Phase-change immersion cooling high power light emitting diodes and heat transfer improvement

A new cooling method of ethanol direct-contact phase-change immersion cooling was proposed in the thermal management of high power light emitting diodes (LED) and the feasibility of this cooling method was investigated. The heat generated by LED was measured firstly using two types of power systems: DC power and LED driver. Then the heat dissipation performance was evaluated under different experimental conditions. The results indicate that startup process of the cooling system is quick and only 450 s is needed to reach steady-state under heat load of 42.78 W. The minimum thermal resistance of 1.233 °C/W is obtained when liquid filling ratio is 33.14%. The junction temperature of LED under different absolute pressures is much lower than the limited value of 120 °C. Baffle with total height of 140 mm, bottom space height of 20 mm and distance away from substrate surface of LED of 8 mm improves heat transfer performance best due to ethanol self-circulating in the cooling receiver. Overall, the ethanol phase-change immersion cooling is an effective way to make sure high power LED work reliably and high efficiently.     

Study on thermal performance of high power LED employing aluminum filled epoxy composite as thermal interface material

This paper elucidates the thermal behavior of an LED employing metal filled polymer matrix as thermal interface material (TIM) for an enhanced heat dissipation characteristic. Highly thermal conductive aluminum (Al) particles were incorporated in bisphenol A diglycidylether (DGEBA) epoxy matrix to study the effect of filler to polymer ratio on the thermal performance of high power LEDs. The curing behavior of DGEBA was studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The dispersion nature of the Al fillers in polymer matrix was verified with Field Emission Scanning Electron Microscope (FESEM). The thermal performance of synthesized Al filled polymer composite as TIM was tested with an LED employing thermal transient measurement technique. Comparing the filler to polymer ratio, the rise in junction temperature for 60 wt% Al filled composite was higher by 11.1 °C than 50 wt% Al filled composite at cured state. Observed also from the structure function analysis that the total thermal resistance was 10.96 K/W higher for 60 wt% Al filled composite compared to 50 wt% Al filled composite. On the other hand, a significant rise of 9.5 °C in the junction temperature between cured and uncured samples of 50 wt% Al filled polymer TIM was observed and hence the importance of curing process of metal filled polymer composite for effective heat dissipation is discussed extensively in this work.     

Thermal characterization of high power LED with ceramic particles filled thermal paste for effective heat dissipation

The next generation packaging materials are expected to possess high heat dissipation capability. Understanding the needs for betterment in the field of thermal management, the present study aims at investigating the package level analysis on a high power LED. In this study, commercially available thermal paste was heavily filled with ceramic particles of aluminium nitride (AlN) and boron nitride (BN) in order to enhance the heat dissipation of the device. Different particle sizes of AlN and BN fillers were incorporated homogenously into the thermal paste and applied as a thermal interface material (TIM) for an effective system level analysis employing thermal transient measurement. It was found that AlN TIM achieve less LED junction temperature by a difference of 2.20 °C compared to BN filled TIM. Furthermore, among D₅₀ = 1170 nm, 813 nm and 758 nm, the AlN at D₅₀ = 1170 nm was found to exhibit the lowest junction temperature of 38.49 °C and the lowest total thermal resistance of 11.33 K/W compared to the other two fillers.     

Thermal characterization of multicolor LED luminaire

The LED based dynamic lighting scheme, require compact and thermally efficient luminaire. This paper presents the thermal investigation on the conceptual design of 36 W multicolor light emitting diode (LED) luminaire. The developed prototype design includes configuration and placement of the multichip LED package, RGBW and single die amber LEDs in a 5 × 3 array on the heat sink. LED configurations with low power input are placed between the LEDs having the high power input. The proposed configuration and placement of LEDs reduces the local heat concentration in the centre region of the heat sink. The temperature of 72 °C at LED chip base plate is reduced to 32.1 °C on the heat sink surface. The numerical results are experimentally validated. The proposed method contributes to a reduction in the size of the luminaire and also enhancement of heat dissipation for improving the longevity of the multicolor based LED luminaire.     

