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An acetylcholinesterase (AChE) biosensor was successfully fabricated with a stable structure and high detection accuracy. Graphene (Gra) nanofragments modified with chitosan (CS) and AChE were successively drip coated on the surface of a glassy carbon electrode via a layer-by-layer assembly method. The concentration range of the sensor to detect dichlorvos was 0.1-100,000 nM, and the limit of detection was 54 pM. CS was used to modify Gra for the first time, which enhanced the mechanical flexibility of these Gra nanostructures, significantly improving the stability and detection accuracy of this sensor.

Graphene/silver nanowire composite films have great potential as transparent conductive electrodes in the field of optoelectronic devices. So far, antioxidant and reducing the junction resistance are two major parameters in the silver nanowire electrode studies. In this paper, a pseudo-biological inspired structure for transparent electrodes was proposed by combining hybrid diameters silver nanowire network with the chemical vapor deposition-grown (CVD-grown) graphene as a passivation layer. Compared with the traditional structure, the silver nanowire network with this novel structure of human vascular tissue network greatly reduced the sheet resistance. An environmentally friendly liquid, deionized water, was selected for the generation of capillary forces at the liquid bridge, thereby improving the wire junction problem. After welding with the capillary force, the increase in the value of the figure of merit (FoM $= \sigma _{DC} \mathord {\left /{ {\vphantom {\sigma _{DC} \sigma _{OP}}} }\right. } \sigma _{OP}$ ) acted a pivotal part in improving conductivity with excellent optical performance. In addition, graphene was chosen as an encapsulation layer to protecting silver nanowires from oxidation while improving electrical properties. Compared with the silver nanowire films with a diameter of 20 nm, the $\sigma _{DC} \mathord {\left /{ {\vphantom {\sigma _{DC} \sigma _{OP}}} }\right. } \sigma _{OP}$ of the graphene/silver nanowire composite film increased by 69.6 % with a sheet resistance of $26.4~\Omega $ /sq. More importantly, graphene is supposed to protect silver nanowires from oxidation and moisture, which makes the composite films promising as electrodes for underwater optoelectronic devices or a possible development in high humidity environments.

An electrochemical acetylcholinesterase biosensor based on silver nanowire, graphene, TiO2 sol–gel, chitosan and acetylcholinesterase has been fabricated successfully for the detection of organophosphate pesticides. The outstanding electrical properties of silver nanowires and graphene, and moreover the self-assembly of these two nanomaterials make the biosensor highly sensitive. Simultaneously, the immobilization efficiency of the enzyme is greatly improved by the action of the TiO2 fixed matrix. Under optimum conditions, the biosensor exhibited excellent performance for the detection of dichlorvos with a linearity in the range of 0.036 μM to 22.63 μM and the detection limit was found to be 7.4 nM. The biosensor was highly reproducible and stable during detection and storage.

Graphene/silver nanowire composite films have great potential as transparent conductive electrodes in the field of optoelectronic devices. So far, antioxidant and reducing the junction resistance are two major parameters in the silver nanowire electrode studies. In this paper, a pseudo-biological inspired structure for transparent electrodes was proposed by combining hybrid diameters silver nanowire network with the chemical vapor deposition-grown (CVD-grown) graphene as a passivation layer. An environmentally friendly liquid, deionized water, was selected for the generation of capillary forces at the liquid bridge, thereby improving the wire junction problem. Compared with the silver nanowire films, the value of the figure of merit (FoM) of the graphene/silver nanowire composite film increased by 69.6% with a sheet resistance of 26.4 ?/sq, and a transmittance was 91.4% at 550nm. Also, graphene was chosen as an encapsulation layer to protecting silver nanowires from oxidation while improving electrical properties, which makes the composite films promising as electrodes for underwater optoelectronic devices or a possible development in high humidity environments. Furthermore, the capillary-welded silver nanowire/graphene composite film was used as the transparent anode to prepare organic light-emitting diode (OLED) devices. The devices emitted green electroluminescence at 516 nm, and the turn-on voltage was about 3.8 V. The maximum brightness was 50810 cd/cm2, which is higher than the indium tin oxide-based (ITO-based) devices with the same configuration. Therefore, it means that this novel silver nanowires/graphene electrode has great potential in optoelectronic device applications.

