Dejin Computer Technology

Electronics Design & Manufacture

PCB thermal design requirements for electronics PCB layout

1. Components should be installed in the best natural heat dissipation position to make the heat transfer path as short as possible. Should as far as possible on the same piece of PCB components according to their degree of heat partition arrangement and the size of calorific value, small heat or heat resistance of components (such as small signal transistor, small-scale integrated circuit, electrolytic capacitors, etc.) on the upstream of the cooling air (entrance), heat or heat resistance good components (such as power transistor, large scale integrated circuit, etc.) on the downstream of the cooling air flow. The lateral surface of the component installation direction is parallel to the wind direction to facilitate thermal convection.


2. Heating components should be placed above the PCB as far as possible, and in the gas channel when conditions permit. Integrated circuit chips with high calorific value are generally placed on the main PCB to avoid overheating of the bottom shell. If placed under the main PCB, some space should be reserved between the chip and the bottom case to make full use of the gas flow to dissipate heat.


3. For the switching power supply with free convection air cooling, the heat flow channel of the components should be short, the cross-sectional area should be large, and there is no adiabatic or thermal insulation in the channel. For switching power supplies with forced air cooling, it is best to arrange the power devices (or other components) in a cross-length fashion to make the heat transfer cross section as large as possible.


4. The heat capacity of PCB should be evenly distributed, and components with high power consumption should not be centrally distributed. Components with high heat should be installed separately. If it is unavoidable, lower components should be placed in the upstream of the airflow, and sufficient cooling air flow through the heat consumption concentration area should be ensured. When the cooling flow velocity is not large, the components are arranged in a fork arrangement to improve the turbulence degree of the flow and increase the heat dissipation effect.


5. The distance between the components and the housing should be greater than 35 ~ 40cm if the components are not installed on the housing. In the horizontal direction, the high-power device should be arranged as close as possible to the edge of PCB, so as to shorten the heat transfer path. In the vertical direction, the high-power components should be placed as close as possible to the top of the PCB in order to reduce the impact of their operation on other components.


6. In the layout of components, the impact on the surrounding thermal radiation should be taken into account, and heat-sensitive components (including semiconductor devices) should be kept away from heat sources or isolated. For heat sources with a temperature higher than 30℃, it is generally required that under natural cooling conditions, the distance between the components and the heat source should not be less than 4mm. Temperature sensitive components are best placed in the lowest temperature area (such as the bottom), do not put it on the heating components directly above, multiple components had better be staggered layout on the horizontal surface.


7. The heat dissipation of switch power PCB mainly depends on air flow, so the air flow path should be studied during the design, and components or PCB should be reasonably configured. Air flow always tends to the place where the resistance is small, so when configuring components on the PCB, avoid leaving large airspace in a certain area. This should also be noted when configuring multiple PCBS.


8. In the shell with vents, the components layout should follow the air flow direction, that is, the inlet → amplification circuit → logic circuit → sensitive circuit → breakdown circuit → low power resistance circuit → heating component circuit → outlet, forming a good heat dissipation channel. The heating components should be above the housing, and the thermosensitive components should be below the housing. The metal housing should be used as the cooling device. Consider installing components with high heat and high radiation on a PCB.


9. The components are designed to ensure the stability of the thermal environment in order to reduce the temperature stress changes caused by thermal cycling and shock. The change rate of temperature shall not exceed 1℃/min, and the range of temperature variation shall not exceed 20℃. This index can be adjusted according to the characteristics of the designed switching power supply.


10. The coolant and cooling method of the components shall be suitable for the selected cooling system and components and shall not result in chemical reaction or electrolytic corrosion.


11. The electric power of the cooling system is generally 3% ~ 6% of the required cooling thermal power. When cooling, the airflow contains water, temperature difference is too large, will produce condensation or adhesion. Water and other contaminants can lead to electrical short circuits, reduced electrical clearance or corrosion. Measures to be taken are as follows.


The temperature difference before and after cooling should not be too large.


Temperature difference will produce condensation parts, water should not cause blockage or water, if there is water, water parts of the material will not corrode.


Heat shrinkable bushing or other shielding insulation for exposed conductive metal.


12. Capacitors (liquid media) should be kept away from heat sources. During PCB layout, as much space as possible should be reserved between components, between IC chips or between components and chips to facilitate ventilation and heat dissipation.


13. As permitted by the rules, the contact pressure between the cooling component and the component requiring cooling shall be as high as possible, and the two contact surfaces shall be in full contact.


14. For the heat dissipation scheme using heat pipes, the area in contact with the heat pipes should be increased as far as possible to facilitate the heat conduction of heating components and integrated circuit chips. Turbulence in space generally produces high frequency noise which has a significant impact on the circuit and should be avoided.


15. When the amount of heating components in PCB is small (less than 3), add a radiator or heat conducting pipe to the heating components. When the temperature cannot be lowered, use a radiator with a fan to enhance the cooling effect. When heating components quantity is large (more than three), can use the radiator cap (plate), it is based on the location of the heating components and high and low that PCB customized special radiator, or in a large, flat pluck out the components of different high and low position on the radiator, the radiator cover whole buckle on the components, contact with each of the components and heat dissipation. However, due to the poor consistency in the installation and welding of components, the heat dissipation effect is not good, so a soft thermal phase-change thermal pad is usually added on the surface of components to improve the heat dissipation effect.