Experimental and numerical investigation of circular minichannel heat sinks with various hydraulic diameter for electronic cooling application

This article presents the experimental thermal and hydraulic performances of heat sinks with various channel diameter for cooling electronic components. A heat sink with the length and width of 60 mm and total height of 16 mm fabricated from aluminum material. The heat sink is designed with four circular minichannels and three different values of hydraulic diameter of channel (D = 4 mm, D = 6 mm and D = 8 mm). The minichannel heat sink is heated with a uniform base heat flux. Also, numerical simulation of the problem is performed using Finite Volume Method (FVM). Comparing the experimental and numerical results show that numerical results are in a good agreement with experimental data. The variation of channel diameter affects the heat transfer and pressure drop characteristics of the circular shaped minichannel heat sink. The experimental results show that the increase of channel diameter reduces the pressure drop in the heat sink. Also, the minichannel heat sink with a hydraulic diameter of 4 mm has a much lower thermal resistance than the minichannel heat sinks with a hydraulic diameter of 6 mm and 8 mm. Furthermore, the optimization is done to have the maximum heat transfer coefficient and minimum of pressure drop along the heat sink.     

U- and L-shaped heat pipes heat sinks for cooling electronic components employed a least square smoothing method

Heat pipes-heat sink modules transfer heat from a heat source to the heat pipes, and then to the heat sink and out into the surrounding ambient, and are suitable for cooling electronic components through a forced convection mechanism. The configuration and thermal performance of the heat sinks with inserted heat pipes were studied in the present paper. This article uses experimental procedures to investigate the thermal performance of two embedded U-shaped heat pipe and six embedded L-shaped heat pipe thermal modules with different fan speeds and heat source areas. And via the superposition method and least-square estimators in experimental data, the performance curves of individual U- and L-shaped heat pipes were derived and predicted. Results show that the lowest thermal resistances of U- and L-shaped heat pipe-heat sinks are respectively 0.246 °C/W and 0.166 °C/W given dual fans operating at 3000RPM and 30 × 30 mm² heat sources. Results for a single U-shaped heat pipe are 0.04 °C/W at 78.85 W, while sequential results for L-shaped heat pipes are 1.04 °C/W, 2.07 °C/W, 2.76 °C/W, 2.19 °C/W and 1.7 °C/W between 34 W and 40 W.     

Thermal characteristics and fabrication of silicon sub-mount based LED package

In this paper, the cost of a light emitting diode (LED) package is lowered by using a silicon substrate as the base attached to the chip, in contrast to the conventional chip-on-board (COB) package. In addition we proposed an LED package with a new structure to promote reliability and lifespan by maximizing heat dissipation from the chip. We designed an LED package combining the advantages of COB based on conventional metal printed circuit board (PCB) and the merits of a silicon sub-mount as a substrate. When an input current 500–1000 mA was applied, the fabricated LED exhibited the light output of approximately 112 lm/W at 29 W. We also measured and compared the thermal resistance of the sub-mount package and conventional COB package. The measured thermal resistance of the sub-mount package with a reflective film of Ag and the COB package were 0.625 K/W and 1.352 K/W, respectively.     

Connecting plugs of high-powered GaN-based lighting-emitting diodes prepared by electroplating

We investigate the fabrication and the characteristics of a gallium nitride-based light-emitting diode (GaN-based LED) with a connecting plug. The connecting plug was prepared by electroplating, connecting the front and back side of the GaN-based LED via a through hole formed by a laser driller to improve the heat dissipation and the yield loss that was caused by the disconnection between the front and the back sides of the GaN-based LEDs because of the edge coverage effect. The junction temperature of the GaN-based LEDs with the connecting plug increased from 19 to 54 °C when the injection current was increased from 100 to 500 mA.     