An acetylcholinesterase biosensor modified with graphene and transition metal carbides was prepared to detect organophosphorus pesticides. Cyclic voltammetry, differential pulse voltammetry, and electrochemical impedance spectroscopy were used to characterize the electrochemical catalysis of the biosensor: acetylcholinesterase/chitosan-transition metal carbides/graphene/glassy carbon electrode. With the joint modification of graphene and transition metal carbides, the biosensor has a good performance in detecting dichlorvos with a linear relationship from 11.31 μM to 22.6 nM and the limit of detection was 14.45 nM. Under the premise of parameter optimization, the biosensor showed a good catalytic performance for acetylcholine. Compared to the biosensors without modification, it expressed a better catalytic performance due to the excellent electrical properties, biocompatibility and high specific surface area of graphene, transition metal carbides. Finally, the biosensor exhibits good stability, which can be stored at room temperature for one month without significant performance degradation, and has practical potential for sample testing.

A new graphene/silver nanowire (AgNW) hybrid transparent conductive films (TCFs) was prepared by wet-transferring a chemical vapor deposition (CVD)-grown monolayer graphene onto pristine spin coati...

Ultraviolet (UV) treatment has been demonstrated to be an effective way to removing the poly-methyl methacrylate (PMMA) residual during the process of the wet-etching transfer of graphene. ...

In this paper, a novel and scalable method to fabricate graphene/carbon nanotube (CNT) hybrid transparent conductive films on Cu substrates, which combines electroplating and chemical vapor deposition (CVD) is proposed and demonstrated. The Cu substrate was electroplated with electrolyte containing conductive CNTs; then, a uniform graphene film was grown on Cu. After a commonly utilized polymethyl methacrylate assisted transfer process, a hybrid graphene/CNT transparent conductive film was obtained at the target substrate. Conventional graphene grown on electropolished Cu was used as the reference sample. The comprehensive characterization using scanning electron microscopy (SEM), Raman microscopy system, and transmission electron microscopy (TEM) selected area electron diffraction pattern show that the CNTs are uniformly covered by a monolayer graphene with comparable quality to graphene grown on electropolished Cu. The hybrid thin films exhibit outstanding surface morphology (RMS of 1.26), obviously enhanced electrical properties (the square resistance decreases from 490 to 394 Ω sq−1), better surface wettability (7° decrease in contact angle), and a negligible transmittance loss (1.3% reduction at 550 nm) compared to CVD graphene that was grown on electropolished Cu. It is anticipated that the graphene/CNT hybrid films that are fabricated using the proposed approach can be a promising alternative to ITO to realize the emerging, particularly flexible optoelectronic devices.

A highly sensitive, stable electrochemical acetylcholinesterase biosensor with silver-graphene modified was fabricated to detect organophosphorus pesticides, dichlorvos. Complete production and test process were characterized by scanning electron microscopy, cyclic voltammetry, differential pulse voltammetry. The electrochemical signal of biosensor got to be well amplified because of the modification of silver-graphene. The better detection performance was got, compared with the sensors that lack or no modified. Moreover, TiO2 sol-gel and high viscous chitosan were used as a highly efficient immobilization matrix for immobilizing acetylcholinesterase. The detection linear range of the biosensor to dichlorvos, a model organophosphorus pesticides compound, was from 0.036 μM (7.9 ppb) to 22.6 μM, with a limit of detection of 7.4 nM (1.6 ppb). The biosensor was highly reproducible and stable in detection and storage.

Both graphene and silver nanowire electrodes are thermoelectric materials that replace ITO as electrodes for optoelectronic devices. In this paper, we studied the effect of silver nanowire concentrations on electrode performance. By taking into account factors such as the sheet resistance and the transmittance of the films, the concentration of silver nanowires solution was finally chosen to be 3 mg/ml. Compared with Joule heat welding, capillary welding has self-limiting, which can prevent silver nanowires from being blown because of excessive heat. Therefore, capillary welding was chosen to improve the junction problem. In addition, graphene was chosen as an encapsulation layer to protecting silver nanowires from oxidation while improving electrical properties. Finally, the graphene/silver nanowire composite electrode had the sheet resistance of 59.7 Ω/sq with the transmission of 91.8 % at 550 nm.

