Put forward requirements for the heat dissipation effect of the heat sink, what measures should be taken in terms of design to meet them?
1. Improve the flatness of the contact surface with the heat sink. In order to improve the heat absorption capacity, it is desirable that the heat sink and the heating element are tightly integrated without leaving any gaps. Unfortunately, this cannot be achieved. Therefore, materials with lower thermal resistance and better adaptability should be used to fill the voids, which requires thermal paste. However, the thermal resistance of the thermal paste is always higher than that of the metal material for processing the heat sink. In order to fundamentally improve the heat absorption capacity of the heat sink base, the flatness of the base surface must be improved. The flatness is measured by the maximum drop of the surface, which is also called frontal degree detection and coplanarity detection. Generally, the flatness of the base of the heat sink can be less than 0.1mm after a little processing. For example, a milling machine or multi-pass wire drawing process can achieve a flatness of 0.03mm, while a CNC milling machine or grinding can achieve better results. We will introduce in detail later. In short, the flatter the heat-absorbing base of the heat sink, the more conducive to heat absorption. But because it can't be perfect, applying thermal paste has become a necessary step to install the heat sink.
2. The specific heat capacity of the material is higher.
The concept of specific heat capacity has been introduced in the previous article, from which we can know:
To increase the temperature of 1 kg of copper by 1°C needs to absorb 93 calories, and to increase the temperature of 1 kg of aluminum by 1°C needs to absorb 217 calories. So whether the heat sink of the aluminum heat-absorbing base can get better heat storage effect? it's not true! Because the heat storage capacity of a specific object is also determined by its quality, specifically to the heat sink base of the heat sink. With the same volume, it depends on the material density-the density of copper is 8933 kg/m3
, and the density of aluminum is 2702 kg/m3
. May wish to calculate the volume specific heat capacity of copper and aluminum according to the following formula:
Cv=ρ x Cm
The volumetric specific heat capacity of copper = 8933 kg/m3
x 93kl/kg * °C ≈ 0.83 x 10^6 kl/m3
Volume specific heat capacity of aluminum = 2702 kg/m3
x 217kl/kg * °C ≈ 0.58 x 10^6 kl/m3
The result is clear: With the same volume of copper and aluminum (including various aluminum alloys), when the same temperature change occurs, copper can absorb about 40% more heat than aluminum, which can better suppress the surge in the temperature of heating equipment. This is the reason why mid-to-high-end radiators do not use all-copper designs, but also use copper-aluminum heat-absorbing base designs.
In addition to choosing materials with higher "volume specific heat capacity" in terms of materials, improvements can also be made in the shape design of the heat sink base —— Keeping the thickness of the base of the heat sink constant can increase the bottom area of the base. Or keeping the area of the base of the heat sink constant, the thickness of the heat-absorbing bottom can be increased. Both can increase the volume of the heat-absorbing bottom base, thereby increasing the heat capacity.
The heat transfer coefficient of the heat sink material should be high. To reduce the internal thermal resistance of the heat-absorbing base, the use of copper with higher thermal conductivity is indeed a better choice than aluminum alloy, and it is also the method used by many middle and high-end radiators. The material of the heat sink base is determined, and the thermal resistance can also be changed by adjusting the shape design of the heat absorption base. At this time, it is faced with the problem of balancing the longitudinal and lateral thermal resistance of the heat absorption base.
According to the basic common sense of heat conduction: The larger the cross-sectional area, the smaller the thermal resistance, and the greater the thickness, the greater the thermal resistance. Specific to the shape design of the endothermic bottom: The larger the area, the thinner the thickness, and the smaller the longitudinal thermal resistance; On the contrary, the thicker the thickness, the smaller the lateral thermal resistance and the larger the effective connection area of the fin.
The longitudinal and transverse thermal resistance respectively put forward contradictory requirements on the shape of the endothermic bottom, which requires the designer to make a trade-off. Choose the appropriate area, thickness and shape, so that the longitudinal and transverse thermal resistance can meet the requirements. If you fail to find a suitable balance point, there may be some serious adverse effects on the thermal conductivity and even the overall performance of the heat sink:
Large thickness, small area-small lateral thermal resistance, which can effectively use the fins connected to it. However, the longitudinal thermal resistance is large, which increases the overall thermal resistance of the heat sink, which is not conducive to the improvement of the overall performance.