Failure modes and effects analysis for high-power GaN-based light-emitting diodes package technology

In this study, nondestructive test is developed to analyze the structure failure of LED package. The relationship between thermal resistance analysis and LED package failure structure is build with T3Ster thermal transient tester and scanning electron microscope (SEM). The failure LED device with defect in the attaching layer and gap between LED chip and copper are designed advisedly. The failure factors of LED package have been measured with thermal resistance analysis and SEM cross-section images. The thermal dissipation performance of LED with defect in the attaching layer is indicated by thermal resistance analysis combined with SEM cross-section images. The blister in attaching layer results in 4.4 K/W additional thermal resistance. The gap between LED chip and copper also makes high additional thermal resistance with 8.6 K/W. Different failures of LED packages are indicated obviously using thermal transfer analysis. The LED package failure structure such as interface defect between solder and cup-shaped copper is able to forecast without destructive measurement.     

Thermal optimization of GaN-on-Si HEMTs with plastic package

In this paper, the degradation of a GaN-on-Si based RF power amplifier is investigated by means of electrical characterization. The reliability issues identified during this work are clearly related to the high thermal resistance between the device and the heat sink, which causes gate-leakage current and output power degradation. Moreover, we have demonstrated a low cost thermal optimization approach by increasing the thermal dissipation area and reducing the device carrier thickness. Measurement results show that the saturated output power can be increased from 1 W up to 5 W without device degradation at 3.8 GHz.     

Heat sink performances of GaN/InGaN flip-chip light-emitting diodes fabricated on silicon and AlN submounts

Fabricating flip-chip light emitting diodes (FCLEDs) with two good thermal conductivity materials of silicon and aluminum nitride (AlN) as submount are investigated on its output power and heat sink capacity. It is known that many advantages exist in FCLED structures. In addition to the upward emitting light, the downward propagating light is reflected up by a high reflectance contact, increasing the light extraction. The heat generated in the LED flows directly through the interconnect metal of the submount, improving thermal conduction. Except blue shift at the low current injection region (0–0.3 A), the heat induced bang gap narrowing (red shift) at high current injection region (0.3–0.7 A) is observed with a red shift of 8.92 nm for conventional LED, 4.62 nm for silicon submount FCLED, and only 2.87 nm for AlN submount FCLED. The light intensity of FCLEDs with silicon and AlN submounts exhibits 1.6 and 7 times at an injection current of 0.35 and 0.7 A, respectively, larger than that of conventional LED.     

Thermal/luminescence characterization and degradation mechanism analysis on phosphor-converted white LED chip scale packages

According to the requirements on minimizing the package size, guaranteeing the performance uniformity and improving the manufacturing efficiency in LEDs, a Chip Scale Packaging (CSP) technology has been developed to produce white LED chips by impressing a thin phosphor film on LED blue chips. In this paper, we prepared two types of phosphor-converted white LED CSPs with high color rendering index (CRI > 80, CCT ~ 3000 K and 5000 K) by using two mixed multicolor phosphor materials. Then, a series of testing and simulations were conducted to characterize both short- and long-term performance of prepared samples. A thermal analysis through both IR thermometry and electrical measurements and thermal simulation were conducted first to evaluate chip-on-board heat dissipation performance. Next, the luminescence mechanism of multicolor phosphor mixtures was studied with the spectral power distribution (SPD) simulation and near-field optical measurement. Finally, the extracted features of SPDs and electrical current-output power (I-P) curves measured before and after a long-term high temperature accelerated aging test were applied to analyze the degradation mechanisms. The results of this study show that: 1) The thermal management for prepared CSP samples provides a safe usage condition for packaging materials at ambient temperature; 2) The Mie theory with Monte-Carlo ray-tracing simulation can be used to simulate the SPD of Pc-white LEDs with mixed multicolor phosphors; 3) The degradation mechanisms of Pc-white LEDs can be determined by analyzing the extracted features of SPDs collected after aging.     

