A Method for Thermal Performance Characterization of Ultrathin Vapor Chambers Cooled by Natural Convection
Summary (2 min read)
Introduction
- A heat pipe or vapor chamber can passively transport heat from a localized generation source to a diffuse heat rejection surface at a low temperature gradient.
- The sealed vapor chamber contains a working fluid, and vapor is generated at the evaporator section located over the hot spot.
- Heat was rejected on the condenser side using a finned heat sink cooled by forced air convection.
- The thermal performance of the vapor chamber was assessed based on its thermal resistance and condenser-side temperature uniformity [5].
- This product sector necessitates a paradigm shift in thermal management, where the external surface temperature threshold is dictated by user considerations, rather than by device operating temperature limits.
Experimental Facility
- An experimental facility is developed to evaluate the performance of ultrathin vapor chambers at low heat loads.
- The intrinsic challenge in vapor chamber characterization under such conditions is estimation of the percentage of heat input rejected through the vapor chamber versus parasitic losses through other pathways.
- From the recorded images, a pixel-by-pixel calibration of the surface temperature versus sensor output was performed.
- Finally, a thermocouple is inserted at the center of the copper heater block to measure the junction temperature.
- Active data processing is performed in a LABVIEW interface to determine when steady-state conditions have been reached, defined as when the standard deviation of the junction temperature for the last 150 data points is less than 0.02 K.
Calibration of the Test-Section Heat Loss
- A calibration procedure is implemented that predicts heat loss from the test section.
- The test section temperatures were recorded for heat loads in the nominal range of 0.15–4 W. Key characteristics of the metal heat spreaders used for the calibration process are listed in Table 1.
- The lateral cell lengths increase in the outward direction from 0.25 mm to 2.25 mm.
- With a sufficient match to the experimental data, the heat transported through the heat spreaders and the heat loss through the insulation block can be easily extracted from the numerical data.
- By evaluating the thermal resistance of both the copper and aluminum heat spreaders, as shown in Fig. 6(b), the influence of the junction-to-ambient temperature on the overall heat loss can be incorporated into the regression.
Results and Performance Metrics
- The copper vapor chamber has 0.2-mm-thick copper walls, uses water as the working fluid, and is lined with a single layer of copper mesh (pore sizes of approximately 50–100 lm).
- The data obtained from the tests were used to assess the behavior of the vapor chamber relative to the solid copper heat spreader of the same dimensions.
- The large thermal resistance contributed by the condenser-side natural convection (in addition to the comparatively smaller thermal resistances of the copper block and conductive epoxy layer) should be omitted from the device thermal resistance assessment for the current configuration, since its inclusion would mask any variations in performance of the actual device under test.
- 1=RMS Ts Ts;m Q VC 1=RMS Ts Ts;m Q Cu (4) For an ideal heat spreader, the temperature profile would be a uniform temperature on the condenser surface at Ts,m if the convective boundary condition on the condenser is uniform.
Conclusions
- A novel approach was developed for characterization of vapor chambers of ultrathin form factor.
- Given their intended application in portable electronics platforms, the experimental facilities are designed to evaluate performance at low heat input powers with heat rejection to the ambient by natural convection.
- The condenser surface temperature distribution was monitored because of ergonomics implications that govern the thermal management requirements for these applications.
- The high thermal resistance due to natural convection in the heat dissipation pathway necessitates careful calibration of the parasitic heat losses from the system.
- The testing methodology developed is an important tool for the development of vapor chambers and heat spreaders intended for use in portable electronics platforms.
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Cites methods from "A Method for Thermal Performance Ch..."
...Numerical investigations have been conducted on the design of TGPs in ultrathin scale with a thickness of submillimeter[6][7][8]....
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"A Method for Thermal Performance Ch..." refers background in this paper
...Visualization of the surface temperature via an IR camera allows for the development of performance metrics based on the surface temperature distribution....
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510 citations
"A Method for Thermal Performance Ch..." refers background in this paper
...A test facility is developed to experimentally characterize performance and analyze the behavior of ultrathin vapor chambers that must reject heat to the ambient via natural convection....
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251 citations
"A Method for Thermal Performance Ch..." refers background in this paper
...An experimental facility is developed to evaluate the performance of ultrathin vapor chambers at low heat loads....
[...]
213 citations
"A Method for Thermal Performance Ch..." refers methods in this paper
...A numerical model of the test section is generated to simulate conduction in the heater block assembly, insulation block, and heat spreader....
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...A sample vapor chamber is tested for heat inputs below 2.5 W. Performance metrics are developed to characterize heat spreader performance in terms of the effective thermal resistance and the condenser-side temperature uniformity....
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119 citations
"A Method for Thermal Performance Ch..." refers background in this paper
...The test section is comprised of the heat spreader sample, with insulation and a centered heater block on the underside; the top side of the heat spreader is exposed to ambient air....
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