Silicone materials with a relatively high refractive index have been introduced for the encapsulation of high power LEDs and LEDs with relatively short wavelengths. However, most of those existing silicone encapsulants, they still suffer thermal and radiation induced degradations and thus cause reliability issues and shorten the lifetime. A new high performance silicone has been developed and its performance has been compared with other silicone products in the packaging of high power blue and white LEDs. The new materials has been found to suffer less loss in the optical output during the aging test, high temperature/high humidity test, as well as JEDEC reliability test. Thus, this material is excellent for the packaging of high power white LEDs and high power colored LEDs, because of its ability in maintaining high transparency and great radiation/thermal resistance.

The potential of an innovation for establishing a simultaneous mechanical, thermal, and electrical connection between two metallic surfaces without requiring a prior time-consuming and expensive surface nanoscopic planarization and without requiring any intermediate conductive material has been explored. The method takes advantage of the intrinsic nanoscopic surface roughness on the interconnecting surfaces: the two surfaces are locked together for electrical interconnection and bonding with a conventional die bonder, and the connection is stabilized by a dielectric adhesive filled into nanoscale valleys on the interconnecting surfaces. This "nano-locking" (NL) method for chip interconnection and bonding is demonstrated by its application for the attachment of high-power GaN-based semiconductor dies to its device substrate. The bond-line thickness of the present NL method achieved is under 100 nm and several hundred times thinner than those achieved using mainstream bonding methods, resulting in a lower overall device thermal resistance and reduced electrical resistance, and thus an improved overall device performance and reliability. Different bond-line thickness strongly influences the overall contact area between the bonding surfaces, and in turn results in different contact resistance of the packaged devices enabled by the NL method and therefore changes the device performance and reliability. The present work opens a new direction for scalable, reliable, and simple nanoscale off-chip electrical interconnection and bonding for nano- and micro-electrical devices. Besides, the present method applies to the bonding of any surfaces with intrinsic or engineered surface nanoscopic structures as well.

A novel printable optically reflective adhesive is introduced to replace optically transparent adhesives for the die bonding of standard mesa type light-emitting diodes (LEDs). The new adhesive shows much lower thermal resistance than the optically transparent ones, and the printed adhesive layer also shows low fillet coverage around the chip, eliminating the absorption due to the surrounding die attach adhesive materials. The improvement in optical output by using printable optically reflective adhesive is found to be 2.3% points over standard adhesive with 33% fillet coverage. This letter suggests a cost-effective and speedy manufacturing method for mid-power mesa type LEDs, as well as chip-on-board packaging with multiple LED chips.

Die bonding is the first step in the packaging of light-emitting diodes and its role in lumen output performance and consistency of the packaged LED emitters has recently been investigated: there is an optimal range of bond-line thickness (BLT) and the fillet coverage has to be minimized. One method which can accurately control the BLT and fillet coverage is to employ the pre-made die attach adhesive (DAA) films, which cost more than the use of conventional DAA pastes. However it is difficult to control BLT as well as fillet by using conventional DAA pastes. In this work, a silicone-based transparent DAA is used for LED chip bonding. DAA layers are formed by using stencil printing, as an alternative method for LED die bonding with an objective of exploring a cost-effective method for LED die bonding with the controlled DAA BLT and fillet coverage. It is demonstrated that with other conditions being the same, a low BLT value of 3.9µm could be achieved by stencil printing. The lumen output of the packaged white LED emitters using printed DAAs with no fillet coverage, is found to be 3.5% higher than using the conventional pin-transferred DAA pastes with 33% fillet coverage, while the reliability of the packaged white LEDs is found not to be altered. The method of printable DAAs for LED die bonding is thus a cost-effective alternative to the one using pre-made DAA films.

Adding optical reflectors to the backside of GaN-based light-emitting diode (LED) chip has been adopted for enhancing the light output. The optical effectiveness of backside reflectors (BRs) in enhancing light output is investigated by studying the light output of packaged mid-power blue and white LED emitters based on BR-based and BR-free chips. It is found that the effectiveness is strongly dependent on the optical properties of die attach adhesive (DAA) materials. When epoxy-silver DAA is applied, the difference in light output between the gold BR-based and BR-free blue LED chips is about 9% points at 40% coverage. However, with optically clear or reflective DAAs, BR-free chips show higher light output than the gold BR-based chips. For the optically clear DAA with 40% coverage, a 20% point enhancement of the output power of blue LED chips is obtained by the high transmittance of the DAA and high reflectance of the silver plating on the leadframe. With the optically reflective DAA, the enhancement for blue LED chips is ~22% points due to the high reflectance of the DAA. This letter suggests a cost-effective method for enhancing the light output of mid-power LED emitters based on relatively low-cost BR-free LED chips by using optically clear or reflective DAAs depending on the applications and with a low fillet coverage for both materials.

The role of die attach adhesives (DAAs) in influencing the lumen output of midpower (30-150-mA input power) blue and white light emitting diodes (LEDs), which currently dominate the LED backlighting and general solid-state lighting, is studied. It is reported for the first time that a replacement of conventional silver-filled epoxy DAA by an optically transparent DAA can lead to a significant enhancement of light output as high as 13% and 21% increase for midpower blue and white LEDs, respectively. The mechanism for the observed enhancement is discussed in this paper.

A thin optical reflector is often introduced to the backside of the standard mesa type light emitting diode (LED) chip with the aim to enhance its light output. However, most of the reported light output enhancements because of backside reflector (BR) introduction might not be relevant. This is because the reported measurement is often from a naked LED chip instead of a packaged LED emitter, and those based on the packaged emitters employing conventional silver based die attach adhesive (DAA). The actual role of BR, which is expected to be greatly influenced by the packaging materials and processes, is investigated for the monotonic blue color and white LED emitters using Monte-Carlo simulations. Contrary to prior reports, it is demonstrated for the first time that the role of BR can be diminished when the optically transparent DAA is used and other key packaging materials and processes are optimized, i.e., the light output for a packaged emitter with a BR-free chip can be as high as that of the packaged emitter using the same chip but with an added BR.

In this study, silver pastes containing different sizes of glass frits were employed to form the front-side electrodes in order to examine the size effect of the glass frits on the interfacial microstructures between the front-side silver contacts and the n-type silicon emitter layers, which will subsequently affect the electrical performance of silicon solar cells. The interfacial microstructures at the Ag/Si interfaces were investigated by advanced electron microscopy techniques. The transfer length method (TLM) was used to measure the specific contact resistivity of silver electrodes screen-printed on the n-type silicon substrates. The particle size of the glass frits was found to strongly affect the interfacial microstructures and therefore resulted in different specific contact resistivities (ρ c) of the fabricated silver ohmic contacts. Nanosized glass frits showed excellent etching ability during engineered thermal treatments. The samples made with silver pastes containing microsized glass frits showed a thick residual glass layer at the Ag/Si interface, while the silver paste with nanosized glass frits was found to form an interface with less glass residue, which led to lower resistance after otherwise identical processing.

Thermal management is of critical importance for high power white light-emitting diodes lamps in which blue chip array is combined with yellow phosphors. In this study, the thermal performance of a 100W white LED package on a heat sink with a CCT of 4500K under different phosphor configuration was investigated by a combination of the ray-tracing simulation and computational fluid dynamics simulation. Key simulation results were verified by experiment at first. The simulation results show that the maximum temperature of the model always exists in the phosphor layer. As phosphors are moved away from the LED chips, the maximum temperature of phosphors increases from 120.4°C (200-μm thick conformal phosphor coating) to 160.6°C (in-cup phosphor), and even to 203°C (200-μm thick remote phosphor). It coincides with an analysis of the thermal resistance network of the model, which shows that the major dissipation path for heat generated in the phosphor layer is through the (encapsulant) chips, substrate, heat sink into the air. Among all studied phosphor configurations, the highest junction temperature is 98.3°C, and exists in the model with conformal phosphor coating. It is due to severe absorption of back scattered light by LED chips with phosphor coating.

Phosphor particle size effect on both lumen efficiency and angular uniformity of correlated color temperature (CCT) is studied for three types of phosphor converted white LEDs using Monte Carlo simulation tool. It is demonstrated for the first time that the size effect is non-monotonic and exhibits minima in the sub-micron size range. In particularly, the minimum lumen efficiency exists at the particle size parameter (πd/λ) around 6, where, however, also lies in the relatively low CCT uniformity. Moreover, the simulation results are verified by an analytical model developed for the estimation of critical particle size. Finally, this study provides important technical implication for the selection of phosphors in WLED manufacturing and development of phosphor materials for WLED applications.

A special liquid encapsulant and the liquid encapsulation process are developed for enhancing passive cooling for the packaging of monochromatic LED emitters. It was observed that the liquid encapsulation process is superior than the conventional solid silicone encapsulation process in terms of the junction temperature for all the packaged emitters of various wavelengths using the liquid encapsulant. An enhanced reliability in terms of lifetime is thus expected for the emitters encapsulated with the liquid encapsulant. Moreover, an enhanced light output is also demonstrated for the emitters packaged with the liquid encapsulant, which can be attributed to be a result of the improved heat dissipation by convection and conduction in the upward direction through the liquid as well as in the downward direction through the contact area between the liquid and the reflective cup.

Thermal performance of phosphor-based white light-emitting diodes (LEDs) under an input current of 350mA is investigated by finite-element simulation in which the thermal and optical interactions are considered. It is demonstrated that the temperature of the phosphor particles, regardless of phosphor placement, is always higher than the junction temperature. It is concluded that the junction temperature, which characterizes the thermal behavior of monotonic color LED emitters, cannot be used alone for characterizing the thermal behavior of white LED emitters. In fact, the phosphor temperature is critical in determining the lumen performance and reliability of white LED emitters. In addition, the phosphor temperature is effectively reduced by coating the phosphors directly on the chip and maintaining a relatively higher phosphor concentration (above 60 wt.%) in the phosphor-silicone mixture layer.

Two types of packaged white light emitting diodes in which one has a flat-top (FT) emitting surface and the other is a flat-top-with-lens (FTWL) type are fabricated by using the same leadframe and investigated on their optical properties, such as optical power, luminous efficiency, correlated color temperature (CCT), chromaticity coordinate, and color-rendering index (CRI), as a function of phosphor concentration in silicone encapsulant. It is found out that the optical power, CRI, and CCT decrease steadily as the phosphor ratio increases, while the luminous efficiency increases up to a level and then drops after a certain value of the phosphor ratio for both types of packages. Due to the totally internal reflection (TIR) at the encapsulant-air interface, the FT package shows a 10∼11% power (in mW) reduction compared with the FTWL package at the same phosphor concentration. However, it is demonstrated that the FT package provides a more efficient way of utilizing phosphor than the FTWL package based on the same targeted chromaticity coordinates due to the TIR effect inside, resulting in a reduced phosphor usage with a lumen output only about 3% lower than that of the FTWL package.

The thermal and optical characteristics of phosphor converted white light-emitting diodes (LEDs) with different phosphor concentrations ranging from 4 wt % to 13 wt % are investigated. The light output of LEDs with higher phosphor concentration is found to have larger degradation in constant current compared with pulse current than that with lower phosphor concentration. In addition, the junction temperatures of phosphor converted white LEDs raise with increasing phosphor concentration, so that the decreased phosphor conversion efficiency is observed both in pulse and constant current modes. The physical mechanisms for these observations are discussed. This study elucidates the phosphor dependent optical and thermal behavior of phosphor converted white LEDs.

In this paper, the influence of the die attach adhesive (DAA) layer on the thermal performance of high power light emitting diodes was first investigated by using finite element analysis, and some key results were verified by the experimental data. Effective thermal management of the studied light emitting diode package can be achieved by selecting a DAA material with a proper thermal conductivity and by manipulating the geometry parameters of the DAA layer, such as the DAA area, and the bond-line thickness. The significance of DAA thermal conductivity to heat dissipation was further demonstrated by an analysis of the bottleneck to heat transfer.

Silicone materials with a relatively high-refractive index have been introduced for the encapsulation of high-power light-emitting diodes (LEDs), and LEDs with relatively short wavelengths. However, most of those existing silicone encapsulants still suffer from thermal and radiation induced degradations and thus lead to reliability issues and a shorten lifetime. A new high-performance silicone has been developed and its performance is compared with other commercial silicone and optical grade epoxy in high-power white LEDs. The new materials had been found to suffer less loss in the lumen output during the aging test and high-temperature/high-humidity test, as well as the Joint Electron Devices Engineering Council (JEDEC) reliability test. It is concluded that this material is excellent for the packaging of high-power white LEDs and high-power colored LEDs, because of its ability in maintaining high-transparency and great radiation/thermal resistance.