Improving cooling effectiveness by use of chamfers on the top of electronic components

A Computational Fluid Dynamic (CFD) study based on Reynolds Averaged Navier–Stokes (RANS) approach is carried out to predict the mean velocity field and the heat transfer rate of an impinging jet in cross-flow configuration on a heated wall-mounted cube. Targeting an electronic cooling configuration, the aim is to investigate the effect of geometrical modification of the component on the cooling effectiveness. For the same cross flow Reynolds number Re_H = 3410, three levels of impinging jets are computed as well as a case without impinging jet that will serve as baseline case for comparison. The results from the RANS computation are compared to experimental data from published scientific literature. The validation shows qualitatively good agreement and almost all flow structures are well reproduced by the computation. In an attempt to optimize the wall heat flux over the cube surface, a new geometry is proposed without sharp corners on the top cube face. Numerical results show that with minor geometrical modification (chamfer), the fluid flow structure around the electronic component is radically transformed and the heat transfer rate can be improved. The highest cooling effectiveness improvement is realize for the highest Reynolds number ratio Re_j/Re_H = 1.5 and for the chamfer height of 4 mm.     

An analytical two-dimensional model for AlGaN/GaN HEMT with polarization effects for high power applications

An analytical two-dimensional model for AlGaN/GaN modulation-doped field effect transistor is developed. The spontaneous and piezoelectric polarization effects have been included. Two-dimensional analysis has been carried out in the high field region. The output characteristics, device transconductance and cut off frequency for 120 nm gate length device are obtained. Peak transconductance of 320 mS/mm and a cut off frequency of 120 GHz has been obtained. The results show excellent agreement when compared with experimental data thereby proving the validity of the model.     

Sapphire substrate-transferred nitride-based light-emitting diode fabricated by sapphire wet etching technique

In order to investigate the influence of a sapphire substrate on the GaN-based light-emitting diode (LED) performance, sapphire-etched vertical-electrode nitride-based semiconductor (SEVENS) LEDs are fabricated by a sapphire wet etching technique. The performance of SEVENS-LEDs is substantially dependent on the presence of sapphire substrate. The light-output power of a SEVENS-LED with a microroughened surface structure and without a sapphire substrate (type-A) is not saturated up to a junction current as high as 300 mA, constituting a notable improvement relative to that (250 mA junction current) of SEVENS-LEDs with a 5 μm-thick sapphire substrate (type-B). The 200 mA light-output power of type-A SEVENS-LED is 1.8 times stronger than that of type-B SEVENS-LED. With increasing junction current, the variation of the peak wavelength is less for the type-A SEVENS-LED than for the type-B SEVENS-LED. These results imply that even a thin sapphire substrate on the SEVENS-LED affects the heat dissipation characteristic at high injection levels.     

Experimental study of microrectangular groove structure covered with multi mesh layers on performance of flat plate heat pipe for LED lighting module

The paper presents a novel concept for a coronary-stent-like model to solve the problem of compactness between wick and copper mesh, which can enhance the performance of the hybrid structure flat plate heat pipe (FPHP) of LED lighting modules. The various wick structures combine axial rectangular grooved structures, manufactured in aluminum extraction, and the concept of a coronary-stent-like model, which provides a supportive copper mesh and wick structure. In this study, the performance of FPHP was experimentally measured at different inclination angles and heating areas. The axial rectangular groove structure and copper mesh layer structures have different permeabilities and capillary pumping forces, and combining these two structures could be beneficial for pumping the required operational fluid across the axial groove structure and from the condenser to the evaporator under different inclinations of the flat plate heat pipe. The exterior wall temperature of the FPHP was measured to evaluate the thermal resistance and vapor heat transfer coefficient at the condenser and evaporator for 31 × 31 and 10 × 10 mm² heating areas. The experimental result showed that the FPHP has better performance in both the junction temperature of the LED light module and the uniformity of the substructure temperature. The highest FPHP temperature was decreased by 28%, as compared to a commercial substrate. In addition, a 200 W LED light module, running for 9 h with FPHP, maintained luminance at about 2080 lux due to its low thermal resistance and high capillary force.     