We have proposed an approach to improve the photovoltaic performance of inverted polymer solar cells (i-PSC) using lithium ion doped ZnO (Li ZnO) as cathode buffer layer (CBL). The Li ZnO CBL was prepared using the diffusion technique, performed by inducing the Li ion of 8-hydroxyquinolatolithium (Liq) to diffuse into ZnO film through annealing the bi-layer ZnO/Liq film. Doping concentration of Li ion was controlled by using various thickness of Liq film and annealing temperature. Based on Li ZnO CBL, the poly (3-hexylthiophene) [6,6]:-phenyl C61-butyric acid methyl ester (P3HT:PCBM) i-PSC device possessed a optimal power conversion efficiency (PCE) of 4.07%, which was 30% improved than that of the device with neat ZnO as CBL. The enhancement of the device performance could be attributed to the enhanced electron mobility and better band matching of the Li ZnO CBL. Our finding indicates that the Li ZnO film fabricated with relatively low temperature treatment has great potential for high-performance i-PSCs.

Highly accelerated decay testing ( HADT ) based on the subsystem isolation method for light-emitting diode ( LED ) lamps is proposed. The proposed test identifies the desired thermal stress limit ( TSL ) in a given humidity stress for LED light engine subsystems. In HADT , a monitoring procedure for the in situ pseudo-junction temperature ( T pj ) of LED lamps and a step-fitting analysis procedure for the monitored parameters are suggested to detect abnormal thermal stress levels. The obtained TSL is applied to step-stress accelerated degradation testing ( SSADT ) to verify the uniform decay mechanisms at various stress levels. Results show that the goodness-of-step fit analysis of the monitored parameters effectively identifies the abnormal thermal stress levels. Consequently, the TSL of LED light engines can be clearly observed through the in situ T pj and optical parameters monitored in HADT . The application study shows that SSADT rapidly and effectively accelerates the decay process of LED lamps, and the tested samples undergo uniform decay mechanisms at three reasonable stresses, which are selected based on the obtained TSL . The proposed HADT has significant potential use in the qualification specification of LED lamps.

We have demonstrated a new composite thin film for encapsulation consisting of polydimethylsiloxane (PDMS) and graphene. Graphene is a kind of hydrophobic material, causing that moisture could hardly penetrate into the surface of graphene. Graphene is usually fabricated by the method of Chemical Vapor Deposition (CVD). The atomic distance of graphene is very small; thus the water vapor channel from one layer to the nearby layer has been cut off, achieving the barrier of oxygen, water vapor and particle. Graphene and PDMS have high transmittance, making this method be widely used in various photoelectric devices. Meanwhile, graphene owes excellent heat dissipation which can reduce the temperature gradient and the junction temperature, thus improves the comprehensive performance and reliability of the device. Furthermore, PDMS has a good cohesiveness property, by which fit graphene very well and meet the requirement of encapsulation at the same time. Using Ca film test, we have found that the composite thin film has the greatest water oxygen barrier effect among them.

The technique of organic light-emitting diodes developed rapidly over the last decade and small size OLED panels has also been commercially applied. However, organic semiconductors exhibit relatively poor electrical and thermal behavior and the device stability is always a tough issue since the high sensitivity to the environment, such as the water, oxygen, temperature, and so on. Therefore, the organic-inorganic hybrid light-emitting devices become research focus for combining both the high electron mobility of inorganic semiconductors and the extraordinary optoelectronic properties of polymers. However, according to previous research results, there are still problems such as low brightness, poor color rendering, low efficiency and low current density etc. Here, we present a novel inverted organic-inorganic hybrid blue fluorescent device with the hybrid p-n junction structure using n-type GaN (n-GaN) and organic semiconductor layers, which was proved to have extremely high luminance and low driving voltage. Single layer n-GaN with thickness of 2.5 um on patterned sapphire substrate (PSS) was used in this paper because it has excellent electrical conductivity and outstanding thermal conductivity. Based on this organic-inorganic hybrid system, we have proved a high-brightness and low turn-on voltage light-emitting devices.

