Investigation on conduction heat dissipation for thermal management in distributed multi-point electronic chips

A conduction thermal dissipation system is designed for the thermal management of distributed multi-point electronic chips. The heat from the chips is conducted to the flow channel on the two sides with the help of heat pipes. For the purpose of improving the heat dissipation performance, the influence of different flow channel structures was analyzed with the help of numerical simulations. When the fin thickness and spacing are 0.4/0.5 mm, the heat transfer performance is optimal with acceptable flow pressure drop. The results indicated that temperature and uniformity meet the requirements of the electronic chips. The temperature of the chips is lower than 80°C at 1500 W under the condition of the 35°C ethylene glycol solution with 200 L/h. The impact of thermal interface materials on thermal performance was also experimentally studied. The results showed that the graphite interface material has an excellent comprehensive performance with good thermal conductivity and convenient application. This method of distributed multi-point chips and two-sided heat dissipation is more effective in fully utilizing the advantage of heat pipes compared to the traditional usage of evaporation and condensation located on the two sides.


Introduction
At present, large-scale integrated circuits are developing rapidly, and the number of transistors per unit area is constantly increasing.At the same time, the performance requirements of electronic devices continue to increase, resulting in a sharp rise in device heat generation.Removing growing heat from a reduced volume is a huge challenge.The conventional heat dissipation methods of electronic devices mainly include natural air cooling [1], forced convection [2], single-phase cooling [3][4], and phase change cooling [5][6].Natural air cooling does not require external power and only relies on the temperature difference to drive airflow.This method has the characteristics of high reliability and low cost but poor performance.Forced convection is driven by the fan, which results in a large flow velocity and improved heat dissipation performance.However, due to the use of fans, there is a lot of noise.The heat transfer performance of liquid cooling is significantly enhanced for the large sensible heat capacity.At the same time, the leakage of liquid causes serious harm to the stable operation of electronic equipment.
In view of the risk of liquid leakage in liquid cooling, a dry conduction cooling technology was developed [7][8], as illustrated in Figure 1.The dry conduction cooling consists of heat pipes, a conduction plate, and a flow channel.The power is conducted from the device to the flow channel through heat pipes.The working liquid and the device are in indirect contact through the conduction plate.It will not directly affect the device once the liquid leakage occurs, which achieves efficient heat transfer and reliable operation.Kim et al. [9] dissipated the CPU heat using heat pipes.The chip and fins are located on the two sides of the heat pipe.Compared to the conventional fin heat sink, the fan speed is smaller, and the noise is reduced under the same heat dissipation conditions.Behi et al. [10] investigated the influence of PCM-assisted heat pipes on the heat transfer performance, in which the heater and cooling device are located at both ends of the heat pipe.The middle adiabatic section is covered by PCM.The test results show that PCM contributes 86.7% to cooling.Mahdavi et al. [11] designed a screen mesh heat pipe.The impacts of filling ratio, inclination angle, and heating power were studied and compared with the simulation results.Therefore, most heat pipes are used for the singleheat source, which can only be conducted from one side to another side.However, the degree of miniaturization and the number of chips in the same space is increasing, which leads to a significant increase in heating power.The conventional usage of heat pipes is unable to meet the heat dissipation need.In this investigation, dry conduction cooling equipment is designed for the multi-point electronic chips.The heat is conducted to the flow channel, which achieves the temperature uniformity needs of the chips.At the same time, the interfacial and convective temperature rise of different thermal interface materials and channel structures were analyzed, respectively.The optimal interface material and channel structure were also selected for the cooling equipment.

Analysis of the cooling requirements
The chips are in a multi-point array layout form.There are 128 chips installed in a 340 mm × 275 mm area.The total heat dissipation is 1500 W. The single-point heat flux is 13 W/cm 2 .The case temperature of the chip should not exceed 100°C.The uniformity is within the range of ±5°C.The cooling power for the electronic chips exceeds the capacity of air-cooling.The liquid cooling may cause leakage issues.It is complicated to design the flow channel for temperature uniformity.Therefore, the dry conduction cooling system is accepted.

The design of the conduction plate
The structure of the conduction plate is shown in Figure 2(a), which is welded from a cover plate, several heat pipes, and a base plate.The bulges in contact with chips are machined on the cover plate.Heat pipes are placed on grooves machined on one side of the base plate.The two ends on the other side are the heat-dissipating area in contact with the flow channel.The heat pipe conduction plate is 340×27×7.5mm 3 .There are 128 heat dissipation bulges with a cross area of 15 mm ×15 mm arranged in 8 columns and 16 rows on the heat pipe conduction plate, which correspond to the position of the heat sources.Several heat pipes with a cross area of 7.1 mm × 4.5 mm flattened from a circular heat pipe with a diameter of 6 mm are sandwiched inside the conduction plate using vacuum welding.

