## Chip heat dissipation area calculation

The tool provides a mutual conversion of the chip heat dissipation area and the temperature rise coefficient.

Estimate the temperature rise coefficient (Celsius/Watt) based on the effective area of the heat sink (cm 2 )

 Formula: ℃/W = 50 / Sqrt(Area cm² )

 Area[A] (cm²) Effective area ℃/W (Temperature rise coefficient)

Calculation of minimum effective fin area of heat sink based on known temperature rise

 Formula: Area [A](cm²) = (50/(℃/W) )²

 ℃/W (Temperature rise coefficient) Effective area Area[A] (cm²)
1. Thermal resistance

Silicon: silicon has excellent electrical properties of semiconductors. The band gap is moderate, which is 1.21 ev. Carrier mobility is high, electron mobility is 1 350 square centimetres per volt. sec, hole mobility is 480 square centimetres per volt. sec. The intrinsic resistivity is as high as 2.3 65507 The lifetime of unbalanced minority carriers in silicon crystals is longer than that in tens of microseconds to 1 milliseconds. The thermal conductivity is large, the chemical property is stable, and it is easy to form a stable thermal oxidation film. In the fabrication of planar silicon devices, PN junction surface passivation and protection can be achieved by using oxide film, and metal oxide semiconductor structures can be formed to fabricate MOS field effect transistors and integrated circuits. These properties make PN junction have good characteristics, so that silicon devices have the advantages of high voltage resistance, small reverse leakage current, high efficiency, long service life, good reliability, good heat conduction and so on.

We often see MOS devices in computers, so what are MOS devices?

The full text of MOS is: Metal Oxide Semiconductor. A field effect transistor fabricated using an oxide film silicon material is called a MOS type field effect transistor, and is a metal oxide field effect transistor.

In the winter, when we put our hands on a piece of wood and put it on an iron plate, we feel that the iron plate is cooler than the wood plate. The iron plate is bigger, the tighter the contact, the colder it feels. This shows that the iron plate has better heat dissipation than the wood board, and the heat dissipation capability is related to the area, volume, geometry, and the tightness of the contact surface.

When the computer is working, the loss of the PN junction of the chip transistor (any integrated circuit chip is composed of N transistors) produces a temperature rise Ti, which is the thermal resistance Rri between the die and the case, and the component case without the heat sink The thermal resistance Rrb between the component and the surrounding environment, the thermal resistance Rrc between the component and the heat sink, and the thermal resistance Rrf between the heat sink and the surrounding environment transfer heat away, so that the temperature difference can meet the requirements of the normal operation of the component.

Since the heat conduction is mainly through the three thermal resistances of Rri, Rrc and Rrf, the total thermal resistance Rrz can be expressed by the following formula:

Rrz＝Rri+Rrc+Rrf

So when the allowable temperature rise and power consumption of the chip have been determined, the required total thermal resistance Rrz can be determined, and then the size of the heat sink is determined from the following formula. This is the purpose of introducing the thermal resistance and its application.

The thermal resistance Rr is the total thermal resistance Rrz from the die of the chip through the outer casing, the contact surface, and the heat sink to the surrounding air, so that it can be calculated by the following formula.

Ti-Ta=Pc（Rti+Rrc+Rrf）=Pc Rrz

Rrz=（Ti-Ta）/Pc

Where: the junction temperature allowed by the Ti chip, the air temperature around the Ta chip environment, and the heat source power loss of the Pc chip

2.Thermal conductivity

Thermal conductivity (also known as "thermal conductivity" or "thermal conductivity") is an important physical quantity that reflects the thermal properties of a material. Heat conduction is one of the three basic forms of heat exchange (heat conduction, convection and radiation), and is a subject in various fields of engineering thermophysics, materials science, solid state physics, energy, and environmental protection. The thermal conductivity of a material depends to a large extent on its microstructure. The transfer of heat depends on the atom, the vibration of the molecule around the equilibrium position, and the migration of free electrons. The electron flow dominates in the conductive metal, and the lattice vibration plays a dominant role in the insulator and most semiconductors.

In 1882, the French scientist J. Fourier established the theory of heat conduction. At present, various methods for measuring the thermal conductivity are based on the Fourier heat conduction law. When there is a temperature gradient inside the object, heat is transferred from the high temperature to the low temperature. This phenomenon is called heat conduction. Fourier pointed out that the heat dQ passing through the ds area in dt time is proportional to the temperature gradient inside the object, and the thermal conductivity of the proportional coefficient is:

dQ/dt＝-λ .dt/dx .ds

Where dQ/dt is the heat transfer rate, dt/dx is the temperature gradient in the direction perpendicular to the area ds, the "-" sign indicates that heat is transferred from the high temperature region to the low temperature region, and λ is the thermal conductivity coefficient, indicating the thermal conductivity of the object. the size of. In the formula, the unit of λ is W.m minus 1 power. K minus 1 power.

For anisotropic materials, the thermal conductivity in all directions is different (commonly used for tensors).

If you don't understand or understand the above formula (the above is a high-level physics course at the university), I will express the thermal conductivity coefficient in plain language.

Thermal conductivity is also known as thermal conductivity or thermal conductivity. A physical quantity that characterizes the thermal conductivity of a substance. It is arranged inside the object perpendicular to the direction of heat conduction to take two parallel faces with an area of 1 m and an area of 1 m2. The temperature difference between the two planes is 1 degree, and the heat is transferred from one plane to the other in 1 second. It is defined as the thermal conductivity of the substance. The unit is: watt / (meter. Celsius), the original engineering unit system is: kilocalories / (m. hour. Celsius), the reciprocal of thermal conductivity is called thermal resistance. When other conditions are constant, the greater the thermal conductivity, the smaller the thermal conductivity is, the greater the thermal conductivity; otherwise, the smaller the thermal conductivity.

According to the above formula and definition, the heat dissipation method of the chip is to dissipate heat through the air flow in the box by the area of the substrate (copper or aluminum) that is in contact with the chip, and the amount of heat dissipation is proportional to the area of the substrate. When the temperature inside the chassis reaches a certain level, it loses its heat dissipation capability. In order to remove the heat radiated from the chip, only strong winds can be used under normal conditions. Therefore, the cooling method of the fanless silent heat pipe is not desirable.

In addition, according to the aerodynamic principle (the formula is not listed), the installation of the chassis fan, the wind direction must be consistent, both: before entering and exiting, forming a duct, in order to take away the heat inside the box, to achieve the purpose of heat dissipation.