The development of silver nanowire electrodes is always limited due to some disadvantages, such as roughness, oxidative properties, and other disadvantages. In this research, a capillary-welded silver nanowire/graphene composite film was used as an electrode for organic light-emitting diode (OLED) devices. As an encapsulation layer, graphene reduced the surface roughness and the oxidation probability of silver nanowires. The composite electrode showed an excellent transmittance of 91.5% with low sheet resistant of 26.4 ohm/sq. The devices with the silver nanowire/graphene composite electrode emitted green electroluminescence at 516 nm, and the turn-on voltage was about 3.8 V. The maximum brightness was 50810 cd/cm2, which is higher than the indium tin oxide-based (ITO-based) devices with the same configuration. Finally, it was proved that the silver nanowire/graphene composite electrodes possessed better heat dissipation than the ITO-based ones under energization. In summary, it means that this novel silver nanowires/graphene electrode has great potential in OLED device applications.

In this paper, we simulate graphene on the surface of substrate, using interconnection metallization to connect merge pin schottky (MPS) power diode chip and substrate, then build a thermal model of the device, and perform the finite-element analysis with ANSYS software. In recent years, graphene has attracted a large amount of interest because of its outstanding electric conductivity, ultrahigh thermal conductivity and large specific surface area. Therefore, if graphene films are used as heat sink, heat will diffuse through graphene surface at breakneck speeds, solving the heat accumulation problem of the device. In this work, we simulate the heat dissipation effect of the device with graphene films or without graphene films. Then, we change the thickness of graphene films, contact area between graphene films and heat dissipation substrate, and contact area between interconnection metallization and graphene films.

In this paper, enhanced photolithography with Al film insertion for large-scale patterning of chemical-vapor-deposited graphene is proposed and demonstrated. An Al film was evaporated onto the surface of graphene, and then traditional photolithography and wet etching were used to make an Al mask. After Ar plasma treatment and removal of the Al mask, patterned graphene was obtained on the target substrate. Graphene patterns fabricated by conventional photolithography were utilized as reference samples. Compared to conventional patterned graphene, the graphene patterns exhibited obviously less residual photoresist (PR), high resolution, and can be applied to various shapes and sizes. It is worth mentioning that the pitch can reach the limit of photomasking utilized in this study. In addition, the graphene also shows a relatively lower work function (~4.2 eV), better photoelectric properties (sheet resistance ~531 Ω/sq, T550 = 94.2%), and outstanding surface hydrophilicity (water contact angle 49°). It is anticipated that graphene patterns realized by the proposed approach could make the application of graphene more promising in emerging and future optoelectronic devices.

In this letter, a theoretical degradation model of LEDs at dynamic working conditions was proposed. The Arrhenius effective temperature (AET) was used to describe the effect of dynamic working temperature. Utilizing the concept of AET, the lifetime prediction of LEDs at dynamic working conditions can be carried out based on the data base at steady state. It was found that the activation energy has a great effect on the numerical value of AET. The extraction of activation energy from steady state measurement made the degradation model available to predict the lifetime of LEDs at various input. The AET was found to be always higher than the arithmetical average temperature. The lifetime of LEDs was found to increase with the frequency and decrease with the duty cycle for the typical dynamic working conditions–pulse input.

In this paper, we present the thermal, electrical, and optical analyses of light emitting diode (LED) packages with different chip sizes. The LED packages under investigation employed the same configuration of package components, except for the chip sizes. The forward current was found to increase with the chip size at the same forward voltage due to the area increase of current spreading. The luminous flux and optical power were found to increase with the chip size at the same current density. The thermal analysis was made by the transient thermal measurement and thermal simulation using the finite volume method. It was demonstrated that the thermal resistance decreased with the chip size under the same package conditions both by simulation and experiment. The bulk thermal resistance and spreading thermal resistance were combined together to give out a quantitative investigation of the partial thermal resistance variation. Moreover, the spreading thermal resistance was found to have a great effect on the total thermal resistance of LED packages.

In this letter, the thermal evaluation of high-power LED packages at pulse conditions was reported. A theoretical calculation model was proposed based on the analogy between the thermal and electrical RC circuits. The thermal performance of LED packages driven by pulse input was calculated using the RC network extracted from transient thermal measurement. The junction temperature fluctuation band decreases with the frequency at certain duty cycles. The saturated average junction temperature rise linearly increases with the duty cycle at certain frequencies. These predictions were verified by the real-time junction temperature measurement using the peak shift method at pulse conditions. The theoretical model was found to be effective and applicable to the evaluation of the thermal performance of LEDs working at pulse conditions.

