Comparative study on thermal gap filler and thermal silicone pads

Comparative study on thermal gap filler and thermal silicone pads

Summary

Efficient management of the heat generated during battery charging and discharging is one of the core factors in high energy density lithium-ion battery packs. Thermal interface material (TIM) is a material used to connect batteries or battery modules and heat sinks. It is mainly used to fill the micro-voids and uneven holes on the surface generated when the two materials are joined or contacted, so as to improve the heat dissipation performance of the device. The TIM materials often used by battery manufacturers are formed-in-place liquid dispersible thermally conductive gap fillers and pre-cured thermally conductive gaskets (also known as gap fillers), both of which have their own advantages and disadvantages. The purpose of this study was to compare the thermal resistance of Liantengda thermally conductive gap fillers and thermally conductive gaskets with comparable thermal conductivity. Data from this and future applications research will allow designers to develop more efficient and cost-effective battery packs.

Introduce

In the field of transportation, electric vehicles (EVs) are one of the main directions of future development. In order for electric vehicles to gain more share in the market, the current major trend is to expand the driving range and enhance the performance of electric vehicles, making them similar to the current performance of internal combustion engine vehicles. This forces battery pack engineers to increase battery energy density. Increased energy density means more heat is generated in a smaller space, so thermal management becomes one of the key metrics for battery pack performance and design.

Figure 1 shows the three ways a battery pack absorbs or releases heat, namely radiation, convection, and conduction. Conduction between the battery pack and the cooling plate is the most widely used method in EV battery packs. The limiting factor for conduction heat transfer is the interface between the battery module and components such as the heat sink. As shown in Figure 2, although the surfaces of these parts appear very smooth to the naked eye, they are actually rough on the microscopic scale.

Surface roughness results in that only a small fraction of the apparent surfaces are in direct contact with each other, thus containing air. To solve this problem, as shown in Fig. 2(b), TIMs material is used to connect the interface, replacing air, so that the microscopically rough interface can be better filled. Equally important, TIMs can provide good electrical insulation properties to prevent high-voltage breakdown between high-energy batteries and commonly used metal heat sinks.

Battery manufacturers typically use liquid dispensing to form one of thermally conductive gap fillers or thermally conductive gaskets in place. But the two processes are very different. The thermal gap filling material needs to be mixed with a metering mixing device first, and then glued to the surface of one substrate, pressurized on another substrate, and compressed to a set thickness. The material is then cured to form a compliant solid interface. In contrast, thermal conductive gaskets need to be cured and formed, then cut to shape, placed between two substrates, compressed to a set thickness, and secured in place. Applying a certain pressure can make the compliant thermally conductive pad come into close contact with the rough substrate surface, but at the same time, it will also have a certain influence on its thermal resistance.

Given the inherent application and physical differences between thermal gap fillers and thermally conductive gaskets, the steady-state heat transfer properties of the two materials can be compared on two solid substrates. Data from this study and future studies will allow designers to develop more efficient and cost-effective battery packs.

Heat Transfer Terms and Definitions

Before discussing the experimental methods and results of this study, a brief description of commonly used heat transfer terms and definitions for thermal interface materials is given. The ability of heat to transfer from the hot substrate to the cold plate will be governed by the thermal resistance of the thermal interface material. This thermal resistance can be defined by the following equation: R= ΔT/Q
Where: R is the thermal resistance of the thermal interface material, in °C/W; ΔT is the temperature difference between the hot plate and the cold plate, in °C; Q is the power of the heat source, in W. Note: The unit of temperature can also be expressed in Kelvin K.
More common is the definition of interface thermal resistance, which is very similar to the thermal resistance equation above, but takes into account the heat flow: θ=ΔT/Q/A, where: A is the cross-sectional area of ​​the interface in m2

The thermal resistance of a thermal interface material reflects two properties, the ability (or inoperability) of the thermal interface material to transfer heat across discrete interfaces of the substrate and the thermal conductivity of the thermal interface material itself. It can be expressed by the following formula: θ=θi+t/k

