Abstract
     Surface-mount IC packages rely on printed circuit boards (PCBS) for heat dissipation. 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 run well, but also can bury the hidden danger of thermal accidents. Careful handling of PCB layout, board structure, and device mounting can help improve heat dissipation performance for medium to high power applications.

introduction
    Semiconductor manufacturing companies struggle to control the systems that use their devices. However, the system on which the IC is installed is critical to overall device performance. For custom IC devices, system designers often work closely with manufacturers to ensure that the system meets the numerous thermal 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 parts, with no contact between the manufacturer and the end application. In this case, we can only use some general guidelines to help achieve a better IC and system passive cooling solution.
    The common semiconductor package type is bare pad or PowerPADTM package. In these packages, the chip is mounted on a metal sheet called a chip pad. The chip pad supports the chip in the process of chip processing, and is also a good heat channel for device heat dissipation. When the bare pad of the package is welded to the PCB, the heat can be quickly dissipated from the package and then into the PCB. After that, the heat is dissipated through the PCB layers and into the surrounding air. Bare-welded disc packages can generally conduct about 80% of the heat, which enters the PCB through the bottom of the package. The remaining 20% of the heat is dissipated through the device wires and various aspects of the package. Less than 1% of the heat is lost through the top of the package. For these bare-welded disc packages, a good PCB heat dissipation design is essential to ensure certain device performance.

Electronic System Design
Figure 1 PowerPAD design
    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 separation between high-power components maximizes the PCB area around each high-power component, helping to achieve better heat transfer. Care should be taken to separate temperature-sensitive components from high-power components on the PCB. Whenever possible, high power components should be installed away from PCB corners. A more central PCB position maximizes the board area around high-power components, helping to dissipate heat. Figure 2 shows two identical semiconductor devices: components A and B. Component A is located at the corner of the PCB and has a chip junction temperature 5% higher than component B because component B is located closer to the center. Due to the smaller area of the panels surrounding the components used for heat dissipation, the heat dissipation at the corner position of component A is limited.


Electronic System Design
igure 2 Influence of component layout on thermal performance. PCB corner components have a higher chip temperature than intermediate components.
    The second aspect is the structure of the PCB, which has the most decisive impact on the thermal performance of the PCB design. The general principle is: the more copper in the PCB, 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. For most applications, this mounting 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 continues to shrink, adversely affecting thermal performance. The larger the PCB, the greater the area that can be used for heat transfer, but also has more flexibility, can leave enough space between the high-power components.
    Wherever possible, maximize the number and thickness of PCB copper-grounded strata. The weight of ground copper is generally large, and it is an excellent heat path for the entire PCB to dissipate heat. For the arrangement of the layers, the total proportion of copper used for heat conduction will also increase. However, this wiring is usually carried out in electrothermal isolation, thus limiting its role as a potential heat dissipation layer. The device ground layer should be wired as electrically as possible to many ground layers to help maximize heat transfer. The cooling through hole on the PCB located under the semiconductor device helps the heat to enter the hidden layers of the PCB and conduct to the back of the board.
    For improving heat dissipation performance, the top and bottom layers of the PCB are "prime locations". The use of wider wires, routed away from high-power devices, can provide a thermal path for heat dissipation. A special heat transfer plate is an excellent way for PCB heat dissipation. The heat transfer plate is generally located on the top or back of the PCB and is thermally connected to the device through a direct copper connection or a hot through hole. In the case of inline packaging (only the package with leads on both sides), this thermal conductive plate can be located on the top of the PCB, shaped like a "dog bone" (the middle is as narrow as the package, the connection copper area away from the package is large, the middle is small and the two ends are large). In the case of a four-side package (with leads on all four sides), the heat transfer plate must be located on the back of the PCB or into the PCB.

Electronic System Design

Figure 3. Example of a "dog bone" shape method for a dual in-line package

    Increasing the heat transfer plate size is an excellent way to improve the thermal performance of PowerPAD packages. Different thermal plate sizes have a great impact on thermal performance. Product data forms provided in tabular form typically list this size information. However, it is difficult to quantify the impact of increased copper on custom PCBS. With some online calculators, users can select a device and then change the size of the copper pad to estimate its impact on the thermal performance of non-JEDEC PCBS. These calculation tools highlight the extent to which PCB design affects thermal performance. For four-sided packages, the area of the top pad is just smaller than the bare pad area of the device, in which case the buried or back layer is the first way to achieve better cooling. For dual in-line packages, we can use the "dog bone" pad style to dissipate heat.
    Finally, systems with larger PCBS can also be used for cooling. In cases where the heat transfer plate and ground layer are connected, some of the screws used to mount the PCB can also be effective heat access to the system base. Considering thermal conductivity and cost, the number of screws should reach the maximum of the diminishing return point. After connecting to the thermal plate, the metal PCB reinforced plate has more cooling area. For some applications where the PCB has a housing, the welding filler has higher thermal properties than the air-cooled housing. Cooling solutions, such as fans and heat sinks, are also common ways to cool systems, but they often require more space or require design modifications to optimize cooling.
    In order to design a system with high thermal performance, it is not enough to select a good IC device and a closed solution. The heat dissipation performance scheduling of the IC depends on the PCB and the capacity of the heat dissipation system to allow the IC device to cool quickly. Using the passive cooling method, the heat dissipation performance of the system can be greatly improved.