In this paper, we present the thermal and mechanical analysis of high-power light-emitting diodes (LEDs) with ceramic packages. Transient thermal measurements and thermomechanical simulations were performed to study the thermal and mechanical characteristics of ceramic packages. Thermal resistances from the junction to the ambient were decreased from 79.6 to 46.7degC/W by replacing the plastic mold with a ceramic mold for LED packages. Thermomechanical stress induced in the heat-block test was simulated using a finite-element method. Higher level of thermomechanical stress in the chip was found for LEDs with ceramic packages, despite less mismatching coefficients of thermal expansion, compared with that with plastic packages. The thermomechanical-stress components in the direction of the thickness were found to be larger than that in other two directions. The results suggest that the thermal performance of LEDs can be improved by using ceramic packages, but the mounting process of the high-power LEDs with ceramic packages is critically important and should be the reason for causing delaminating interface layers in the packages.

Degrading characteristics of InGaN/GaN-based blue light-emitting diodes (LEDs) in the optical increase stage (stage I) were investigated. The LEDs were under an ageing with dc current of different levels. Different ageing phenomena were observed for both the electrical and the optical characteristics. The tunnelling components of the forward current were found to change with different trends for the samples aged under current stresses of 100 mA and 1000 mA, respectively. The former decreased monotonically, while the latter decreased in the early period, and then kept increasing. Different ageing rates but the same trends were observed for the optical degradation of the samples aged under different stresses. The ageing factors induced by the dopant activation and the change of defects competed with each other in the ageing process and were proposed to be responsible for the ageing characteristics.

Ceramic packages with different paths of thermal dissipation were designed by a simulation using the finite-volume method. Thermal resistances of the packages were obtained from the calculation of temperatures of LED (light emitting diode) chips. It was shown that the thermal characteristics of LED packages were strongly affected by the contact area of a thermal via in contact with a LED chip rather than that in contact with a ceramic mould. Ceramic LED packages were fabricated and characterized. A deviation of simulated results from experimental results was found. The difference was attributed to the poor contact roughness between the LED chip and the thermal via.

In this paper we report on the thermal analysis of high-power light-emitting diodes (LEDs) as a function of input power. The transient thermal measurement method using structure function was employed to investigate the thermal behavior of LEDs at different working conditions. It was shown that a 5 W LED package installed with four chips exhibited an increase of thermal resistance from 7.06 K/W to 8.83 K/W with the input current ranging from 50 mA to 700 mA at the ambient temperature of 25 °C. However, the thermal resistance for a 1 W LED package with one chip was decreased from 20.33 K/W to 19.54 K/W with the input current ranging from 50 mA to 350 mA. Based on our improved LED model, the different trends for the two packages are attributed to the difference in the power dissipated by internal series electrical resistances. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper we present the analysis of delamination in light-emitting diode (LED) packages by transient thermal measurement and thermo-mechanical simulation. The LED samples were subjected to various moisture preconditioning treatments, and subsequently heat block testing. Transient thermal measurements were used to investigate the thermal behavior of the delaminated LEDs. Thermal resistance from chip to lead frame was increased from 41.7 to 70.9 K/W by a preprocessing for 24 h. Blocking of thermal path induced by delamination was regarded as being responsible for the increased thermal resistance. The maximum thermal stress for the die attach obtained from FEA simulation is 91.5 Mpa, and the thermal stress distribution is consistent with the observed micrograph for the delamination in the lead frame. Thermal stress induced in the reflow process caused delamination and moisture absorption expanded the delamination in the LED package investigated.

Solder joint qualities of light-emitting diodes (LED) lamps have a significant effect on their lifetime. Due to variations in LEDs, the product lifetime is determined by the lowest performing component of the product when multi-LEDs are used. In this paper, we propose a quick electrical inspection method of solder joint quality for LED products, which is obtained by the heating curve measurement for a short time with the forward voltage as a temperature sensitive parameter. The utility of this quick inspection method is verified by the measurement of a solder voids ratio based on the most commonly used approaches in the industry—x-ray. The proposed method is cost effective and only needs around 1–5 s for each LED, which is about one third of the x-ray. Therefore, the quick electrical test method is both meaningful and promising especially when carrying out quality tests on large quantities of solder and can be a highly recommended method to be adopted by the LED lighting industry.

