The heat generated when the electronic device is working , so that the internal temperature rose rapidly , if not the heat dissipation , the device continues to heat up , the device will fail due to overheating , the reliability of electronic equipment will decline. Accordingly , heat treatment of the circuit board is very important .
A printed circuit board temperature factors
The direct cause of the temperature rise is due to the presence of PCB circuit power devices , electronic devices exist in power consumption, heat intensity with power changes in the size varying degrees .
PCB temperature rise of two kinds of phenomena :
( 1) local temperature rise or a large area ;
( 2 ) short-term or long-term temperature rise .
In analyzing the PCB thermal power, generally to analyze the following aspects .
An electrical power consumption
( 1 ) Analysis of the power consumption per unit area ;
( 2 ) Analysis of the distribution of power on the PCB .
2 PCB structure
( 1 ) PCB size ;
( 2 ) PCB material.
3 PCB mounting
( 1 ) Installation ( eg mounted vertically, horizontally mounted ) ;
( 2 ) from the housing and the seals distance.
4 thermal radiation
( 1 ) radiation coefficient PCB surface ;
( 2 ) the temperature difference between the absolute temperature , and their adjacent surfaces of the printed circuit board ;
5 heat conduction
( 1 ) Install the heat sink ;
( 2 ) Other mounting structure of conduction.
6 thermal convection
( 1 ) natural convection ;
( 2 ) forced convection cooling .
From the analysis of the factors mentioned above PCB is an effective way to solve the PCB temperature , often in a product and systems of these factors are interrelated and dependent on , the majority of factors that should be analyzed according to the actual situation , and only for a specific to compare the actual situation correctly calculate or estimate the parameters such as temperature and power consumption .
Second, the circuit board cooling mode
1 . High plus radiator heating devices , thermal plate
When the PCB in a small number of larger devices hair when heat (less than three ) , you can add a radiator or heat pipes in heating devices , when the temperature can not drop down , can be used with fan heatsink to enhance heat dissipation effect. When a large amount of heating devices (more than three ) , can be large heat shield ( board ) , which is based on the location and level of the PCB heating devices and custom dedicated radiator or radiator in a large flat different components on the level of the position of the cutout . The overall buckle in the heat shield element surface, while cooling in contact with each component. However, due to the level of consistency is poor component assembly and welding , the cooling effect is not good. Usually components plus a soft surface thermal phase change thermal pad to improve heat dissipation.
2 . Heat through PCB board itself
Current sheet is widely used PCB laminates / epoxy glass cloth material or phenolic resin glass cloth material , as well as a small amount of paper-based materials used CCL . Although these substrates have excellent electrical properties and processing performance , but poor heat resistance , high thermal path as heating elements , almost can not expect a resin transfer heat from the PCB itself , but the heat from the surface element to the surrounding air. However, with the electronic member has entered the small , high -density mounting and high heat of the assembly times, if only by a very small surface area of the surface of the component is insufficient to dissipate heat . And because extensive use of QFP, BGA and other surface mount components , a large amount of heat generated by components transmitted PCB, therefore , the best solution is to increase the heat dissipation capability of the PCB itself in direct contact with the heating element through the PCB board conduction out or dissipate .
3 . Adopt reasonable alignment designed to achieve thermal
Since the resin sheet material of poor thermal conductivity , and the holes are copper lines and a good conductor of heat , thus increasing the thermal conductivity and increasing the rate of residual copper is the primary means of cooling holes .
PCB cooling capacity evaluation , we need the thermal conductivity of the composite material composed of a variety of different PCB is calculated using one equivalent thermal conductivity of the insulating substrate ( nine eq).
4 . For a free- convection air cooling equipment , it is best integrated circuit ( or other device ) arranged by longitudinal , or arranged in horizontal long way .
5. Devices should be aligned as far as possible in accordance with district heat and cooling degree of the same size on a piece of PCB , heat a small or poor heat resistance of the device ( such as small signal transistors, small- scale integrated circuits , electrolytic capacitors, etc. ) on the cooling airflow the most upstream ( inlet) , heat , or high heat resistance devices ( such as power transistors, LSI , etc.) on the most downstream cooling flow .
6. On the other device when the temperature in the vertical direction, the power devices are arranged as close to the top of the PCB , in order to reduce these devices work ; in the horizontal direction, the power devices are arranged as close to the edge of the PCB , in order to shorten the heat transfer path .
7 . Temperature sensitive device is preferably placed in the coldest regions ( such as the bottom of the device ) , do not put it above the heating device , multiple devices are interleaved best layout in the horizontal plane .
8. PCB cooling equipment mainly depends on the air flow , so the design to study the air flow path , the rational allocation of components or printed circuit boards. Always tend to place less resistance when the flow of air flow , so the printed circuit board configuration device , to avoid leaving a large airspace in an area. Printed circuit board machine in multi-block configuration should also pay attention the same problem.
9. Focus on the PCB to avoid hot spots , as the power is evenly distributed to the PCB , the PCB surface to maintain a uniform and consistent temperature performance . Often the design process to meet strict uniform distribution is more difficult , but be sure to avoid high power density areas , avoid hot spots affect the normal operation of the entire circuit . If the conditional, the thermal efficiency of the printed circuit analysis is necessary, such as increasing the number of professional PCB design software is now in the hot performance metrics analysis software module , can help designers to optimize circuit design.
10. The maximum power consumption and heat maximum cooling device arranged near the optimum position. Do not place a higher heating device in the corner and around the edge of the PCB , unless it is arranged in the vicinity of a cooling device . In the design of power resistors when selecting larger devices as possible , and when adjusting the PCB layout so that there is enough space for heat dissipation .
11. Heat dissipation in the device connected to the substrate due to reduce the thermal resistance between them. In order to better meet the requirements of thermal characteristics , the underside of the chip can use some of the thermal conductivity of the material ( such as smear layer thermal silica ) , and maintain a certain contact area for heat dissipation devices .
12. The device connected to the substrate :
( 1 ) to shorten the length of the device leads ;
(2 ) selection of high -power devices , you should consider the thermal conductivity of lead material , if possible , try to choose a cross- section of the surface leads the largest ;
( 3 ) Select the number of pins more devices.
13. Select the device package :
( 1 ) Consider the thermal design should pay attention to the device 's package instructions and its thermal conductivity ;
( 2 ) should be considered to provide a good thermal conduction path between the substrate and the device package ;
( 3 ) on the heat conduction path off the air should be avoided , if such a situation can be filled with thermally conductive material .
