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    When to Use PCB Thermal Pads or Paste for Thermal Management Design

    March 31, 2017

    Thermal pad replacement

    It’s rare that one will offhandedly know which thermal interface material will work the best for their PCB thermal pad. This article weighs the advantages and disadvantages of thermal pads vs. pastes with these helpful guidelines. Read on to find out more!

    I used to live in Alabama, which I casually referred to as Hell in the summertime. Lots of heat, lots of humidity, lots of sweat. I am thankful, though, that I don’t have to experience the heat inside high power circuit boards. While I was able to jump in the pool to cool off, your PCB layout probably uses a heat sink to keep its high power integrated circuits (ICs) from melting. While I’m torn choosing between trunks or a Speedo, you might be torn between using thermal pads/films or thermal paste/grease for your heat sink. Now, you may not want to take fashion advice from an Alabamian, but I do have some good tips for deciding on the perfect thermal interface material (TIM) and solder mask.

    Board with large SMT heat sinks
    That’s one cool looking board.

    Pads vs. Paste: Advantages and Disadvantages

    Both thermal pads and thermal paste have various advantages and disadvantages. The list is almost as long as a Southern summer so you might want to grab a cool drink to help you make it through.

    Thermal Relief Pads

    Advantages:

    • Easily handled

    • Can be cut into specific shapes

    • No-mess, easy application

    • No pump out

    • No dry out

    • Range of materials with various specifications

    Disadvantages:

    • High manufacturing costs

    • Intensive application

    On first glance, thermal relief pads sound like an instant winner. More form fitting than my Speedo and easy to apply. Versatile material possibilities also mean you can select a pad’s electrical, thermal, chemical, and physical attributes for your specific application. However, just like a sleek inground pool costs more than an above ground eyesore, the price tag on thermal pads can make you sweat. PCB thermal pads are normally hand applied during manufacturing. This will boost manufacturing costs for large orders of boards.

    Thermal Paste

    Advantages:

    • Reliable

    • Inexpensive

    • Easy application

    • Fills gaps well

    • Thin layer

    Disadvantages:

    • Messy

    • Pump out

    • Dry out

    • Requires pressure

    Thermal paste is like an old swimming hole out in the woods. You know it’ll always be there, and it’s cheap, but things might get a bit messy. However, just like sunscreen is cheaper than skin cancer treatment, thermal paste is less expensive than PCB thermal pads.

    Thermal paste on computer CPU
    Like I said, messy application.

    When to Use What

    Some people run to the ocean when it gets hot, some run to the mountains. In most printed circuit board applications, there’s not a clear answer as to which TIM will work the best. However, there are some specific applications where there’s a clear winner.

    Thermal Pads

    One of the big advantages that PCB thermal pads have over thermal paste is they have no risk of pump out. Printed Circuit Boards that have rapid temperature cycles are the most susceptible for this. If you expect your Printed Circuit Board to turn on and off rapidly, you will likely need to use a thermal pad for your heat sink. More so, if the area you need to cool is oddly shaped, thermal pads may be the answer for you. Thermal pads can be cut into precise shapes, making their application a breeze.

    They say that heat and pressure turn carbon into diamonds, but they just make me wilt like a flower in the sun. Your ICs may be a little more like me than diamonds. Thermal paste requires a large amount of pressure to transfer heat effectively. If the ICs you’re cooling can’t withstand mechanical stress, thermal pads are the right choice.

    Thermal pads are also very useful when TIM properties need to be carefully chosen. Thermal pads can be made from a very wide variety of materials. This gives you more control over TIM properties like; chemical resistance, conductivity, and physical properties.

    Thermal Paste

    If cost is a hot topic for your PCB, thermal paste will be your most inexpensive option. If I’m going outside in the heat, I like to wear the thinnest shirt I have. The same principle applies to TIMs, as the thinner the TIM layer, the better the thermal conductivity. Thermal pads are almost always thicker than thermal paste. As such, thermal paste will usually provide the best heat transfer. If your heat sink requires a TIM with a very high thermal conductivity, thermal paste could be the best option.

    Thermal paste also bests thermal pads when it comes to uneven surfaces. Since thermal paste is a liquid, it can fill large gaps more evenly than thermal pads. Some heat sinks or ICs have uneven surfaces where the TIM will be applied. If that’s the case for your PCB, thermal paste will provide better thermal conductivity than a thermal pad.

    Remembering which of your components need heat sinks can be a pain. Instead of straining your brain, let your PCB design software do it for you. Most design programs will let you add in comments on components. Attach thermal concerns to components to keep your PCB out of hot water. CircuitStudio® has documentation showing how to edit components.

    Keeping yourself cool in the summer can sometimes be a full time job. Making sure you choose the right TIM to keep your PCB cool can also take up a lot of time. Keeping track of the above TIM recommendations will let you focus on your own temperature rather than your PCB’s.

    Want more information on TIMs? Talk to an expert at Altium or if you happened to be reading this but don't use Altium yet, be sure to check out your own free trial to find out for yourself why Altium is the best professional PCB Design software. 

    We would like to hear your thoughts and feedback. Comment and share below.

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