PCB Thermal Management Considerations for Assembly

There are fewer things in life that make me happier than being surprised by a nice warm, sunny day. Swimming, boating, or just being outside is a joy. Even if I’m inside working I would rather see the sunshine through the window than the rain and dark skies. But if you are unprepared for it, the summer heat can easily turn on you, and it pays to be prepared. Last year, across the United States, people spent over 1.24 billion dollars on suntan products in order to be prepared.

Your circuit board design needs to be prepared for the heat as well, but slathering some Coppertone on the power supply components just won’t cut it. PCBs that aren’t prepared for the heat of manufacturing and regular operation are going to have a lot of problems. We’ll take a look at some of the issues with PCB thermal management, and how you can design a board that will thrive in the heat instead of withering.

PCB Thermal Management Design Considerations for Best Performance

There’s a lot of electrical activity in an operating circuit board, and all of that activity is going to produce some heat. While heat is expected in the normal operation of a board, too much heat can cause the breakdown of the dielectric materials in the components, resulting in their eventual failure. To avoid this, here are some PCB design techniques that can help control the heat generated by the board:

• Component placement:Components that run hot from high amounts of current, such as power parts or microcontrollers, should be placed towards the center of the design. This will allow the heat to diffuse out through the board as opposed to being placed on the edges where the heat will tend to accumulate. You should also space hot components out from each other, and place them to take advantage of the airflow for cooling. Also, keep in mind to place sensitive components some distance from those that are generating more heat.
• Traces: Using more metal (copper) on the board will also help to dissipate heat. This can be done by increasing the copper weight (depth) that the board is built with, and the width of the traces as well. This will likely increase the thickness of the board, which is also helpful for devices that run with high currents.
• Thermal pads: Heat can be conducted through the board using plated through holes in thermal pads and vias. These holes will help to distribute the heat into the board instead of allowing it to remain around the hot devices.
• Board structure: The printed circuit board itself ends up acting as a heat sink, and the thicker the board is, the more thermal energy it will absorb. And while standard FR-4 board materials can handle a lot of heat, it may become necessary to look into other board materials such as polyimides or metal cores for excessive amounts of heat.
• Cooling devices: Finally, there are many external cooling devices that can be used for thermal management on a printed circuit board. These include thermal paste under hot components, heat sinks, and cooling fans.

By carefully using some or all of the above methods, PCB designers can better manage the heat generated by the normal operation of their board. But there is another type of heat that can cause problems for the board that PCB designers must be aware of as well.

Design Techniques to Manage PCB Heat During Manufacturing

When a circuit board is manufactured, it will be exposed to a great deal of thermal stress. PCB fabrication requires both heat and pressure in order to laminate the layers together, while the soldering process will heat the board up during assembly. Wave soldering will run the exposed components leads on the bottom side of the board through a molten wave of solder, and solder reflow requires the entire board to be baked in an oven. Here are some of the thermal problems that are associated with circuit board manufacturing:

• Bow and twist: If the metal coverage throughout the different layers of the board is not consistent, it can cause the board to bow or twist during fabrication. For instance, if the first three layers of a six-layer board have a higher percentage of metal than the bottom 3 layers, the board will be out of balance. This imbalance can result in the board warping as the high heat and pressure are applied during fabrication.
• Bad solder joints: These often happen because the metal around the pin to be soldered behaves as a heat sink and robs heat from the connection during wave solder. As a result, the solder doesn’t melt correctly, leaving a cold and unreliable solder joint. The solution in cases like this is to use a thermal relief pad to concentrate the heat on the solder joint.

A thermal relief pad like this will help focus the heat on the pin to create a better solder joint

• Tombstoning: This is a condition where a thermal imbalance between the two pins of a small surface mount component causes the part to stand up vertically during solder reflow, just like a tombstone. The thermal imbalance occurs when one of the pins is connected to a large area of metal, which acts as a heat sink. This allows the solder on the other pin to melt faster pulling the part up with it. To prevent this, PCB designers should use thermal relief type connections when connecting pins to large areas of metal.

Even with careful design practices, there are still some components that are more susceptible to thermal problems during circuit board assembly than others. Thankfully there are some steps that your contract manufacturer can take to protect those components. These include protective shields during soldering and modified soldering processes. In addition to that, your PCB CM can also help you with other areas of thermal management.

How Your PCB Contract Manufacturer Can Help You with Thermal Management on your Board

Usually, PCB thermal issues are related to power, and a PCB contract manufacturer will typically review the bill of materials (BOM) on your board for components with high power requirements. This will often trigger a series of the following questions to make sure that your design is adequate for the heat that it will be generating during operation:

• Will the layout support the expected current and power requirements?
• Is there a preferred airflow direction to cool high-power components?
• Are you expecting any future component changes that might alter the airflow that should be taken into account now?
• Are there any sensitive components that could have their components impacted by being too close to high-powered and hot components?

At VSE we want your design to be successful just as much as you do. To this end, our engineering team will spend time with you to make sure that your design will operate within its specified thermal limits, and that it can withstand the rigors of PCB manufacturing.