Mutharasu Devarajan

Influence of AlN Thin Film as Thermal Interface Material on Thermal and Optical Properties of High-Power LED

An AlN thin film was fabricated and used as a thermal interface material (TIM) for a high-power light-emitting diode (LED) application. The AlN thin film was stacked with an Al thin film on Al substrates and used as a heat sink. The thermal behavior of a 3-W green LED mounted on the AlN and AlN/Al stack thin-film-coated substrates was tested using a dual interface method. A low value in junction temperature (TJ) of the LED was observed with an AlN/Al stack used as the interface. The observed total thermal resistance (Rth-tot) was low for the AlN/Al-stack-coated Al substrates compared to the bare and thermal-paste-applied Al substrates. The interface thermal resistance (Rth-b-hs) was influenced by the driving currents and observed a low value with the AlN-based coatings. Improved luminosity was observed from the LED with the AlN/Al stack as the TIM. The observed correlated color temperature (CCT) was low for the AlN/Al stack at all driving currents. No significant change was observed in the color rendering index (CRI) and peak wavelength (λpeak).

Thermal resistance studies of surface modified heat sink for 3W LED using transient curve

Purpose – Surface configuration at the interface between two materials makes a huge difference on thermal resistance. Thermal transient analysis is a powerful tool for thermal characterization of complex structures like LEDs. The purpose of this paper is to report the influence of surface finish on thermal resistance.Design/methodology/approach – Surface of heat sink was modified into two categories: machined as channel like structure; and polished using mechanical polisher and tested with 3W green LED for thermal resistance analysis.Findings – The observed surface roughness of rough and polished surface was 44 nm and 4 nm, respectively. Structure function analysis was used to determine the thermal resistance between heat sink and MCPCB board. The observed thermal resistance from junction to ambient (RthJA) value measured with thermal paste at 700 mA was lower (34.85 K/W) for channel like surface than rough surface (36.5 K/W). The calculated junction temperature (TJ) for channel like surface and polished ...

Estimation of Optical Power and Heat-Dissipation Factor of Low-Power SMD LED as a Function of Injection Current and Ambient Temperature

Optimization of light-emitting diode (LED) design is highly dependent on the relationship between photometric, electric, and thermal parameters of the device. The photoelectrothermal (PET) theory connects all the features of LED device. In this paper, wall-plug efficiency equations of PET theory were extended to estimate heat-dissipation factor and optical power of low-power surface-mounted device (SMD) LED. The wall-plug efficiency as a function of ambient temperature and injection current is used to predict the heat-dissipation factor and the optical power of the SMD LED. The TeraLED and T3ster systems were utilized to provide optical and thermal test reports of the LED parameters at a certain range of injection current and ambient temperature. The presented equations were verified by testing on low-power SMD LED. Results demonstrate that the estimated and measured values were in good agreement for all the considered parameters. This validates the extension of PET theory equations for low-power SMD LED.

Thermal Transient Analysis of High-Power Green LED Fixed on BN Coated Al Substrates as Heatsink

Boron nitride (BN) thin film was deposited over Al substrate and used as a heatsink for high-power LED. The rise in junction temperature TJ was evaluated from transient cooling curve and observed as a low value for BN coated substrates rather than for bare and thermal paste (TP) applied substrate measured at 700 mA. Total thermal resistance (Rth-tot) was also observed as a low value for BN samples. The BN thin film interface influenced the wavelength peak shift toward a higher value. Noticeable increment on luminosity of the LED was also observed for BN coated Al substrate with respect to driving currents. The observed results were suggested the use of BN thin film as thermal interface materials as an alternative for TP.

Thermal characterization of a high power infrared emitter as a function of input current

In this paper, thermal characterization of a high power infrared emitter as a function of input current is reported. Thermal behavior of the high power infrared emitter is investigated by employing thermal transient method. For measurement, input current is varied from 0.2A to 1.0A at constant ambient temperature of 27°C under still-air environment. It is found that junction-to-interface thermal resistance, RthJI of the high power infrared emitter increases from 2.50 KfW to 3.17 K/W with the increase of input current from 0.2A to 1.0A at an ambient temperature of 27°C. From the findings, increase of junction-to-interface thermal resistance, RthJI for the high power infrared emitter as a function of input current is due to the variation of optical efficiency with input current as presented in this paper.

