How to design PCB for heat dissipation?

- Feb 16, 2020-

IC package depend on PCB to dissipate heat. In general, PCB is the main cooling method for high-power semiconductor devices. A good PCB heat dissipation design has a huge impact, it can make the system work well, but also can bury the hidden danger of heat accidents. Careful handling of PCB layout, board structure and device mount helps to improve the heat dissipation performance of medium and high power applications.

 

Semiconductor manufacturers have difficulty controlling systems that use their devices. However, the system that installs the IC is crucial for the overall device performance. For custom IC devices, system designers usually work closely with manufacturers to ensure that the system meets the numerous heat dissipation requirements of high power devices. This early collaboration can ensure that the IC meets the electrical and performance standards, while ensuring the normal operation within the customer's heat dissipation system. Many large semiconductor companies sell their devices as standard parts, and there is no contact between manufacturers and end applications. In this case, we can only use some general guidelines to help achieve a better IC and system passive heat dissipation solution.

 

The first aspect of PCB design that can improve thermal performance is PCB device layout. Whenever possible, the high power components on the PCB should be separated from each other. this physical interval between high-power components can maximize the pcb area around each high-power component, thus contributing to better heat conduction. Care should be taken to isolate the temperature-sensitive component on the PCB from the high power component. The installation position of the high power components should be away from the PCB corner whenever possible. The more intermediate PCB position can help heat dissipation by maximizing the area of the panel around the high power component. Two identical semiconductor devices are shown: components A and B. Component A is located at the corner of the PCB, with a chip junction temperature of 5% higher than that of Component B, because the position of Component B is a little more centered. Since the area around the component for heat dissipation is smaller, the heat dissipation at the corner position of component A is limited.

 

The second aspect is the structure of the PCB, which has the most decisive effect on the thermal performance of the PCB design. The general principle is: the more copper PCB, the higher the thermal performance of the system components. The ideal heat dissipation for semiconductor devices is that the chip is mounted on a large piece of liquid-cooled copper. For most applications, this mount method is not practical, so we can only make some other changes to the PCB to improve the heat dissipation performance. For most applications today, the total volume of the system is constantly shrinking, which has a negative impact on the heat dissipation performance. larger pcb, the larger the area it can be used for heat conduction, and also have greater flexibility to leave enough space between high-power components.

 

For improving the heat dissipation performance, the top and bottom layers of the PCB are “golden lots ”. Using wider conductors, wiring away from high-power devices, can provide a thermal pathway for heat dissipation. Special heat conduction plate is an excellent way for PCB to dissipate heat. The heat conducting plate is generally located at the top or back of the PCB and is thermally connected to the device through a direct copper connection or through a hot hole. In the case of inline packaging (with leads on only the two sides of the package), this heat conduction plate can be located at the top of the PCB, shaped like a “dog bone ”(the middle is as narrow as the package, and the copper area connected away from the package is large, and the middle is small and the two ends are large). In the case of four-sided packaging (with leads on all sides), the heat conduction plate must be located on the back of the PCB or into the PCB.

 

Increasing the size of the heat conducting plate is an excellent way to improve the thermal performance of the powerpad type package. different thermal conductive plate sizes have a great effect on thermal performance. Product data forms provided in tabular form typically list these dimensions. However, it is difficult to quantify the impact of custom PCB copper increases. Using some online calculators, the user can choose a device and then change the size of the copper pad to estimate its effect on the heat dissipation performance of the non-JEDEC PCB. these computing tools, highlighting the extent of the impact of PCB design on heat dissipation performance. For the four-side package, the area of the top pad is just less than that of the bare pad of the device, in which case the concealed or back layer is the first way to achieve better cooling. for the double column straight insert package, we can use the “dog bone ” type pad style to dissipate heat.

 

Finally, the system of the larger pcb can also be used for cooling. in the case of screw heat dissipation connected to the heat conduction plate and ground layer, some screws used to install the pcb can also be an effective thermal path to the system base. Considering the heat conduction effect and cost, the number of screws should be the maximum value to reach the point of diminishing returns. After connecting to the heat conduction plate, the metal PCB stiffener has more cooling area. For some applications where the PCB has a shell, the type-controlled soldering material has higher thermal performance than the air-cooled shell. Cooling solutions such as fans and heat sinks are also commonly used for system cooling, but they typically require more space or need to modify the design to optimize the cooling effect.