With the continuous improvement of the electron component integrating, the packet size and weight of micro electro-devices are continue shrinking. The convenient and low-temperature processing techniques are the trend of bonding techniques. Glass frit bonding technology has advantages which include a relatively low joining temperature, and less stringent requirements for contact surface smoothness. The laser bonding is widely used due to the advantages of non-connect, local heating and flexibility. Both of the techniques are applied in Micro Electro-Mechanical System (MEMS), modern electronic, electro-optic, and photovoltaic. In the paper, we investigated the influence of the copper oxide (CuO) fillers to laser energy absorbing by the glass frit. Three groups of glass frit with CuO ratios of 10%, 15% and 20% were researched. The characteristics of samples were evaluated. The mechanical strengths of bonded samples were tested by tensile stress gauges. The surface morphology of the films was observed by optical microscope and scanning electron microscope (SEM). The sheet resistance was measured by a resistivity meter. The results showed that the sheet resistance was complied with the requirements of low conductivity. The 15% CuO sample showed the better shear strength than the other two groups. The glass frit with 10%, 20% of CuO fillers exhibited poor features.

The reliability evaluation of Light Emitting Diodes (LED) is analyzed by lifetime test, which is usually derived from the accelerated test. In order to find the problem why the LED cannot reach the claimed lifetime, the reliability issue of the LED is investigated by the accelerated temperature tests. One of the accelerated test environment temperature is 55 °C and the environment is an oven, while the other is the room temperature of 23 °C. All the HP-LED samples are driven by the same current 450 mA. Some of the samples in the oven showed the phenomenon of current leakage since 200 hours aging, but the samples in the room temperature did not. In addition, luminous flux of the samples in the oven decreased faster after the same aging period. The HP-LED with current leakage shows faster luminous flux descending. In a word, the degradation behaviors of GaN based light-emitting diodes under high-temperature stressing is investigated.

High-power LEDs have been widely used and have a big potential market due to the many advantages compared to conventional light sources. Reliability is one of the most important issues for the application of LEDs. In order to study the degradation mechanism and the stability of LED withstanding cyclical exposures to temperature variations, life test was carried out under temperature cycle testing. The experiment was designed basing on the EIAJ ED-4701/100. Four groups of 1 W high power LEDs adopting different material and structures were utilized. The temperature cycle accelerated life test under the condition of −25°C/125°C. After the accelerated test, the optical performance of the four kinds of devices were compared and analyzed, and the thermal performance of the LEDs was also tested, analyzed and discussed. At last, the reliability differences of high-power LEDs were analyzed and degradation mechanism of high power LEDs under temperature cycling was discussed.

In this paper, GaN-based blue light emitting diodes with double roughened surfaces were fabricated and compared with planar LEDs and these with single roughened layer. The double roughened surfaces were fulfilled by patterned sapphire substrates (PSS) and roughened p-GaN surface. Magnesium treatment during the growth process was used to produce p-GaN surface roughening. After the epitaxial growth, the four group samples were fabricated using the standard process (four mask steps) with a mesa area of 10×23 mil2. At last, 25 LED devices were packaged for each group using the same packaging material and by the same method. It was found that import of PSS and roughened p-GaN surface may lead to a little higher voltage by simply copying the epitaxial process of planar LED. However, even fabricated by the non-optimized epitaxial process, comparing with planar LEDs, the optical power of the package can be increased by 40.1% by using the patterned sapphire substrate, 10.2% by using p-GaN surface roughing, and 44.9% by using double roughing technologies.

In this paper, the reliability issue of the LED lamp in high temperature and moisture condition was investigated. Thermal stress and hygroscopic stress in the LED package were obtained by coupled-field analysis using Finite element analysis (FEA). The results demonstrate that the temperature of the module can reach saturation in tens of seconds while the moisture field needs longer to reach balance. 85°C/85RH can reach the same distribution of relative humidity with 65°C/65RH, but the time spent is half. So temperature is the important factor to affect the humidity distribution. The moisture concentration is better than the relative humidity response to the material moisture level. So we made further research on hygroscopic stress. Both the thermal stress and hygroscopic stress contributed to the delaminations in LED packages.

