PCB News - How to design a PCB system with high thermal performance

IC packages rely on PCB for heat dissipation. In general, PCB board is the main cooling method for high power semiconductor devices. A good PCB heat dissipation design has a great impact, it can make the system run well, but also can bury the hidden danger of thermal accidents. Careful handling of PCB layout, board structure, and device mount can help improve heat dissipation performance for medium and high-power applications.

Semiconductor manufacturers have difficulty controlling systems that use their devices. However, a system with an IC installed is critical to overall device performance. For custom IC devices, the system designer typically works closely with the manufacturer to ensure that the system meets the many heat dissipation requirements of high-power devices. This early collaboration ensures that the IC meets electrical and performance standards, while ensuring proper operation within the customer's cooling system. Many large semiconductor companies sell devices as standard components, and there is no contact between the manufacturer and the end application. In this case, we can only use some general guidelines to help achieve a better passive heat dissipation solution for IC and system.

How to design a PCB system with high thermal performance

Common semiconductor package type is bare pad or PowerPADTM package. In these packages, the chip is mounted on a metal plate called a chip pad. This kind of chip pad supports the chip in the process of chip processing, and is also a good thermal path for device heat dissipation. When the packaged bare pad is welded to the PCB, heat is quickly exited from the package and into the PCB. The heat is then dissipated through the PCB layers into the surrounding air. Bare pad packages typically transfer about 80% of the heat into the PCB through the bottom of the package. The remaining 20% of the heat is emitted through the device wires and various sides of the package. Less than 1% of the heat escapes through the top of the package. In the case of these bare-pad packages, good PCB heat dissipation design is essential to ensure certain device performance.

One 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 separation between high-power components allows the PCB area around each high-power component to be regionalized, which helps achieve better heat transfer. Care should be taken to separate temperature sensitive components from high power components on the PCB. Wherever possible, high-power components should be located away from the corners of the PCB. A more intermediate PCB position allows for greater board area around the high-power components to aid in heat dissipation. Figure 2 shows two identical semiconductor devices: components A and B. Component A, located at the corner of the PCB, has A chip junction temperature 5% higher than component B, which is positioned more centrally. The heat dissipation at the corner of component A is limited by the smaller panel area around the component used for heat dissipation.

The second aspect is the structure of PCB, which has a decisive influence on the thermal performance of PCB design. As a general rule, the more copper the PCB has, the higher the thermal performance of the system components. The ideal heat dissipation situation for semiconductor devices is that the chip is mounted on a large block of liquid-cooled copper. This is not practical for most applications, so we had to make other changes to the PCB to improve heat dissipation. For most applications today, the total volume of the system is shrinking, adversely affecting heat dissipation performance. Larger PCBS have more surface area that can be used for heat transfer, but also have more flexibility to leave enough space between high-power components.

Wherever possible, the number and thickness of the PCB copper layer should be changed. The weight of grounding copper is generally large, which is an excellent thermal path for the entire PCB heat dissipation. The arrangement of the wiring of the layers also increases the total specific gravity of copper used for heat conduction. However, this wiring is usually electrically insulated, limiting its use as a potential heat sink. The device junction layer should be wired as electrically as possible to as many junction layers as possible to aid in heat conduction. Heat dissipation holes in the PCB below the semiconductor device help heat enter the embedded layers of the PCB and transfer to the back of the board.

The top and bottom layers of a PCB are "prime locations" for improved cooling performance. Using wider wires and routing away from high-power devices can provide a thermal path for heat dissipation. Special heat conduction board is an excellent method for PCB heat dissipation. The thermal conductive plate is located on the top or back of the PCB and is

thermally connected to the device through either a direct copper connection or a thermal through-hole. In the case of inline packaging (only with leads on both sides of the package), the heat conduction plate can be located on the top of the PCB, shaped like a "dog bone" (the middle is as narrow as the package, the copper away from the package has a large area, small in the middle and large at both ends). In the case of four-side package (with leads on all four sides), the heat conduction plate must be located on the back of the PCB or inside the PCB.

Increasing the size of the heat conduction plate is an excellent way to improve the thermal performance of PowerPAD packages. Different size of heat conduction plate has great influence on thermal performance. A tabular product data sheet usually lists these dimensions. But quantifying the impact of added copper on custom PCBS is difficult. With online calculators, users can select a device and change the size of the copper pad to estimate its effect on the thermal performance of a non-JEDEC PCB. These calculation tools highlight the extent to which PCB design influences heat dissipation performance. For four-side packages, where the area of the top pad is just less than the bare pad area of the device, embedding or back layer is the first method to achieve better cooling. For dual in-line packages, we can use the "dog bone" pad style to dissipate heat.

Larger PCB systems can also be used for cooling. The screws used to mount the PCB can also provide effective thermal access to the base of the system when connected to the thermal plate and ground layer. Considering thermal conductivity and cost, the number of screws should reach the point of diminishing returns. The metal PCB stiffener has more cooling area after being connected to the thermal plate. For some applications where the PCB housing has a shell, the TYPE B solder patch material has a higher thermal performance than the air cooled shell. Cooling solutions, such as fans and fins, are also commonly used for system cooling, but they often require more space or require design modifications to optimize cooling.

To design a system with high thermal performance, it is not enough to choose a good IC device and closed solution. IC cooling performance scheduling depends on THE PCB and the capacity of the cooling system to allow IC devices to cool quickly. The passive cooling method mentioned above can greatly improve the heat dissipation performance of the system.