PCB Tech - some methods of PCB thermal design

1 Heat dissipation through the PCB board itself

At present, the widely used PCB boards are copper-clad/epoxy glass cloth substrates or phenolic resin glass cloth substrates, and a small amount of paper-based copper-clad boards are used. Although these substrates have excellent electrical properties and processing properties, they have poor heat dissipation. As a heat dissipation path for high-heating components, it is almost impossible to expect heat from the resin of the PCB itself to conduct heat, but to dissipate heat from the surface of the component to the surrounding air. However, as electronic products have entered the era of miniaturization of components, high-density mounting, and high-heating assembly, it is not enough to rely on the surface of a component with a very small surface area to dissipate heat. At the same time, due to the extensive use of surface mount components such as QFP and BGA, a large amount of heat generated by the components is transferred to the PCB board. Therefore, the best way to solve the problem of heat dissipation is to improve the heat dissipation capacity of the PCB itself, which is in direct contact with the heating element, through the PCB board. To be transmitted or emitted.

2 High heat-generating components plus radiator and heat conduction plate

When a small number of components in the PCB generate a large amount of heat (less than 3), a heat sink or heat pipe can be added to the heating device. When the temperature cannot be lowered, a heat sink with a fan can be used to enhance heat dissipation Effect. When the number of heating devices is large (more than 3), a large heat dissipation cover (board) can be used, which is a special heat sink customized according to the position and height of the heating device on the PCB or a large flat heat sink Cut out different component height positions. The heat dissipation cover is integrally buckled on the surface of the component, and it is in contact with each component to dissipate heat. However, the heat dissipation effect is not good due to the poor consistency of height during assembly and welding of components. Usually, a soft thermal phase change thermal pad is added on the surface of the component to improve the heat dissipation effect.

3 For equipment that adopts free convection air cooling, it is best to arrange integrated circuits (or other devices) vertically or horizontally.

4 Use reasonable wiring design to achieve heat dissipation

Because the resin in the plate has poor thermal conductivity, and the copper foil lines and holes are good conductors of heat, increasing the residual rate of the copper foil and increasing the heat conduction holes are the main means of heat dissipation.

To evaluate the heat dissipation capacity of the PCB, it is necessary to calculate the equivalent thermal conductivity (nine eq) of the composite material composed of various materials with different thermal conductivity-the insulating substrate for the PCB.

5 The devices on the same printed board should be arranged as far as possible according to their calorific value and degree of heat dissipation. Devices with low calorific value or poor heat resistance (such as small signal transistors, small-scale integrated circuits, electrolytic capacitors, etc.) should be placed in cooling At the top of the airflow (at the entrance), devices with large heat or heat resistance (such as power transistors, large-scale integrated circuits, etc.) are placed at the most downstream of the cooling airflow.

6 In the horizontal direction, high-power devices are placed as close to the edge of the printed board as possible to shorten the heat transfer path; in the vertical direction, high-power devices are placed as close as possible to the top of the printed board to reduce the temperature of other devices when these devices are working. Influence.

7 The heat dissipation of the printed board in the equipment mainly relies on air flow, so the air flow path should be studied during the design, and the device or printed circuit board should be reasonably configured. When air flows, it always tends to flow in places with low resistance, so when configuring devices on the printed circuit board, avoid leaving a large airspace in a certain area. The configuration of multiple printed circuit boards in the whole machine should also pay attention to the same problem.

8. The temperature-sensitive device is best placed in the lowest temperature area (such as the bottom of the device). Never place it directly above the heating device. It is best to stagger multiple devices on the horizontal plane.

9 Arrange the components with the highest power consumption and the highest heat generation near the best position for heat dissipation. Do not place high-heating devices on the corners and peripheral edges of the printed board, unless a heat sink is arranged near it. When designing the power resistor, choose a larger device as much as possible, and make it have enough space for heat dissipation when adjusting the layout of the printed board.

10 The RF power amplifier or LED PCB adopts a metal base substrate.
11 Avoid the concentration of hot spots on the PCB, distribute the power evenly on the PCB board as much as possible, and keep the PCB surface temperature performance uniform and consistent. It is often difficult to achieve strict uniform distribution during the design process, but areas with too high power density must be avoided to prevent hot spots from affecting the normal operation of the entire circuit. If possible, it is necessary to analyze the thermal efficiency of the printed circuit. For example, the thermal efficiency index analysis software module added in some professional PCB design software can help designers optimize the circuit design.
Four, summary
3.1 Material selection
(1) The temperature rise of the conductors of the printed board due to the passing current plus the specified ambient temperature should not exceed 125 degree Celsius (commonly used typical value. It may be different depending on the selected board). Since the components installed on the printed board also emit some heat, which affects the working temperature, these factors should be considered when selecting materials and the design of the printed board. The hot spot temperature should not exceed 125 degree Celsius. Choose thicker copper clad as much as possible
(2) In special cases, aluminum-based, ceramic-based, and other plates with low thermal resistance can be selected.
(3) The multi-layer board structure is helpful for PCB thermal design.

3.2 Ensure that the heat dissipation channel is unblocked
(1) Make full use of the components arrangement, copper skin, window opening and heat dissipation holes to establish a reasonable and effective low thermal resistance channel to ensure that the heat is smoothly exported to the PCB.
(2) The setting of heat dissipation through holes Design some heat dissipation through holes and blind holes, which can effectively increase the heat dissipation area and reduce the thermal resistance, and increase the power density of the circuit board. For example, set up via holes on the pads of LCCC devices. Solder fills it in the circuit production process to increase the thermal conductivity. The heat generated during circuit operation can be quickly transferred to the metal heat dissipation layer or the copper pad on the back through the through holes or blind holes to be dissipated. In some specific cases, a circuit board with a heat dissipation layer is specially designed and used. The heat dissipation material is generally copper/molybdenum and other materials, such as printed boards used on some module power supplies.
(3) The use of thermally conductive materials In order to reduce the thermal resistance of the thermal conduction process, thermally conductive materials are used on the contact surface between the high power consumption device and the substrate to improve the heat conduction efficiency.
(4) The process method is likely to cause local high temperature in some areas where the device is mounted on both sides. In order to improve the heat dissipation conditions, a small amount of small copper can be mixed in the solder paste, and there will be a certain amount of solder joints under the device after reflow soldering. high. The gap between the device and the printed board is increased, and the convection heat dissipation is increased.

3.3 Requirements for the arrangement of components

(1) Perform software thermal analysis on PCB, and design and control the internal maximum temperature rise;

(2) It can be considered to specially design and install components with high heat and radiation on a printed circuit board;

(3) The heat capacity of the board is evenly distributed. Be careful not to place high-power components in a concentrated manner. If it is unavoidable, place the short components upstream of the airflow and ensure sufficient cooling air flow through the heat-consumption concentrated area;