Thermal Impedance Measurement on Different Chip Arrangements for Various Multichip LEDs Application

The effect of thermal impedance of three chip arrangements in a multichip light-emitting diode (LED) package has been discussed with each chip attached to a heat sink with thermal interface material. Each orientation represents different existing lighting applications such as T8 tube, MR 16 down light, and spotlight. In this paper, the emitter and sensor current are constantly maintained. One LED of each orientation is driven, while other LEDs are sensed until all LEDs are driven with the emission current of all the orientations. Comparisons are made between the thermal impedance matrix pairs of each LED arrangement to illustrate the difference in the thermal dissipation. Thermal impedance curves show the fluctuation in the thermal flow inside the package, implying the reciprocal behavior of each orientation. This paper further explains the reciprocal behavior of the chips and the importance of the orientation based on junction temperature. Moreover, the principle of linear superposition is implemented to estimate the junction temperature matrix. The theoretical and the experimental results are compared using thermal transient tester. Among three orientations, the square-shaped LED arrangement shows an average junction temperature of 116 °C within the package while exerting the highest change of junction temperature of 4 °C, comparing with the linear superposition method.

Structural and surface analysis of AlInN thin films synthesized by elemental stacks annealing

This paper presents the synthesis of AlInN thin films on Si (100) substrates using elemental stacks annealing (ESA) process. Single stack InN films were grown on Si (100) substrates by reactive radiofrequency (RF) magnetron sputtering using pure indium target in Ar–N2 environment and then an Al stack layer was deposited on the InN films by direct current (dc) sputtering of pure aluminum target in Ar atmosphere at room temperature. Annealing of the deposited films was carried out at 400 °C for 2, 4 and 6 h in a tube furnace under N2 atmosphere. X-ray diffraction (XRD) results reveal that annealing for 2 h does not produce a well-defined AlInN film, however, with the increase of annealing time to 4 h and to 6 h, (002) and (103) oriented highly crystalline AlInN films are formed with wurtzite structures. Field emission scanning electron microscopy (FESEM) results indicate a uniform film structure with grains growth by increasing the annealing time. Energy dispersive x-ray (EDX) analysis shows higher Al (atomic %) in the film as compared to In and N. Atomic force microscopy (AFM) results show a decrease in the surface roughness with increase of the annealing time.

Influence of phosphor packaging configurations on the optical performance of Chip on Board phosphor converted Warm White LEDs

The optical properties of high power Warm-White Light Emitting Diodes (WW-LEDs) in Chip-on-Board (COB) package of Dam-and-Fill encapsulation with three different phosphor packaging configurations were investigated in this paper. The phosphor configurations studied were remote phosphor, double phosphor layers and normal phosphor configuration. Analysis of experimental data shows that LED samples in COB package with normal phosphor configuration had the highest efficiency compared to the other two phosphor configurations, differing from usual results obtained in other comparison studies of phosphor configurations in LED packages. This investigation also shows that LED samples with remote phosphor configuration has the highest relative intensity at red-yellow emission region, while LED samples with normal phosphor configuration has the highest relative intensity at blue emission region.

Estimation of Luminous Flux and Luminous Efficacy of Low-Power SMD LED as a Function of Injection Current and Ambient Temperature

Light-emitting diode (LED) luminous flux and efficacy as a function of temperature and current is important to qualify and optimize the design of an LED system. In this paper, the extension of light output equation was used to estimate luminous flux and efficacy of low-power surface-mounted device LED. The photometric and thermal test reports of LED parameters at a certain range of injection current and ambient temperature were obtained by the combination of T3ster and TeraLED systems. The luminous flux and efficacy as a function of injection current and ambient temperature were predicted. The agreement between the experimental and theoretical results validates the extended equation used in this paper. The optimized operating conditions of LED under test are determined.

Comparison of Radio Frequency and Microwave Plasma Treatments on LED Chip Bond Pad for Wire Bond Application

The efficiencies of argon radio frequency [Ar(RF)] and argon microwave [Ar(MW)] plasma treatments were compared in terms of contaminant removal and wire bond interfacial adhesion quality in this paper. The efficiency in contaminant removal was analyzed by applying Ar(RF) and Ar(MW) plasma treatments with operating frequencies of 13.56 MHz and 2.45 GHz, respectively, onto the light-emitting diode chip bond pad prior to the wire bonding process. Surface characterization results show that Ar(MW) plasma treatment is able to remove the bond pad surface contaminant and improve the bond pad surface roughness more effectively than Ar(RF) plasma treatment. Moreover, contact angle measurement results show that bond pad samples after Ar(MW) plasma treatment have higher surface free energy compared with Ar(RF) plasma treatment. To study the improvement of wire bond interfacial adhesion after Ar(RF) and Ar(MW) plasma treatments, the bond pad samples were wire-bonded via the thermosonic wire bonding method and examined with the ball shear test. The ball shear test results show that Ar(MW) plasma treatment leads to a better improvement of wire bond interfacial adhesion compared with Ar(RF) plasma treatment.