Experience in Thermal Design of Electronic Devices
The experience in and notes on the thermal design of electronic devices are summarized on the basis of years of design experience and experimental analysis summary, specifically as follows:
1: Enough natural convection space should be ensured. There should be a certain distance at least 13mm, between components and
structures to facilitate air flow and enhance the heat dissipation of natural convection.
2: The gap between the components on a vertically mounted board and surrounding PCB should be at least 19mm.
3: In terms of the heat conduction path, the metal or heat conduction material with high heat conduction coefficient can be used to
connect components with the shell or the cold plate for heat dissipation via larger surface area.
4: The emissivity can be as high as 1 by radiant heat transfer and powder coat. In the same sealed shell, the temperature rise of painted
components is 10% lower than that of unpainted ones on average.
5: If large radiator cannot be installed due to high heat flux components and limited assembly space, heat pipe may be used; or the heat
35: When forced cooling is adopted, it is suggested to add corrugation on the fin. The addition of corrugation on the surface can increase
may be led to another location where there is enough space to install a radiator.
6: Within the design life of the product, the failure rate of cooling system, such as the fan, should be lower than that of components.
7: The safety margin must be considered upon thermal design. At least a safe temperature of 20-30 ℃ should be reserved; and a safe
design margin should also be reserved in consideration of an increase in heat consumption and flow resistance caused by the failure
of electronic devices during use.
8: Natural convection, low speed fan and other highly reliable cooling modes should be used as far as possible.
9: When using the fan, attention should be paid to noise parameters, which must meet the design requirements for noise. Meanwhile,
dust prevention and the protection device in front of the fan should be considered.
10: Product cost should be considered upon thermal design to ensure optimized cooling solution, simple product structure, high reliability,
small volume and low cost.
11: Cooling system, such as the fan, should be easy to monitor and maintain.
12: Each air duct should have clear air inlet and air outlet. Different air ducts should be isolated. Air flow short circuit between different air
ducts should be avoided for example, the air outlet of air duct A should not be the air inlet of air duct B.
13: In case of any vacancy of standard cabinet, a guide plate should be installed to prevent air flow short circuit for the air flow short circuit
in the cabinet has great impact on the heat dissipation of components.
14: The cooling by natural convection is applicable to sealed electronic devices with small volume and dense components.
15: The air is a poor conductor of heat. When the distance between heated surfaces is very short, the heat conduction by surrounding
air actually exists. In such case, The air is a poor conductor. A large number of our experiments proved that if the distance between
two surfaces < 6.35mm, the heat transfer between the two surfaces is mainly realized through the air and the heat transfer by natural
convection can be neglected. If the distance between two surfaces > 12.7mm, heat dissipation is mainly realized by convection and radiation
16: The black anodize surface can ensure excellent heat absorption and high emissivity, while polished surface has low emissivity and can be
used to shield heat radiation, for example, the emissivity of polished copper plate is 0.023 and that of black anodize can be as high as 0.95
17: The reflectivity of metal surface is associated with roughness: the reflectivity of shinny surface is higher than that of rough surface If a
polished metal surface is powder coat black, its reflectivity will be improved.
18: In order to realize the maximum radiation heat transfer, it is suggested that the surface of the fin be black. It certainly should not be
understood as all surfaces should be black, for example, at 100 ℃, the emissivity of glass is equivalent to the reflectivity of black powder
coating. With the increase of temperature, the emissivity of glass may be 5-10% higher than that of black coating.
19: Do not choose high-density fins for natural convection
20: At least a 30mm clearance should be reserved for the air inlet of the fan. If the air inlet is blocked, the noise will be 2-3 times higher.
Besides, at least a 10mm clearance should be reserved for the air outlet for exhaust
21: Fans with different interfaces and different flows should not operate in parallel.
22: If the fan adopts a filter, the loss of pressure drop should be considered.
