CC BY NC photo by fir0002
It’s summer, and your town’s experiencing record temperature highs. What’s the first thing that you do to combat the sweltering heat? Whether your answer was to open a window, turn on the air conditioner, point a fan at your face or jump into a pool, you’re using some combination of conduction and convection through a fluid medium to remove excess heat.
With electronics, one way to tackle your thermal management is with heat sinks. These are not end-all, one-stop solutions, of course, and come with their own lists of constraints and design challenges. Below you’ll find an understanding of heat sinks, as well as current trends in use-cases for them.
What is a Heat Sink
While traditionally, one of the easiest ways to cool down a hot device is slapping a cooling fan down on top of it and letting the fans go to work, depending on the form factor, this may not be the most available of choices. Furthermore, it may not serve for quick-heating devices or singular component areas whose heat is particularly centralized in an area.
A heat sink is a passive heat exchanger that removes heat from a surface via conduction and dissipates it to the surrounding atmosphere. In electronics, a heat sink is typically a solid block of copper or aluminum with multiple fins that increase available surface area for heat transfer. With the never-ending quest to pack more processing power into smaller form factors, heat sink innovation is more important than before.
Heat Sink Materials
Cheap, durable, and ubiquitous—aluminum and copper remain the de facto “gold standard” for heat sink materials. Innovation in this area has more to do with alloys, and how you play off of the strengths and weaknesses of these two materials than exotic materials (although we’ll get to those a little later).
Copper: The thermal conductivity of pure copper is 386 W/mK, with a density of 8.96 g/cm3, and a price of approximately 6,000 USD per metric ton as of August 2018.
Aluminum: The thermal conductivity of pure aluminum is 204 W/mK, with a density of 8.96 g/cm3, and a price of approximately 2,300 USD per metric ton as of August 2018.
While copper has better thermal conductivity, aluminum has the cost and weight advantage. Figuring out which material to use has a lot to do with the unique needs of your project. Innovation in this area often takes the form of balancing the best of both materials. For example, using a copper base plate to efficiently conduct heat away from a CPU into a cheaper, lightweight, aluminum heat sink. It’s also worth noting that heat transfer between copper/aluminum to air is similar since the thermal conductivity of air is the actual bottleneck. The heat transfer advantage of copper over aluminum is not as high as one would expect from their thermal conductivities, which is why aluminum is the more popular heat sink material.
Working through EMI with particularly power sensitive boards is a challenge
Thermal conductivity is only one piece of the puzzle to designing a good heat sink. Squeezing out performance gains on heat sinks in the cutting edge chips found in smartphones and tablet PCs require innovation in the geometry and layout of the fins. The key concept to grasp here is fin efficiency: the ratio of the fin heat transfer rate to the heat transfer rate of an ideal fin (one where Tfin = Tbase).
The better the fin efficiency, the better the heat transfer via convection to the surrounding atmosphere. Optimizing the geometry of the fins helps maximize airflow through the heat sink. There are two primary fin configurations:
Pin Fin: Erecting a forest of pins from a flat base is a popular way to maximize surface area; they work best when air flows axially along the pins.
Straight Fin: Also known as plate fin heat sinks, straight fins run the entire length of a flat base to maximize surface area; they work best when air flows tangentially to the heat sink.
There are also numerous modifications you can employ to optimize airflow or enhance cooling:
Active cooling: Cutting edge GPUs such as the Nvidia GeForce GTX 1180 use a combination of fans, heat pipes, and vapor chambers to keep temperatures in check.
Cross-cutting flat fins: Taking a straight fin and cross cutting it into multiple short sections can improve the heat transfer coefficient at the fin surface at the cost of reducing surface area exposed to the air stream and inducing a pressure drop; best used in response to unpredictable airflow.
Augmented (curved) fins: Adding a curvature to the leading and trailing edge of a straight fin can scrub “dead” air molecules away from the surface of the fin, improving flow.
A finned heat sink with thermal paste on a CPU
In order to pick the correct fin geometry, you’ll want to use your favorite CFD software to better understand the fluid dynamics at play within the envelope of your electronic device.
You may not realize it, but even the way a heat sink is manufactured can have an impact on thermal performance.
Extruded: The cost effective option, highly automated, extruded heat sinks are typically aluminum and allow the manufacturer to produce the fins and base as one continuous part.
Bonded Fin: The base and fin are made separately and must be bonded together. While heat transfer between the bonds and base take a hit, separate construction allows for more elaborate geometries that can lead to significant gains in surface area that can more than makeup for the drawback.
Zipper: The most cost effective option for high density heat sinks, fins are stamped from metal and folded into shape with interlocking grooves before attaching to a base.
Folded Fin: Fins are made from bending aluminum and copper sheets before bonding to a base. These fins can be much thinner than typical extruded or bonded fins, and provide higher densities and surface areas.
Skived: Both fins and base are cut from a single piece of metal by repeatedly slicing the top off of a belt of metal and folding it back onto the base. Skived heat sinks can be made with thin fin and high density geometries while benefiting from the thermal conductivity of a uniform piece of metal (no impedance from bonding).
Advances in technology have made folded and skived processes more viable, phasing out older techniques such as extrusion. The demand for ever higher thermal conductivities and surface areas will likely continue this trend.
The Future of Heat Sinks
When it comes to the heat sinks themselves, researchers have been cooking up some exciting materials over the years that might change the game. As I promised earlier, here are some of the more exotic materials on the horizon of heat sink design:
Carbon Nanotubes (CNT) boast impressive electrical and thermal properties (thermal conductivity of 3000-6000 W/mK), so it’s no wonder researchers are experimenting with CNT heat sinks and copper CNT composites. Sadly it will be a while before they are commercially available.
Diamond heat sinks are already here. Metal-diamond composites sport a reduced coefficient of thermal expansion (6-12 ppm/K) with a high thermal conductivity (300-650 W/mK).
Graphene is another one of those carbon based metamaterials that have captivated the minds of scientists. Here’s a paper on graphene-carbon-metal composite films for flexible heat sinks.
HOPG (highly ordered pyrolytic graphite) is an anisotropic material with a thermal conductivity around (2000 W/mK).
The prevailing trend for improving heat sink materials is to take good old fashioned copper or aluminum and dope the metal matrix with a high thermal conductivity metamaterial (often carbon). While it’s impossible to predict which of these materials will dominate the future of heat sinks, one thing shall always remain for certain: the best thermal management solutions take holistic approaches that combine heat sinks with other cooling techniques such as heat spreaders, heat pipes, and vapor chambers.
No matter which cooling method or heat sink you want to use, the right ECAD/MCAD tools will help you design a heat sink for power electronics and implement the best cooling strategy. If you have STEP, DFX, IDX, or IDF files for a standard or customized heat sink, you can import these models into OrCAD and incorporate it into your board design. You’ll also be able to add a fan and supporting electrical components as needed. And to verify the effects of the heatsinks, it’s important to use proper simulation tools.
Whether you’re working on a high-power device and need extensive cooling, or working on a more traditional electrical design that requires a regular device, Cadence's suite of design and analysis tools will give you all you need. With OrCAD, you’ll have the layout tool you need for any thermal considerations in your electrical designs.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.
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