Effect of inclination angle on the performance of a kind of vapor chamber
© Ma et al.; licensee Springer; licensee Springer. 2014
Received: 3 February 2014
Accepted: 23 June 2014
Published: 6 September 2014
Vapor chamber is an effective heat spreader for decreasing the hotspot phenomena in LED, and there are many parameters influencing its performance. In this paper, a vapor chamber was experimentally investigated. Both the thermal resistance and the temperature uniformity of the condensing surface under different inclination angles were tested. As the inclination angle changed from 0° to 50° with 10° as an interval, the maximum relative change rate of thermal resistance and temperature uniformity were 9.1% and 4.5%, respectively. And the relative standard deviations of them were 5.5% and 3.7%.
As electronic products become faster and incorporate more functions, they are simultaneously shrinking in size and weight. This results in a significant increase in heat generation. For such a case, when very small size heat sources exist, usually there was a large spreading thermal resistance from heat source to substrate or metal heat sink. It is necessary to find some substrates or heat sinks which can effectively decrease hotspot phenomena. The vapor chamber is such an effective solution to decrease the spreading thermal resistance because of its characteristics such as good temperature uniformity.
Many investigations were conducted to study the performance of the vapor chamber. Go  evaluated the thermal performance of a vapor chamber heat sink containing new microwick structures and found that it had a heat removal capacity of 80 (W/cm2) at the junction temperature of 85(°C) and an ambient temperature of 24(°C). Koito et al. described the effect of heat source size on the thermal resistance of the vapor chamber in their experiments and simulations. They found although the thermal resistance is hardly affected by the heat generation rate and the heat flux of the heat source, it increases as the heat source become smaller. Wu et al. made a fundamental thermal analysis of the vapor chamber, and put forward the factors that would impact the performance of heat sink. Zhang et al. designed a novel grooved vapor chamber. The influence of heat flux, filling amount and gravity on the thermal performance was investigated by experiments. Huang et al. experimentally investigated the thermal performance of the vapor chamber module applied to high power light-emitting diodes in natural convection. The results showed that the spreading resistance and the corresponding temperature difference of the flat-plate vapor chamber at 30 W were lower than those of the copper plate by 34% and 4(°C) respectively and the flat-plate vapor chamber could effectively lower the spreading resistance and diminish the hotspot effect. Wong et al. experimentally investigated the performance of a novel vapor chamber with different working fluids of water, methanol and acetone. And they found water presents the smallest vapor chamber resistance while acetone has the largest.
The present authors have already done some researches on a vapor chamber which was applied to the thermal management of high power LED light source . Experimental tests, numerical simulation were conducted and the results showed that the vapor chamber coupled fin heat sink could make the temperature of the fin base plate uniform and the thermal resistances of vapor chamber was 0.654 (K/W) at the ambient temperature of 27(°C). However, the vapor chamber in that study was fabricated by conventional machining methods, thus the vapor chamber was large and heavy. Then we continued this part of work. The vapor chamber was fabricated by copper powder sintering and diffusion bonding processes. Its size sharply decreased. These samples will be used for the experimental investigation in this paper.
Based on the literature review on the existing vapor chamber work, it is found that most of the papers do not investigate the temperature uniformity under inclination condition, actually, this is very important in many application cases. For example, when we use vapor chamber to solve the hotspot phenomena and decrease the spreading thermal resistance in lighting emitting diodes packaging or other high heat flux chip packaging, the application situations often require the vapor chamber operation in inclination condition. In this case, if the temperature uniformity greatly changes or loses, thermal resistance is different with that in normal horizontal case, the cooling performance will sharply decrease, some of the chips will fail because of high temperature gradient.
In this paper, we will test the temperature characteristics of our small size vapor chamber under inclination conditions. The experimental setup was built, and the experimental results proved that our new version vapor chamber could achieve good thermal performance even under great inclination conditions.
2.1 Vapor chamber and its principle
Size specifications of the vapor chamber
70 × 80 × 6 mm3
60 × 60 × 4 mm3
Top and bottom plate thickness
Wick sheet thickness
Wick (support) column radius
Wick (support) column height
4.5 mm (based on bottom surface)
2.2 Experimental setup
In the experiments, the heat flux and the temperature distribution of vapor chamber were needed for evaluation. In order to obtain the heat flux of the vapor chamber, four thermocouples were inserted in the four holes of the copper rod with a depth of 0.5 cm, achieving the centre, and a same distance from each other. As shown in Figure 4(a). Thus, four temperatures in the central axis of copper rod could be obtained.
The temperature distribution of both bottom surface and top surface of the vapor chamber was also tested. Figures 4(b) and (c) show the test points on them. When all the temperatures were obtained, the thermal resistance including spreading thermal resistance and bulk thermal resistance could be calculated. Also, the temperature uniformity of condensing surface could be evaluated. The concrete principle was as follows.
After all the thermal resistances and temperature distribution on condensing surface under different inclination angles were obtained, the effect of inclination angle on the performance of the vapor chamber could be evaluated effectively.
2.3 Evaluation method
2.4 Accuracy analysis
Based on the thermal resistance equation, the heat flux and temperature differences influence its calculation accuracy. In other words, the measurement accuracy of the heat flux and temperature is the key.
According to Figure 5, because of good thermal insulation, the accuracy of the linear fitting is about 99.72%. This shows that the heat flux measurement has very high accuracy. In addition, according to the present evaluation method, we compared the thermal resistance and the temperature under different inclination angles, in all tests, the heating and thermal insulation are exactly the same. The variations of the heat fluxes under different inclination cases nearly do not exist. Therefore, the measurement error of the heat flux nearly can be ignored for making conclusions in this paper.
In respect to the measurement accuracy of temperature, the errors mainly lay in the measurement error of the thermocouples and the reading error of the digital data acquisition system. When the temperature was in the range from −30 to 150 , the measurement error of T-type thermocouple was about 0.2. The data acquisition system had a reading error of 1 since the cold junctions of the thermocouples used the default setup supplied by the system, and not the ice bath with constant 0. Therefore, the total error of the temperature measurement was about 1.2°C. According to the work of Huang et al. and Wang et al., which have almost the same test points, the maximum error for the thermal resistance is about ±5%.
3 Results and discussion
Thermal resistance and temperature uniformity under different inclination angles
Inclination angle (o)
R VC (K/W)
SDof top surface temperature (°C)
In this paper, experiments were conducted to analyze the effect of inclination angle on the performance of a kind of vapor chamber. The thermal resistance and temperature uniformity under different inclination angles were evaluated. The test results showed that the temperatures at all test points for various angles were very close, the largest difference was less than 2. It demonstrated that the present vapor chamber has ability to effectively decrease hotspot phenomena in high heat flux electronic packaging.
The authors would like to express their great thanks to the support from 973 Project of The Ministry of Science and Technology of China (2011CB013105) and in part by National Science Foundation of China (51376070).
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