A chip scale package (CSP) LED is defined as an LED package that has a close ratio between the volume of the LED chip and the total volume of the LED package. A CSP package is essentially a bare LED die (chip) on which a phosphor layer is coated, with the underside of the die metallized with the P and N contacts to form the electrical connection and thermal path . In the semiconductor industry, a chip scale package was initially defined as a package with no more than 1.2 times the size of the chip. Soon the term CSP has been used to promote a miniature package that achieves its small footprint through design innovation. In the case of CSP LEDs, package miniaturization is achieved by eliminating the plastic submount or ceramic substrate found on a conventional mid-power or high power LED package.Light emitting diodes take advantage of the direct and wide bandgap of gallium nitride (GaN) to create electroluminescence. Gallium nitride is impregnated or doped with donor impurity atoms and acceptor impurity atoms to form an N-type semiconductor layer and a P-type semiconductor layer, respectively. These GaN layers are epitaxially grown on a carrier wafer (also called substrate) which is made of sapphire, silicon carbide (SiC), silicon (Si), or GaN. When a bias is applied across the doped layers, electrons from the conduction band of the n-type semiconductor layer and holes from the valence band of the p-type semiconductor layer flow toward the p-n junction which is sandwiched between two layers. The intracrystalline recombination of these holes and electrons releases energy in the form of photons, which are projected outwards as light. The short wavelength monochromatic light is then converted to broad-spectrum white light by a phosphor layer which is deposited over the LED chip.As you can see, an LED package has three essential elements: GaN layers, a wafer that supports the epitaxial growth of the GaN layers, and a phosphor layer to down-convert part of the blue or other short wavelength emission from GaN LEDs in order to produce white light. However, in practical applications an LED package has a more complex architecture. Today the majority of LEDs are designed in surface-mount plastic leaded chip carrier (PLCC) packages. In this type of packages, the LED die is mounted on a silver (Ag)-plated metal lead frame. The lead frame sits in a plastic cavity formed by highly reflective material such as PPA, PCT or EMC. The positive and negative electrodes of the lead frame are wire bonded to the anode ohmic contact formed on the P-type GaN layer and the cathode ohmic contact formed on the n-type GaN layer, respectively. In high power LED packages , the LED die is mounted a metallized ceramic substrate which has positive and negative terminal pads on the bottom of the substrate. Wire bonding is needed to connect cathode of the junction to the negative terminal via an electrical interconnect layer. The substrate may have an array of thermal vias and a thermal pad on the underside that help improve the thermal performance.Conventional wire-bonded, submount-underlaid LED packages were overdesigned, which incurs a high package cost and introduces a number of parasitics. Packaging represents a significant portion in the typical cost breakdown of an LED package. In general, this cost element can constitute 60% of the total cost for high power LED packages and over 50% of the total cost for mid-power LED packages . The overdesigned architecture is accompanied by a large package form factor as well as increased lumen depreciation and color shift factors. The mid-power PLCC packages, in particular, are prone to accelerated lumen decay and color shift because the plastic housing has poor thermal and photo stability. The silver plated lead frame is susceptible to interaction with corrosive gases such as hydrogen sulfide (H2S) and may eventually lead to intermittent or open wire bond stitch. Wire bonding that connects electrodes of the chip to the lead frame also limits the packing and power density of the LED. Furthermore, a common open failure in conventional LEDs is often due to a broken wire bond.CSP LEDs are the latest incarnation of flip-chip LEDs which are designed to prevent light loss due to the mounting of electrode pad on the upside of P-type GaN layer while improving heat transfer efficiency and package reliability. In a flip-chip CSP LED, photons are pumped from the junction through the a light-transmissive wafer (substrate), instead of the P-type GaN layer as with the PLCC type mid-power LEDs and ceramic-based high power LEDs. The transparent wafer, N-type and P-type GaN layers grown on the wafer are flipped downward the bottom. In conventional LED architectures, the wafer is mounted on the plastic submount or ceramic substrate and faces upward. This design makes the wafer an obstructive element which blocks the electrical path to the electrodes and adds thermal resistance to the thermal path from the LED junction to a PCB.The epitaxial P-type GaN layer of a flip-chip CSP LED is the bottom layer which thermally and electrically interfaces the anode electrode on the underside of the CSP package. Electrical connection to the N-type GaN layer is made through the insulated, metal-deposited vias which pass through the P-type GaN layer and active layer. Chip scale packaging goes a step further by eliminating the submount that comes with flip-chip packages. The bottom surface of the flip-chip LED is exposed to allow solder connections to the anode and cathode electrodes. A conformal phosphor coating is applied directly onto flip-chip LED die, either just over the top surface of the wafer, or on all five facets of the chip.The design of CSP LEDs drew on packaging know-how acquired by the semiconductor component industry to remove as many of the superfluous packaging elements as possible. CSP LEDs are also called package-free LEDs because the submount or package substrate are stripped out and no bonding wires are required, which translates to significantly reduced BOM cost. Wafer level packaging eliminates the need to go through a multitude of steps in a packaging line, allowing the processing cost to be cut down drastically. The technology scales down the size of an LED package to a size hardly larger than that of the LED die itself and thus saves considerable space.The advent of CSP LEDs has an epoch-making significance in the history of LED lighting , in particular when it comes to fact that the notoriously unreliable PLCC-type mid-power LEDs has been occupying a dominant market share for their cost advantage as compared with the ceramic-based high power LEDs. Mid-power LEDs can often exhibit more rapid lumen degradation and undergo more chromaticity shift mechanisms than high-power LEDs. This faster degradation in light output and color stability is largely due to the use of the plastic resin