Fundamental

The key issue in heat treatment is to effectively conduct the heat generated by the heat source to the heat sink.

  Thermal interface materials TIMs=Thermal Interface Materials have been widely used to improve the efficiency of heat transfer. The heat generated by the device is conducted to the metal heat sink through its surface, and the heat is dissipated into the air through the heat sink. However, the surface of the component is usually made of plastic, ceramic or metal, and the surface is not completely flat (the surface of the radiator is also not completely flat). If the component is directly in contact with the radiator, a gap will inevitably remain in the middle, and air will remain in between. The contact surface will be reduced, and the heat will not be effectively conducted. The softness of the TIM material allows it to fill the gaps therebetween, helping to conduct heat transfer effectively.

Figure 1 Schematic diagram of using thermal interface materials to improve heat transfer efficiency

As shown in Figure 1, after the semiconductor is mounted on the circuit board, its thermal resistance at its junction (porous surface) will increase significantly. If the TIM material is filled in between, it can be seen that it effectively eliminates the contact thermal resistance of the semiconductor device mounted on the circuit board, forming a higher gradient temperature difference. Even if it is a polished surface, the two surfaces cannot be in full physical contact. To reduce thermal resistance, it is necessary to achieve complete physical contact, so as to form an efficient heat conduction path.

Thermal conductivity and testing

 The rate of heat conduction through the material is proportional to the normal area of the heat flow and the temperature gradient along the heat flow path. For a one-dimensional, stable heat flow, the rate can be expressed by the Fourier equation:

among them:

K=thermal conductivity, W/m·K

Q=heat flow rate, W

A=contact area

d = heat flow distance

ΔT = temperature difference

Thermal conductivity is an inherent property of a material, and has nothing to do with the size, size, thickness, or direction of the material.

Beihua New Rubber adopts thermal conductivity tester DRL-III to measure the thermal conductivity of products in accordance with ASTM D5470 standard.

Figure 2 Thermal conductivity tester DRL-III test bench

 Thermal resistance

The resistance encountered by the heat in the heat flow path, and the size of the reaction medium or the heat transfer capacity between the media.

The relationship between material thermal resistance and thermal conductivity is:

Thermal resistance is proportional to thickness

Since the actual surface cannot be truly flat or smooth, the contact surface between the surface and the material will also produce resistance to heat flow. This resistance is called contact thermal resistance.

The total thermal resistance in heat transfer is equal to the sum of the thermal resistance of the material and the contact thermal resistance between the contact surfaces.

(2) R total = R material + R contact

Surface smoothness, surface roughness, and clamping pressure have an important influence on contact thermal resistance.

For example, the relationship between thermal resistance and interface thickness of our company's high thermal conductivity gasket BH-T3160 is shown in Figure 3:

Figure 3 The relationship between thermal resistance and interface thickness of thermal pad BH-T3160

Determination of thermal conductivity

The thermal conductivity of the material is:

(4) K=d/R material

Substituting formula (4) in formula (3) will get:

(5) d=KR total-KR contact

     Fitting is performed by measuring the thermal resistance (Rtotal) under different interface thicknesses (d). After removing the influence of the contact thermal resistance, the thermal conductivity of the material can be calculated. As shown in Figure 4, the thermal conductivity of BH-T3160 is 6.05W/m·K.

Figure 4 Fitting the thermal conductivity of thermal pad BH-T3160 by linear regression

Table 2 Product types and applicable interface thickness

T: Recommended typical applications

A: Applicable

Use thin TIM thermal pad to obtain lower thermal resistance and improve thermal conductivity

Advantages of thick TIM thermal pad:

If there are multiple heating components, a thick TIM thermal pad can cover multiple heating components with one piece, even if the height of each component is different.

Thick TIM thermal pad has greater heat capacity. If the heat source generates a large amount of heat instantly, the thick TIM thermal pad can effectively absorb the large amount of accidental heat generated in this part, thereby protecting the components.

Ÿ TIM compression capability

The thermal interface material is usually placed between the heat source and the heat sink, supplemented by a certain pressure. Therefore, in the thermal design management, the compressibility of the TIM material will become an important consideration for our TIM material thickness tolerance design.

Ÿ Glass fiber and surface hardened non-stick treatment

For ease of use, you can choose a thermal pad with glass fiber cloth or with a surface hardness enhancement layer (non-stick) treatment.

Ÿ Product model description: