A Guide to Designing Your Board to Avoid Thermal Mistakes

The cost of not considering your thermal management early in the development process is more than money. Although the financial impact can be staggering, the consequences of an insufficient cooling solution include missed deadlines, product failures and recalls, costly redesign and rework, scrapped materials, safety concerns, and endless amounts of frustration.

The repercussions of a subpar cooling solution include unfavorable product reviews, lower reliability, shorter product lifetime, noise issues, higher maintenance costs, and overall poor product performance. Additionally, it does not allow for forward compatibility. A poor solution cannot move forward into next generation products, but an optimized solution makes that next generation more likely.

In short, it is important that you take the time and effort to optimize your cooling and include it early in your design process.

Designing your board with proper cooling in mind can change the entire layout of your board. It is a common occurrence for engineers to have their board designed and fabricated only to realize that there is no room for mounting their heat sink or that the solution required is simply too large for their final product.

Thermal Management is often an afterthought, and many believe that they just find a heat sink with the correct footprint and simply stick it on top. The reality is that there are many factors to choosing the right heat sink and to designing your board to correctly accommodate it.

The location of your heat sources and other components impact the size and shape of our cooling solution. Conversely, looking at the system from another angle, the size and shape of your cooling solution influence where you should place your heat sources. Mounting holes will impact the routing of traces and may prevent mounting in certain locations. RAM location can be a deciding factor in heat sink size, but the size of your heat sink then impacts where you can place your RAM.

Throughout the entire process, you must be considering your final product. Touch temperature, air flow, direct ambient air contact are all important factors. The size and shape of the enclosure along with size and weight requirements of your product each affect your thermal management choices as well as your board layout.

All too often, engineers design and spin their board with only the electronic function in mind. This can easily lead to forgetting that, in many cases, your thermal solution may end up being the largest component on your board or that it may require a flow path or ventilation holes. So, before you spin your board, plan to make space for your thermal management solution and consider your full board and packaging. While planning, be sure to ask yourself the following questions:

If you cannot include a heat sink due to design constraints for your final product, consider utilizing thermal vias in your board. Thermal vias are conductive traces in your PCB that help spread heat away from your heat source.

Make sure there is enough volume available to place a heat sink. If you might require a heat sink larger than your heat source, you will want to ensure that there are no taller components nearby that would interfere with your heat sink placement. For example, don’t put RAMs (vertical over the board) very close to a processor because it may need a heat sink.

If you have multiple devices that require cooling, you may want to consider putting them close enough to be able to use one heat sink across multiple devices. This helps decrease product complexity and assembly time. Keep in mind that this only works up to a certain point. If the devices will be dissipating higher power, you will want to space them out on the board and ensure room for separate, larger heat sinks.

If you will be using mounting hardware, be sure to route vias and other devices away from areas where you may need to place holes or solder hooks to accommodate larger heat sinks. Be aware that if you are using tape, it will not be as robust as physical hardware. Tape will not provide the same thermal performance as thin bond line interface material combined with a mechanical attachment method.

Whether or not you will need a fan is dependent on factors such as the power density, total power, and the availability of ambient air. Consider if your entire board needs to be cooled or just a single device. This will help determine your board layout and the placement of your fan.

Do not neglect the air pre-heating effect in both passive and active cooling. Keep the most temperature sensitive devices upstream of airflow. Try to stagger heat dissipation devices along the direction of airflow to minimize one heating another.

Your cooling solution becomes more complex when you need to dissipate heat through an enclosure or provide ventilation. You may need to consider external heat sinking or using your enclosure as your heat sink. Air to air heat exchangers are also a viable option.

Your heat sink’s fin pitch needs to be optimized to best take advantage of the chimney (or stack) effect. This is when the amount of heat generated combined with the orientation and spacing of your heat sink fins causes the heat to move more quickly from the heat source. Improper orientation will inhibit the thermal performance of your heat sink.

• Not allowing enough room for a “Keep Out” area around the device to accommodate the heat sink.
• Not considering the system level air flow and air mover placement. An example of this would be placing tall or bulky devices in the flow path, prohibiting air flow.
• Routing circuitry in an area where mounting holes would need to go.
• Sub-optimal RAM placement that hinders airflow, natural convection, or heat sink placement.
• Not considering the back of the PCB. For example, not allowing space for push pins and mounting hardware.

• Adding a fan to a natural convection thermal solution will quickly boost its performance. Although this fix does come with its own considerations. These include power consumption, noise, and additional volume.
• Provide additional ventilation by cutting openings into your enclosure. Be aware that this option may impact IP ratings.
• Adding a small copper spreader, sometimes known as a “thermal band-aid”, can help when you just need to increase your thermal performance slightly. These solutions work best for lower power, low profile applications.
• Switch out your current heat sink for one with a similar footprint but better thermal properties. Examples would include switching an aluminum heat sink for a copper one or replacing your heat sink with an advanced fin type solution to allow for more surface area. In this scenario, cost may be a limiting factor.

Treating thermal management as an afterthought can be disastrous. Although there are a number of enhancements that can be applied to “fix” your solution, nothing takes the place of designing your board with your cooling solution in mind. By thinking ahead, you can avoid costly errors and ensure that your product is optimized for performance, reliability, and lifetime.

Boyd Design can help. Boyd offers engineering services as early on as concept development and as late as a full retrofit. Our engineers can provide continuous support throughout the entire product development cycle from concept to manufacturing and future generations. By bringing in Boyd at the beginning, not only can your team avoid the consequences of insufficient thermal management, we can ensure that your product is fully optimized for success.