Electrical and optical characterization of GaN-based light-emitting diodes fabricated with top-emission and flip-chip structures

We investigated the electrical and optical characteristics of GaN-based light-emitting diodes (LEDs) fabricated with top-emission and flip-chip structures. Compared with top-emission LEDs, flip-chip LEDs exhibited a 0.25 V smaller forward voltage and an 8.7 Ω lower diode resistance. The light output power of the flip-chip LED was also larger than that of the top-emission LED by factors of 1.72 and 2.0 when measured before and after packaging, respectively. The improved electrical and optical output performances of flip-chip LEDs were quantitatively analyzed in terms of device resistance and ray optics, respectively.     

Electromagnetic field intensity triggered micro-biopsy device for active locomotive capsule endoscope

For an active and precise diagnosis, we developed an active locomotive intestinal capsule endoscope (ALICE), which can be wirelessly driven and controlled using an electromagnetic actuation (EMA) system. Since then, there has been a need to develop a biopsy device integrated into ALICE which can take a biopsy sample inside the gastrointestinal tract for a historical analysis of cancer disease. Toward this goal, this paper proposes a smart-triggered biopsy device for the ALICE using a micro-reed switch, where the integrated micro-reed switch is turned on using a strong magnetic field, and the biopsy device mechanism is activated by a micro-reed switch. To execute the biopsy process, first, the ALICE with the biopsy device is driven by an EMA system, where a moderate intensity magnetic field is used for driving the ALICE to reach a target region on the intestinal wall. After that, by increasing the magnetic field above a critical value, the ALICE is pushed hard against the target lesion, the micro-reed switch is turned on, and the biopsy device is triggered. The biopsy process, therefore, is totally wirelessly controlled by the external magnetic field of the EMA system, without an additional controller module. The prototype of the biopsy device, with dimensions of 12 mm in diameter and 5 mm in length, was integrated into the ALICE and the prototype of the ALICE, with the biopsy device having dimensions of 12 mm in diameter and 32 mm in length. The working principle and mechanism of the proposed biopsy device are introduced and the feasibility of ALICE with the biopsy device is demonstrated through in-vitro experiments.     

Effect of ethyl cellulose on thermal resistivity of thixotropic ZnO nano-particle paste for thermal interface material in light emitting diode application

A large amount of heat trapped inside Light Emitting Diode (LED) is the consequence of large thermal resistance between the heat source and the heat sink. Zinc oxide (ZnO) thick film was screen-printed from thixotropic paste that consisted of binder, filler and solvent to act as thermal interface material. Structural, surface morphology, vibrational and thermal properties of the samples were studied by means of Field Emission Scanning Electron Microscopy (FESEM), Atomic Force Measurement (AFM), Fourier Transform Infrared Spectroscopy (FTIR) and Thermal Transient Tester (T3ster). XRD analysis revealed that the formation of hexagonal wurtzite ZnO powder, which is free of hydroxide. FESEM results indicated that 50 wt% of filler loading in the thick film had created longer thermal transportation chain. The surface roughness of thick film displays variation in the range of from 64.8 to 218 nm. The presence of ZnO and binder were confirmed by FTIR spectrum at 518 cm⁻¹ and 668 cm⁻¹ to 2974 cm⁻¹, respectively. Thermal characterization reveals a drop in film’s resistivity with the higher content of filler loading of 50 wt% and 55 wt%. The lowest rise in junction temperature of tested LED is reported to be 14.7 °C of 50 wt% of filler loading.     

Long-wave (10 μm) infrared light emitting diode device performance

Electroluminescence in the range of 6–12 μm is observed from an Sb-based type-II interband quantum cascade structure. The LED structure has 30 active/injection periods. We have studied both top-emitting and flip-chip mount bottom emitting LED devices. For room temperature operation, an increase, saturation and decrease in light output occur at successively higher injection currents. An increase of about 10 times in light output occurs when device is operated at 77 K compared to room temperature operation. This increase is attributed to reduced Auger non-radiative recombination at lower temperatures. The peak-emission wavelengths at room temperature and 80 K operation are 7 and 10 μm, respectively. These devices can be used for high-temperature simulation in an infrared scene generation experiment.