In this paper we present the thermal and mechanical analysis of high-power light-emitting diodes (LEDs) with ceramic packages. Transient thermal measurements and thermo-mechanical simulation were performed to study the thermal and mechanical characteristics of ceramic packages. Thermal resistance from the junction to the ambient was decreased from 76.1 oC/W to 45.3 oC/W by replacing plastic mould to ceramic mould for LED packages. Higher level of thermo-mechanical stresses in the chip were found for LEDs with ceramic packages despite of less mismatching coefficients of thermal expansion comparing with plastic packages. The results suggest that the thermal performance of LEDs can be improved by using ceramic packages, but the mounting process of the high power LEDs with ceramic packages is critically important and should be in charge of delaminating interface layers in the packages., Submitted on behalf of TIMA Editions (http://irevues.inist.fr/tima-editions)

In this paper we present the thermal and mechanical analysis of high power light emitting diodes with ceramic packages. Transient thermal measurements and thermomechanical simulations were performed to study the thermal and mechanical characteristics of ceramic packages. Thermal resistances from the junction to the ambient were decreased from 76.1 °C/W to 45.3 °C/W by replacing the plastic mould to the ceramic mould for LED packages. Higher level of thermomechanical stress in the chip was found for LEDs with ceramic packages despite of less mismatching coefficients of thermal expansion comparing with plastic packages. The results suggest that the thermal performance of LEDs can be improved by using ceramic packages, but the mounting process of the high power LEDs with ceramic packages is critically important and should be in charge of delaminating interface layers in the packages.Copyright © 2007 by ASME

This work reports on the moisture inducing delamination in light-emitting diode (LED) packages and its effects on thermal characteristics. The LED samples were subjected to moisture preconditioning followed by heat block testing. Transient thermal measurements were performed to investigate the thermal behavior of the delaminated LEDs. Increase of thermal resistance with the degree of delamination was observed from the transient measurement. The thermo-mechanical calculated from coupled-field FEA simulation agree well with the micro graphical evidence. The calculated hygro-mechanical stress increased with the preconditioning time. It was found that the thermo-mechanical stress plays more important role than the hygro-mechanical stress for the development of delamination in the LED packages. Moisture pre conditioning for 3 hrs and 6 hrs under 8 5/spl times/C/8 5 RH conditions was found to make little contribution to the delamination between the chip and lead frame.

In this paper we report on the mechanism of delamination in light-emitting diode (LED) packages and its effects on thermal characteristics of LEDs. The LED samples were subjected to moisture preconditioning treatments followed by heat block testing. Transient thermal measurements were performed for the investigation of the thermal behavior of the delaminated LEDs. Increase of thermal resistance with the degree of delamination was observed from the transient measurement test. Thermo-mechanical stress and hygro-mechanical stress in the LED package were obtained by coupled-field analysis using FEA. The calculated thermo-mechanical and hygro-mechanical stress distributions agree well with the micrographical evidence. It was found that the thermo-mechanical stress plays more important role than the hygro-mechanical stress for the development of delamination in the LED packages. Moisture preconditioning for 3 hrs and 6 hrs under 85°C/85RH conditions was found to make little contribution to the delamination between the chip and lead frame. Significant delamination was expected for the samples experienced the moisture preconditioning for 24 hrs.

The degradation of high power GaN/InGaN blue light-emitting diodes (LEDs) was investigated by considering the electrical, optical and thermal ageing characteristics. The LED samples were stressed at the elevated temperature of 85 °C with an injection current of 350 mA. Changes in the tunnelling current and series resistance for the electrical characteristics and an initial increase followed by a gradual decrease for the optical power were observed. Variations of the thermal resistance in the chip and package were found to be 2 °C W−1 and 0.3 °C W−1, respectively. The responsible factors were proposed to be: (a) the dopant activation and changes of defects in the chip level; (b) the yellowing of the optical lens and structural degradations such as generating voids or delaminations in the package level. The changes in the electrical, optical and thermal characteristics were found to depend on and affect each other. The internal relationship for the characteristics of the three aspects was explained.