23: For transformer, attention must be paid to the maximum temperature value of winding group. If the maximum temperature is exceeded,
the transformer will be damaged. Once the transformer is damaged, the whole device cannot work. If the transformer has a shielding case
it should be connected to the base as far as possible. The shell,magnetic core and base should be wrapped with copper strips to enhance
24: Experiments showed that the heat consumption of hollow coil is low, thus no serious problem of heat dissipation will occur for natural
convection may be adopted. Upon thermal simulation, hollow coil can be taken as a cylinder. If only the natural convection of the external
surface can be calculated when the coil is horizontally installed, when it is vertically installed, both the internal surface and the external
surface will cool down naturally.
25: During circuit design, long components should be distributed vertically (along the direction of gravity). Vertically distributed components
should be alternatively arranged in horizontal direction.
26: The heat emitted from the human body is about 100W, thus such heat should be considered during the thermal scheme calculation for
outer space capsule.
27: For electronic device to be used outdoors in harsh environment, protector must be considered and installed in the protective shell as far
28: For a natural convection electronic device with an opening, the design of the shell should ensure that the air can flow in from the bottom
and flow out from the top and that the width of the air opening is between 6.35 – 12.7mm.
29: Sometimes, radiation must be controlled. Since the requirements for temperature rise are low, components with poor temperature
tolerance should be protected to avoid damage when heated. Generally, the use of smooth metal material as a shielding case can effectively
protect such components from thermal radiation, but there should be a permanent connection between the thermal radiation and the PCB
or the shell to form a feat transfer path.
30: Thermal design should consider ambient temperature, if a natural convection electronic product is installed in a cabinet, with the increase
of cabinet temperature, the design of natural convection probably cannot meet the ambient temperature in the cabinet. If two forced
convection devices are place in parallel, the distance between them should be considered, otherwise the hot air discharged from one device
may enter the air inlet of the other device, causing temperature rise.
31: For electronic devices, ambient humidity should be considered. In addition, waterproof and dustproof designs should be completed, or
components will be damaged. Of course, attention should also be paid to the min temperature. If the min temperature is lower than the
min operating temperature of components, a heating unit should be equipped.
32: During natural convection, the temperature boundary is thick; if the fin spacing is too small, thermal boundary crossing will occur, affecting
surface convection. Therefore, it is suggested that the fin spacing > 12mm during natural cooling. If the fin height is below 10 mm, the fin
spacing can be calculated from the fin spacing > 1.2 times of the height.
33: During natural convection, the surface heat transfer ability of the fin is weak, the addition of corrugation on the fin has no impact on natural
heat dissipation, thus the addition of corrugation on the fin is not recommended. Generally, during natural convection, black anodizing is
recognized for it can increase the emissivity and radiation heat transfer on the heat-transfer surface.
34: Since the thermal balance time of natural convection is long, the thickness of fin bottom plate and the fin adopting natural convection
should be considered to resist the impact of instantaneous thermal load. The thickness of fin bottom plate has great impact on thermal
capacity and thermal resistance. If the bottom plate is too thin, the thermal capacity is too small. If it is too thick, thermal resistance will
increase instead. According to experimental recommendation, the bottom plate should be 3-6mm thick and the fin 2-5mm thick.
10-20% of heat dissipation capability. If the corrugation is less than 0.5mm in height and is 0.5-1.0mm wide, the effect of heat convection
can be enhanced.
36: when the wind speed is higher than 1m/s, the natural cooling of the fin can be neglected.
37: When fluid cooling solution is adopted, the pipe must be sealed. Pipe bending should be avoided as much as possible to reduce the loss
of resistance; and the sharp expansion or contraction of the pipe should be avoided. Rectangular or square cooling plate should be used
as much as possible, so as to polish the surface and reduce the thermal resistance.
38: The material of air duct should be as smooth as possible on the surface to reduce air friction and minimize the airflow resistance.
39: For electronic device installed on walls, the air inlet and outlet should not be designed on the back of the device. If the air inlet and outlet
must be designed outside, the clearance between the device and the wall should be greater than 100mm.
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