The design of the liquid cold plate
A liquid cold plate is shown in Figure 2(b), which is welded from two top plates and a bottom plate.Flow channels are machined at the two sides to dissipate heat from the conduction plate.The width of the flow area on each side is 21 mm, which is designed as a three-fold serpentine channel.For the improvement of the heat transfer performance, fins are processed inside the channel.The optimization methods of channel structure mainly include increasing the channel width and height and reducing the fin thickness and spacing.In order to study different parameters on the performance, the commercial simulation software FIoEFD is used for analysis.The dimensions of the flow channel structure are listed in Table 1.The geometric model is established according to the dry conduction heat dissipation system.The equivalent thermal conductivity of the heat pipe is set to 13000 W/(m K).The contact thermal resistance is set to 0.5 and 3 ℃•cm 2 /W for the cold plate and bulges.The boundary conditions are consistent with the experiment with a 1500 W heating power and 200 L/h flow rate of 35 ℃ temperature.The effect of channel width on heat dissipation performance was shown in Figure 3(a).It can be seen that when the height of the channel is 9 mm, the channel wall temperature decreases with the increase of channel width.However, the improvement is limited for the channel width further increased to 35 mm.Besides, the flow pressure drop increases when the channel width increases from 21 mm to 25 mm.That is because the average velocity between fins is almost unchanged.The fin spacing decreases from 1.4 mm to 1.1 mm, resulting in an increase in flow resistance.When the width of the channel further increases, the average flow velocity decreases significantly, and the pressure drop decreases.The performance at different channel height is shown in Figure 3(b).The temperature of the channel wall is reduced within 1 ℃, which means the increase of fin height has basically no effect.Due to the thermal resistance, the temperature difference between the fin and fluid along the height direction decreases, as shown in Figure 4.With the fin height increases to a certain degree, the temperature difference is almost zero.The fin efficiency is constantly reducing, and the performance is difficult to further enhance.For the flow pressure drop, the increase in fin height results in the expansion of the flow section area and reduce in flow velocity.The flow pressure drop reduced by about 50% when height increase.The impact of spacing and thickness on temperature and pressure drop is shown in Figure 3(c).The reduction of fin thickness and spacing significantly improves the channel performance.For the 21 mm channel width, the channel wall temperature is reduced by about 7℃ when the fin thickness reduces to 0.4 mm.For 25 mm channel width, the wall temperature is only reduced by about 1℃ when the fin spacing is further reduced from 0.5 mm to 0.3 mm while the flow pressure drop is significantly increased to 2 bars.Therefore, when optimizing the flow channel structure, the flow resistance should be in an appropriate range.
Through the simulation analysis of the channel parameters, increasing fin height has a limited effect on the performance.The efficiency reduces with the increase of the fin aspect ratio.Besides, due to the limitation of the size of the electronic equipment, it is difficult to further increase the channel width.Therefore, channel width and height remain unchanged.As for the optimization of the fin parameters inside the flow channel, the spacing and thickness are taken as 0.5/0.4mm considering the machinability and flow resistance, which are arranged intermittently to reduce flow resistance.The fins were manufactured by a shoveling process, which is shown in Figure 5.

Experimental apparatus
The performance testing apparatus of the heat pipe conduction plate is presented in Figure 6(a), which is composed of a heat dissipation component, a liquid cooling source, a DC power supply, thermocouples, and a temperature acquisition system.The heat dissipation component consists of a heat pipe conduction plate, a liquid cold plate, and a substrate equipped with heating resistances.The conduction plate is connected to the liquid cold plate by six screws.Different thermal interface materials are pasted on connected surfaces to reduce the temperature difference.The heating resistances are bonded to the substrate by silicone adhesive to simulate the electronic chips, as shown in Figure 6(b).The substrate is mounted through screws on the conduction plate.The heating side is in contact with the bulges on the heat pipe conduction plate.The contact surfaces are also pasted with heat-conducting gaskets to reduce temperature difference.A DC power supply is used to heat the heating resistances.The working liquid, 65# ethylene glycol, flowing through the liquid cold plate is provided by a liquid cooling source.48 thermocouples are attached to the bulge root to measure the temperature distribution of the conduction plate, as illustrated in Figure 7.A temperature acquisition system is used to accord the temperature.Four different thermal interface materials were used, as shown in Figure 8, including thermal grease, graphite, graphene, and thermal sheet.Thermal grease is the TC-5026 compound produced by Dow Corning with a thermal conductivity of 2.9 W/(m• K).Owing to the good fluidity, the gap between the contact surfaces can be fully filled, which results in an extremely low thermal resistance.Graphite is the TIM graphite film with a thickness of 0.2 mm produced by Jones Tech company, which is fabricated by pyrolysis and annealing of hydrocarbons.The thermal conductivity is 800~1300 and 10~15 W/(m• K) in-plane and vertical.Graphene is a Soft-Gray Gold Thermal-Interface Material prepared by Gaoxi Technology with a thickness of 0.2 mm and 0.5 mm, which only consists of carbon elements.This product has strong tensile resistance and is not easy to break.The thermal conductivity is 200~600 and 38W/(m• K) in-plane and vertical.The thermal sheet is the DT-T with a thickness of 0.5 mm fabricated by POLYMATECH, which is a silicone rubber product with high thermal conductivity (20 W/(m• K)) and excellent flexibility.

