As electronic products continue to be miniaturized, as more functions are loaded into smaller devices, the heat dissipation requirements of these systems also increase. This is especially true for PCBs operating at high current. Especially the power supply system with heavy load, such as lithium-ion batteries used in electric vehicles, needs a power management system integrated on PCB. As well as the driving circuit with high current, heat dissipation should be emphasized. Designers need to implement creative strategies to manage the heat generated in high-current PCB. The heat generated by the loss of power consumption in the circuit carrying large current should be combed away from the heating device to resist the temperature rise. Everyone may be familiar with the fans and heat sinks used in computer processors. All these measures can transfer heat from the circuit board and exchange heat with the circulating air. However, in some PCB devices, especially small-sized devices, fans or heat sinks may not be installed. It is necessary to consider other ways of heat dissipation.
Using thicker copper for the resistance of high-current copper traces and vias will lead to significant power loss and heat generation of PCB-based devices, especially when they carry high current. An electrical connection with a larger cross-sectional area has a lower resistance, which reduces the amount of heat loss. The amount of copper used in most PCBs is equivalent to about 1 ounce per square foot. When it is inconvenient to use the fan or the heat sink, the copper thickness can be increased. A high-current PCB should use at least twice the amount of copper. Circuits with a current of more than 10 amps should be as high as 3 or 4 ounces per square foot. The copper thickness of PCB is calculated by oz, which means that the average weight of copper foil is 28.35g in an area of 1 square foot. oz is the abbreviation of ounce, which is transliterated as “ounce”. It is an English unit of measurement, and it is also called English two when used as a weight unit. It represents the average thickness of copper foil by the weight per unit area. Expressed by the formula, that is, 1OZ=28.35g/ FT2. Unit weight: 1oz=28.35g (g) 1oz =16 dram) 16 oz =1 pound (pound) Conversion method introduction: The copper foil thickness is the weight of copper foil divided by the density and surface area of copper. 1 square foot =929.0304 square centimeters, Copper density = 8.9 kg/dm 3. Let the copper thickness be x, and solve the equation:
X * 929.0304cm2 * 8.9g/cm3 = 1oz = 28.35g x = 0.0034287cm =34.287um so 1oz=34.287um The thickness of 1OZ copper foil is about 35um or 1.35mil.
The above picture shows the curve table of current and wire width on a 1oz thick copper PCB. For reference. Using more copper requires increasing the width of the traces on the PCB. In order to avoid occupying too much space, the wires can also be embedded deeper in the circuit board. For example, placing copper bars. This also helps to dissipate heat to the circuit board itself and any nearby heat dissipation holes. Of course, this may require the use of a thicker circuit board, which is suitable for high-current equipment.
If the air around the heating device using the heat dissipation hole and the heat dissipation gasket does not flow, it cannot effectively dredge and dissipate heat. Heat can be transferred from the key electronic components in the circuit board by using heat dissipation holes. The heat dissipation hole is a good heat conduction element between the top layer and the bottom layer of the circuit board. Heat can be transferred to the heat dissipation through holes through simple conduction, and then the heat dissipation through holes can be evacuated from key electronic components. Heat dissipation gasket is usually a metal plate installed at the bottom of the circuit board. After the heat dissipation hole transfers the heat from the hottest spot of the circuit board itself, it must go to other places to further dissipate the heat from the hottest spot of the circuit board. Generally, the heat dissipation holes transfer heat to the heat dissipation gasket for large-area heat dissipation. The following figure shows the infrared image of PCB running at high current:
Layout of high-power devices High-current electronic components such as microcontrollers will generate a lot of heat. It is a good idea to install these components near the center of the circuit board. If the components are installed near the edge of the circuit board, the heat generated will accumulate and the local temperature will be very high. However, if the components are installed in the middle part of the circuit board, the heat will spread to the whole circuit board, and the temperature of the circuit board will be lowered. Multiple high-power components should be distributed on the whole circuit board, rather than concentrated in one place. If the external dimensions of the device allow, different components can even be laid out separately on different PCB boards. The layout of components should be weighed again and again, because it is related to the functional realization of the whole circuit board on the one hand, the heat dissipation and mechanical matching on the other hand, and the possible impact on your manufacturing budget on the other.
When the components are operated at extreme temperature, the life of the electrical connection, components and circuit board itself will be shortened accordingly. The computer industry has used cooling fans to reduce the risk of this problem. But when the fan doesn’t work, most of the heat goes directly into the circuit board and surrounding components. At this time, if the circuit board is very thin, everything will heat up to a very high temperature. Thicker circuit boards will require more heat energy when the overall temperature is raised. Such a thicker circuit board helps to keep the temperature at the top of the circuit board low. If the circuit board is directly mounted on the housing, heat can be conducted to the outside of the equipment. However, this solution may lead to higher production and processing costs. Therefore, appropriate trade-offs should be made in application. JTAG-USB adapter for FPGA hardware development, and other signal conversion modules, non-high current and high heating circuits. 1.0mm thickness is adopted. As a small module of similar devices, the daughter board on the board has a half hole and gold-plated edges, and the whole board is welded to the motherboard as a module. Both the daughter board and the mother board are 1.0mm thick. The conventional power board is 1.6mm thick. This thickness of 1.6mm is the default thickness of the general board factory. Unless otherwise specified, it will be 1.6mm by default. And the processing cost of 1.6mm is the critical point. Within 1.6mm, there is no additional surcharge for special thickness.
The motherboard power board is 2.0mm thick. 2.0mm is the manufacturing process of thickened plate. The plate thickness takes into account the situation of large current and high heat generation in small space. The best cooling strategy to adopt depends on many factors. Not all design or form factors can adapt to all the above strategies. For example, the heat dissipation gasket is not suitable for double-sided printed circuit boards. If there are a large number of components on the circuit board, some of them will inevitably be placed near the edge of the circuit board.
