cost, size, and complexity of adding a fan are far preferred over an equivalent performing larger heat sink relying on non-forced or natural convection alone.
Where a machine can make use of a directional flow of air already as in an HVAC system, car or watercraft, it is simply a matter of orientation and ducting.
Keep Heatsinks Clean!
Here enters one issue very significant to heatsink design that is so often overlooked. Systems are designed with clean surfaces and unobstructed flow. While service factors or lowered efficiencies can be assumed over time, it can be critical to protect fans and heatsinks from clogging debris such as dust, loose fibers or hair. Screens and filters can prevent some of this, but visible easy access for cleaning can be just as important to long term efficiency and product life. Don’t block everything off. Provide some access for cleaning.
How are Heatsinks Made?
Heatsinks are very commonly made by an aluminum extrusion process. Even the natural convection pin-style heatsinks start as fin-style extrusions that are subsequently cross-cut. The extrusion process is very well-suited for the geometry preferred for heatsinks since the extrusion material flows in the same direction that fluid will flow across the heatsink.
You Win Some. You Lose Some.
Copper is a better conductor of heat than aluminum. It would be preferred, but it is also much more difficult to manipulate in forming and machining processes. Aluminum is mechanically better suited, so it wins. Likewise, aluminum is actually a better conductor of electricity than copper, though the same mechanical properties differences give copper the advantage for most electrical wires. Copper wins here.
How Do I Design a Heatsink for My Product?
When it comes to heatsink designs, not every application involved drawing heat away from a flat plate to an open area. The same principles are at play, though size and shape constraints may require an engineer to creatively work around obstacles. Even if a run-of-the-mill flat plate, finned heatsink is selected, the airflow through a machine enclosure may not provide the ideal conditions on preliminary calculations.Engineers will use computational flow dynamics and transient thermal analysis modeling software to evaluate designs. Fluid flow characteristics and related thermal energy flow can be much more accurately visualized with these tools. The benefits of these are usually well worth the effort even though it can require significant time for proper setup and calculation processing time.
Heatsinks are simple little optimized devices that maximize convection for cooling machines and electronics. Use good engineering principles when designing and applying heatsinks to be sure they minimize cost and maximize cooling. Look for opportunities to incorporate these features into another part like an enclosure casting or structural rail. Where possible, a good designer can achieve the same end results with less parts or tooling. Don’t forget to guard from debris and plan for some cleaning down the road to keep everything working well.
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