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    Overview of Heat Sink Design Basics and Principles

    June 13, 2017

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     I played a musical instrument in college, and my teacher always told, “don’t botch the basics!” That translated into practicing my scales for hours on end until I could play any of them and their variations with barely a thought. It’s important for electrical engineers to remember the basics and how they affect our work. I usually work with high-level systems, and it’s easy to forget the simple principles that inform the advanced applications. When it comes to thermal management and heat sinks the three main things to remember are: convection, conduction, and radiation. These three fundamentals will affect things like fin placement and orientation, thermal interface materials (TIM), and heat sink surface treatment. Once you’re refreshed on how all these mesh together, heat sink design will be a breeze.

    Convection 

    Another golden nugget from my teacher was, “music shouldn’t feel forced.” Well, that doesn’t necessarily apply to convection. You can have two kinds of convection on your board: natural and forced. Natural convection does not use any fans or outside force to move air. Instead, it relies on the convection currents that naturally occur in a differentially heated fluid. This passive process draws no power, but can also be a bit slow in cooling things down. Forced convection is just the opposite, it uses an outside force to move air. Usually, this force would be something like a fan. With this method, you have to power the outside force, but you get faster cooling in exchange. Interestingly enough, the form you choose will have an effect on your heat sink design guide. 

    As far as natural convection goes, you need to make sure your heat transfer and heat sink and fins are placed so that they don’t inhibit air movement. The flow is quite weak in a natural convection scheme, so if it’s obstructed in any way your cooling will be severely inhibited. When placing the heat sink, you should be sure to orient it so that air can rise parallel through the fins. Having your fins perpendicular to airflow is like trying to play an instrument while standing on your head, it doesn’t work. For the fins themselves, you should use a sparse placement pattern. If the fins are densely grouped they will inhibit convection. 

    When dealing with forced convection things are at once more simple and complex. Airflow is guaranteed, it’s just a question of optimizing it. Like before, you’ll want to orient your heat sink so that air passes parallel to the fins. Designing the fins is where things get a little tricky. The main concerns with forced convection are pressure drop and loss. If your fins are too tall or too dense, they’ll cause an excessive pressure drop across the heat sink, resulting in a lossy system. If you want to find the perfect fin size and placement, you’ll need to crunch some numbers.

    Conduction

    In an orchestra, the conductor transfers musical instruction over the air via his baton. It’s almost like a radio antenna. Conduction in circuits is the exact opposite. It transfers heat between objects through direct contact. When dealing with conduction you’ll need to think about where your heat sink will go, what it will be made of, and what TIM you’ll use to attach it to the board.

     shutterstock_566074117.jpg
    Don’t try to conduct heat the way and orchestra conductor conducts musicians!
     

    Placing your heat sink is important. You want to maximize cooling and minimize space usage at the same time. In fact, you’d probably be most happy if you didn’t have to use a heat sink at all. However, if you have to use one, best to do it right. The optimal heat sink location is on a hot spot, such as a powerful integrated circuit (IC) or a thermal spreader that collects heat from several sources. 

    When choosing your material there are two main factors to consider: weight and thermal conductivity. Aluminum has excellent weight characteristics and decent thermal conductivity. Copper has excellent thermal management conductivity but can be a bit heavy for your board. If your heat sink is too heavy, it can stress your PCB and cause early failure. 

    Last but not least in conduction are TIMs. Without a good TIM, your heat sink may be as useless as a tuba player with no lips. There is a wide variety to choose from, and you should do your research to see which one is right for you.

    Radiation

     shutterstock_540176980.jpg
    These go to 11! 

    Everyone in our band was always told to radiate as much sound as possible to fill our concert spaces. Everyone except the saxophones that is, their default volume is 11. In the same way, you want your heat sink to radiate as much heat as possible. In order to maximize radiation, you’ll want to maximize the surface area and emissivity of your heat sink. 

    The more surface area you have, the more your heat sink will radiate. Remember, though, that sometimes an increase in surface area will cause more convection losses. So you’ll need to balance surface area against system efficiency to find the optimal solution. 

    Emissivity is the measure of how effectively a surface transfers heat to air. Luckily, you can maximize emissivity without affecting convection or conduction. You can easily treat the outside of a heat sink to increase its emissivity. The surface treatment can have a positive large effect on your heat sink’s thermal transfer characteristics, so I would definitely recommend it. 

    When you’re playing in a concert and you forget the basics you might miss a note or two. Your director might get a little hot under the collar, but that’s about it. Losing track of the fundamentals when choosing a heat sink can leave with you a PCB so hot that it catches fire. That’s why it’s important to effectively use convection, conduction, and radiation to keep your board cool. 

    After you’ve got your heat sink picked out, you might want some help laying out the circuit it’s going to cool. CircuitStudio® is a great design tool that has tons of advanced features to help you with design. Your work could go so fast, you might even have time to pick up an instrument. 

    Have more questions about heat sinks? Call an expert at Altium.

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