In the formula: θ is the thermal resistance of the adhesive layer of the thermal interface material; θi is the interface impedance of the top and bottom interfaces of the adhesive layer of the thermal interface material; t is the thickness of the adhesive layer of the thermal interface material; k is the thermal conductivity of the thermal interface material. In practice, the thermal resistance of a thermal interface material is determined by measuring ΔT for a given steady-state heat flux. As shown in Figure 3, thermal conductivity and interfacial impedance can be determined by measuring thermal resistance over a range of TIM bondline thicknesses. As previously mentioned, these individual parameters are particularly important in evaluating heat transfer at discrete interfaces and through the volume of the TIM itself. For example, a thermal interface material with high thermal conductivity will still have a high thermal resistance at a thinner bond thickness. This is often due to poor physical contact between the TIM material and one or both substrates, resulting in a higher interfacial thermal resistance θi. It is for these reasons that it is necessary to compare the thermal resistance performance of thermally conductive gap fillers and thermally conductive gaskets.

Use thermal resistance and thickness to measure material thermal conductivity k (reciprocal of slope), and interface thermal resistance θi (intercept of y)

Experiment

In order to compare the difference in interface thermal resistance between thermally conductive gaskets and thermally conductive gap fillers, we selected Liantengda LC300 thermally conductive silicone sheet and LCF300 two-component thermally conductive gel (gap filler) for this research experiment.
(1) First, according to the ISO 22007-2 test standard, we used the Swedish Hot Disk TPS2500S thermal conductivity analyzer to test the thermal conductivity of the two products.

( 4 ) Sweden Hot Disk TPS 2500S Thermal Conductivity Analyzer
Product ModelProduct formThermal conductivity (w/m·k)Hardness
LC300Thermal pads3.012OO-60
LCF300Thermal gel3.025OO-45
Table 1 Test results

From the test results in Table 1, the thermal conductivities of the two are very close, which is beneficial to our follow-up research, excluding the influence of different thermal conductivities on the interface thermal resistance θi.
(2) According to ASTM D5470 test standard, use Xiangtan Xiangyi Thermal Conductivity Tester DRL-Ⅲ (hereinafter referred to as Xiangtan Thermal Conductivity Tester) to conduct this thermal resistance test comparison. Because copper has a very small thermal resistance contribution to the metal-TIM-metal measurement, the thermal resistance of the copper pad (Figure 5) can be neglected in this test analysis.

( 5 ) Thermal conductivity meter DRL-III copper plate
( 6 ) Test sample

Before the analysis and test, the preparation of the thermal silicone pad sample was a circular sample with a diameter of 30mm, and the thicknesses were selected from four thicknesses of 0.5mm, 1mm, 2mm, and 3mm (Figure 6). The thermal gap filler is manually dispensed on the copper plate of the instrument, and four thicknesses of 0.5mm, 1mm, 2mm, and 3mm are prepared by changing the pressure. After being heated and cured by the instrument, it is directly tested, which simulates the actual use situation to the greatest extent. Test error is reduced.

Test Results and Discussion

Under the same pressure of 35N (50Kpa), use Xiangtan Xiangyi thermal conductivity meter DRL-Ⅲ (Fig. 7) to test the thermal resistance for comparison, and test the thermally conductive silicone pad with thickness of 0.5mm, 1mm, 2mm, 3mm and thermally conductive gap filler. , 2 groups a total of 8 times, the following (Table 2) data were obtained.

ModelLC300LC300LCF300LCF300
ThicknessConductivity
(w/mk)
Resistance
K·in2/W
Conductivity
(w/mk)
Resistance
K·in2/W
0.5 (mm)2.380.3893.040.236
1 (mm)2.720.6343.050.464
2 (mm)2.811.0993.020.882
3 (mm)3.031.4873.051.344
Table 2

First compare the thermal conductivity of the two materials, the difference between the DRL-Ⅲ and Hot disk values of Xiangtan Xiangyi Thermal Conductivity Tester. From the data, the smaller the thermal conductivity thickness of LC300, the greater the difference, and the thermal conductivity of LCF300 is close. From the comparison of different thicknesses, the smaller the thickness of LC300, the lower the thermal conductivity test value and the lower the thermal resistance; the thermal conductivity of LCF300 is not affected by the thickness, and the thermal resistance is also the smaller the thickness, the lower the thermal resistance.

