The idiom “out of the frying pan into the fire” is intended to warn of the perils of escaping one danger by jumping into something worse. However, taken at face value, it also clearly portrays that excessive heat can be damaging, which is undoubtedly true in electronics. Heat is the expected by-product of a normal functioning circuit board. But if heat isn’t carefully planned for during its design, the PCB may have problems during its manufacturing and operational performance. Here we’ll take a look at PCB thermal resistance and conductivity and see how good thermal management is ultimately essential to the success of the circuit board.
Defining PCB Thermal Resistance and How to Work With It
Just as the flow of electricity is affected by the conductivity and resistivity of the materials it is running through, so is the transfer of heat in a circuit board. The thermal properties of the materials used in fabricating a circuit board will determine how conductive or resistant the board is to heat. Those properties with greater thermal conductivity will allow the movement of heat away from its source, while the inverse of this action is considered thermal resistance.
Circuit boards are a combination of conductive and resistant materials. Copper, for instance, not only conducts electricity but also has high thermal conductivity, as do most electrically conductive materials. These conductive materials provide a low resistance path in the circuit board for heat transfer. On the other hand, FR-4 and other substrate materials resist heat transfer and electricity. Thermal conductivity can be determined by measuring the temperature on both sides of a finished circuit board and inputting the parameters of this heat transfer into the thermal analysis software. The thermal calculations will give you the conductivity of heat through the board, and its inverse will provide the board’s thermal resistance.
Circuit board designers need to understand the PCB thermal resistance of their design before it gets into production. This process requires developing a robust thermal strategy for the design of the circuit board. A good strategy will involve the correct configuration of the board layer stackup, the arrangement of power and ground planes, and the location of different components. Designers often employ the power delivery network (PDN) tools in their PCB design CAD tools to help develop this strategy to identify and prevent potential thermal problems during design. Next, we’ll look at some of the problems designers need to avoid with their PCB design thermal strategies.
Circuit Board Problems if a Thermal Management Strategy Isn’t Developed
A circuit board can suffer from various thermal problems if care isn’t taken during the design phase. These problems can manifest themselves during manufacturing or operation:
Fabrication:
A circuit board can bow and twist during fabrication if the copper used in the board isn’t balanced between the layers in the stackup. For instance, plane layers should be configured symmetrically throughout the layer stackup, and copper should be spread evenly throughout the layers. A stackup with more metal on the top layers than the bottom could warp from the heat and pressure of fabrication. Designers may add additional copper pours and fills to balance the copper on circuit boards with areas void of circuitry.
Assembly:
Thermal problems can also affect the assembly of a circuit board during soldering. Thru-hole pins connected to large amounts of copper may be robbed of the heat they need without the proper thermal relief pads. The copper will act as a heatsink and deny the necessary heat required to form a good solder joint. Likewise, a small discrete surface mount part with uneven amounts of metal connected to its two pins could create a thermal imbalance during the solder reflow process. This condition could result in the component being pulled off one of its pins and standing up if the solder on the other pin melts faster.
Operation:
Electrical activity will produce heat, and as the current increases, the excessive heat can break down the dielectric material in the components on the board. This condition can lead to signal integrity issues, intermittent circuit problems, and outright component failures. High current levels in traces not wide enough to handle the load could also lead to enough heat to burn through the trace.
These problems can all be addressed by executing a good thermal management strategy, and we’ll look at design recommendations that can help with that next.
Design Recommendations to Avoid Thermal Problems
Here are some ideas to keep in mind as you are developing your own PCB thermal resistance management strategy for your design:
- Board materials: While FR-4 is the standard material used in most circuit board fabrication, other materials such as polyimides or metal cores are better at dissipating heat.
- Board layer stackup: Not only is a ground plane necessary for the signal integrity of the board, but it can also help with heat dissipation. Be sure to configure your stackup to accommodate these different design requirements.
- Component placement: Microcontrollers and other hot-running parts should be placed more towards the center of the board to help with heat dissipation. Also, provide enough clearance between hot components, and ensure adequate airflow across the board’s surface for cooling.
- Trace routing: Wider traces are necessary for high-current circuits such as power supplies to help with power integrity and heat dissipation.
- Thermal reliefs: Ensure that thru-hole parts have thermal relief pads to prevent soldering heat from dissipating through planes or large filled areas of copper.
- Thermal pads and vias: Using a via in a large pad directly under a hot component can help channel its heat into the ground and power planes to be dissipated throughout the board.
- Fans, paste, and sinks: Using additional cooling devices such as fans, thermal paste, and heatsinks can also help manage the heat on your circuit board.
At VSE, we have ample experience working with our customers to help them understand PCB thermal resistance and provide working thermal solutions for their circuit boards. We will look for areas of thermal problems in your design and recommend alternative layer configurations, parts, component placement, or routing strategies as needed. Our goal is the same as yours, to develop a circuit board that will perform at the highest levels of quality, so your electronics perform as expected.