Results and discussion
Based on the heat transfer path in dry conduction, the temperature rise mainly includes the convection temperature rise between the working liquid and cold plate, the interface temperature rises between the cold plate and conduction plate, and the conduction temperature rise of the conduction plate itself.Since the heat pipes embedded in the conduction plate have efficient thermal conductivity through the phase change of the working liquid inside, the conduction temperature rise is very small.As a consequence, the temperature rises of the interface and convection have a more remarkable impact on the heat dissipation performance.The convection temperature rise is related to fin parameters inside flow channels.The optimized fin thickness, spacing, and height were selected as 0.4 mm, 0.5 mm, and 9 mm, considering machinability and flow resistance according to the simulation results.A detailed discussion can be found in the previous section.The analysis of temperature rises of different interface materials is carried out as follows.

Experimental results
Under the condition of 1500 W heat power, 35℃ liquid temperature, and 200 L/h flow rate, the bulge root temperature ranges from 49.82℃ to 59.47℃, which basically meets the temperature uniformity requirements of ±5℃.According to our previous experience, the temperature rise from the bulges to the heating source is about 20℃.Therefore, the highest case temperature of the heating resistance is about 79.47℃, which meets the temperature requirements.
The temperature distribution on the heat pipe conduction plate is shown in Figure 9, which presents the characteristics of symmetry.The average temperature difference between the middle and two sides of the same column is about 2.93℃.The heat pipe equivalent thermal conductivity is calculated to be about 13000 W/(m• K).

The influence of thermal interface materials
Thermal interface material between the conduction plate and cold plate should meet the requirements of high thermal conductivity and large contact area.Several kinds of interface materials with different thicknesses, such as thermal conductive sheets, graphite, graphene, and thermal grease, were selected to test the thermal conductivity under the same conditions.The highest temperature on the base of the bulges with different interface materials is shown in Figure 10.The results showed that the temperature using the DT-T sheet was about 10℃ higher than that of other interfacial materials, which means that the thermally conductive sheet has the highest interfacial thermal resistance.The interface thermal resistance of the grease is minimal.However, coating and cleaning operation is difficult during the assembly and disassembly.The temperature using the graphite was only about 4°C higher than that of grease, and it has great advantages in convenient operation.As for the graphite, the temperature was 2℃ lower than that of graphene while the price doubled.Therefore, graphite has the best performance considering different factors.

Conclusion 1)
The dry conduction cooling is designed to meet the temperature requirement of multi-point electronic chips, which are uniformly arranged along the heat pipes.Heat is conducted toward two sides of the conduction plate through the heat pipes with high thermal conductivity.
2) The fin structure inside the flow channel is numerically optimized to reduce the conduction plate temperature.The fin thickness, spacing, and height are taken as 0.4 mm, 0.5 mm, and 9 mm, considering machinability and flow resistance.
3) The testing data demonstrated that case temperature and uniformity meet the requirements of electronic chips.The heat pipe equivalent thermal conductivity is approximately 13000 W/(m• K).The thermal performance of the heat pipe conduction plate was tested under different heat dissipation powers.
4) Graphite was selected as the thermal interface material inside the heat pipe conduction plate and the liquid cold plate, considering the thermal conductivity and installation portability.

Figure 1 .
Figure 1.Schematic diagram of the dry conduction cooling.

Figure 4 .
Figure 4.The temperature difference along the fin height.

Figure 5 .
Figure 5.The fin structure inside the liquid cold plate.

Figure 6 .
Figure 6.(a) The experimental apparatus and (b) the heat dissipation component.

Figure 7 .
Figure 7.The location of thermocouples.

Figure 9 .
Figure 9.The temperature distribution of the bulges.

Figure 10 .
Figure 10.The highest temperature with different thermal interface materials.

Table 1 .
The size of the channel parameters.