Draw the following chart based on the above data

(8) LC300 Resistance and Thickness
(9) LCF300 Resistance and Thickness

According to Figure 8 and Figure 9, the following chart is obtained

ModelLC300LCF300
Measured interfacial thermal resistance – Y
intercept(K·in2/W)
0.187Approaching 0
Table 3

Why is there such a big difference in thermal conductivity test results.

In principle, the thermal conductivity calculated by Xiangtan thermal conductivity meter does not remove the contact thermal resistance. The thermal conductivity is calculated according to the thickness and thermal resistance. Under the same thickness, the thermal resistance and thermal conductivity tend to be inversely proportional, that is, the more The larger the value, the lower the thermal conductivity calculation result.

Thermal resistance = material thermal resistance + interface thermal resistance, LCF300 thermal resistance = material thermal resistance + 0, LC300 thermal resistance = material thermal resistance + 0.187, so the inaccurate thermal conductivity value of LC300 Xiangtan test is due to high interface thermal resistance.

The Xiangtan thermal conductivity meter calculates the interface thermal resistance and the thermal resistance of the thermal conductive material as a whole. This calculation method is more inclined to the actual application. The thermal contact resistance of the thermal pad is much higher than that of the thermal gap filler, so the actual thermal calculation results will vary greatly. Then as the thickness increases, the thermal resistance of the thermal pad itself becomes larger and larger, the proportion of the interface thermal resistance θi to the total thermal resistance θ becomes smaller and smaller, and the measured thermal conductivity value is getting closer and closer to the actual value. . Figure 10 illustrates the contact conditions between the thermally conductive material and the surface in two different states.

( 10 ) Schematic diagram of the tightness of surface contact (a) solid thermal pad (b) liquid thermal gap filler (thermal gel)

In conclusion

Steady-state thermal analysis shows that the two-part thermally conductive gap filler provides lower thermal resistance than thermally conductive materials of similar thermal conductivity and thickness. The main reason is that the liquid thermal conductive gap filler material is easy to flow to the small gaps on the rough surface, and the contact with the adjacent interface is better, which reduces the thermal resistance of the interface, so that the thermal conductivity will be better.
Table 4 summarizes the advantages and disadvantages of thermally conductive silicone sheets and liquid thermally conductive gap filler materials in detail. In addition to the different thermal conductivity, the production process is also different. The thermal pad needs to be cut to the actual required shape or size, and the residual scrap cannot be used, resulting in waste. Thermal pads need manual operation, and products with too soft hardness will be deformed when taken out. These situations can be avoided with liquid thermally conductive gap fillers, and liquid thermally conductive gap fillers can be used automatically.
Good thermal conductivity, easy to use, low surface stress, low material waste, liquid thermal conductive gap filler should be the first choice for TIM.

CompareThermal padsThermal gelParticulars
Relative costHighLowDue to the need to cut the thermal pad, the residual part is wasted
Heat dissipationGoodBetterLiquid thermally conductive gap fillers can flow into rough surfaces and gaps in thermal management systems with low interfacial thermal resistance. The molecular chain of the cured thermal conductive gap filling material is in a network structure, which can improve or even eliminate interfacial precipitation and TIM removal.
BubbleOftenFew
Design flexibilityFixedFlexibleThe hardness and working time of the liquid thermal conductive gap filler can be adjusted by changing the mixing ratio. The dispensing position and method of liquid thermal conductive gap filling material can be flexible to fixed-point dispensing. Unlike liquid thermally conductive gap fillers, the thermal conductivity of thermally conductive gaskets is sensitive to localized pressure differences due to high surface unevenness.
RepairYYBoth thermally conductive materials can be repaired, but compared with the thermally conductive gap filler, the thermally conductive gasket has a poorer contact with the interface and is easier to repair.
Production application processHardEasyIt is easy to generate air bubbles when the thermal pad is applied to a large area. Automated production with thermal pads is difficult. Easily automated production with liquid thermally conductive gap fillers.

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