from which the LED housing is molded. Despite the use of epoxy mounting compound (EMC) which has relatively higher resistance to thermal degradation and photo oxidation, the most common polymer matrices used for molding reflective cavity are cheap polyphthalamide (PPA) and polycyclohexylenedimethylene terephthalate (PCT). These synthetic plastic resins will discolor at high temperatures, which not only leads to color shift in the blue direction, the reduced reflectivity due to resin discoloration and lead frame oxidation also substantially compromises luminous efficacy.CSP LEDs make a lot of sense not just because the package-free design allows the cost savings from stripping of superfluous elements to be passed on to end-users. The excellent lumen maintenance, color stability and package reliability of CSP LEDs enable these low cost packages to deliver a more sustainable solution and a longer ROI than the spectrally short-lived, thermally unstable mid-power PLCC LEDs . The internal construction of mid-power PLCC packages includes a reflective cavity to prevent the otherwise leakage of light from the underside of transparent sapphire substrates. While CSP LEDs still require reflective coating of the bottom and sometimes side walls to redirect the scattered light toward the top, the flip-chip structure turns the transparent sapphire substrate into a light extraction window, rather than a light loss surface. This feature, combined with the direct thermal and electrical interfacing of with the N-type and P-type GaN layers with the bottom anode and cathode electrodes, allows CSP packages to completely remove the LED housing and lead frame assembly.Removal of plastic submounts and lead frames results in the absence of one of PLCC-LED's dominant lumen depreciation and color shift factors in CSP packages. The wire-bonding-free design further contributes to the reliability and performance of CSP LEDs. Bonding wires that connect the lead frame to LED electrodes can break due to internal stress in the encapsulating resin, excessively large flows of electrical current, or electromigration. No bonding wire connections allow CSP LEDs to eliminate the risk factor of open circuit failures within the package. In CSP LEDs, the P-type GaN layer is in direct contact with the anode pad and the N-type GaN layer is connected to the cathode pad through metalized vias. This design reduces both the length of thermal path and the thermal resistance along the thermal path, which allows effective thermal management. Thermally efficient design and a robust electrical conduction path allow the LEDs to be driven at higher currents than conventional LED packages. As a result, CSP LEDs deliver high flux density in an ultra-compact form factor and redefine lumen maintenance, color stability and reliability.Although the advantages of the CSP approach are not in doubt, CSP LEDs aren't without their challenges. The flip-chip structure requires the top-mount wafer to have high optical transparency so that a good photon extraction efficiency can be achieved. The optical property of sapphire fits in with this specification, but this wafer material has a high dislocation density (13%) in the epitaxial films. High lattice mismatch to GaN results in a low internal quantum efficiency (IQE) that the LED industry has been struggling with. IQE improvement, however, is a universal challenge to the LED industry. To bring package features down into the chip level, wafer scale packaging is employed for today's CSP architectures. For LED packaging at this level, wafer configuration is critical to phosphor integration.Wafer level chip scale packaging (WLCSP) is typically used to produce surface emitters (light is emitted from the top surface, as opposed to volume emitters which produce emission from all five facets). In this process, phosphor coating is made on the entire epitaxial wafer before it is diced into individual CSP packages. Blue photons, however, will escape from the sidewalls of sapphire-based devices. This means implementing WLCSP on GaN-on-Sapphire flip-chip LEDs becomes a challenge as the epitaxial wafer has to be diced into individual dies to ensure good side wall coverage on the chip with phosphor dispensing. Chip binning is required to ensure minimal within-wafer pump wavelength variation across the individual dies before they're batch dispensed with phosphor.To eliminate blue leak from wafers and further drive down the wafer cost, silicon has been introduced into WLCSP as a transitory wafer material. The silicon wafer is removed after the epitaxial growth is completed. Phosphor is coated as a film directly over the GaN layer. With this method, the problem of blue leak is addressed, and no additional steps such as chip dicing, binning and transferring are required. But the challenge comes from growing high quality GaN epilayers on silicon, which has 17% lattice mismatch and 54% CTE mismatch to GaN. High density threading dislocations due to lattice mismatch and cracking or bowing due to the thermal mismatch impede the use of cost competitive silicon wafers for mass production of GaN LEDs.The major challenge to the LED industry at the package level is the fundamental Stokes losses which occur as blue photons are down converted to green and red photons. As a result, the overwhelming majority of the lighting products for interior lighting applications have a mediocre 80 CRI and a high color temperature . Faithful reproduction of colors to create a visually pleasing environment, and productive implementation of visually demanding tasks, including reading tasks, detail work and color-critical tasks all rely on a high CRI light source. Ever since the phase-out of 90+CRI incandescent lamps, the highly saturated colors of everything around us under artificial lighting are significantly distorted. The lighting industry continues to churn out poor color quality LED products with an extremely high correlated color temperature (CCT) for residential lighting applications . High CCT light contains a significant portion of blue in its light spectrum. People who are exposed to blue-rich spectrum light in the evening can have their circadian rhythms disrupted because a high dosage of blue will suppress the release of melatonin, a critical hormone that signals to the human body to prepare for a regenerative sleep. Circadian disruption affects cell metabolism and proliferation, and is linked to increased incidence of diseases in modern society.Narrowing the full width half maximum (FWHM) bands for the red and green SPDs (spectral power distributions) thereby reducing Stokes loss is the key to elimination of the trade-off between luminous efficacy (or source efficiency) and color quality. The opportunity is thus to develop phosphors with narrower emission linewidths for high efficiency down-conversions. The use quantum dots (QDs) as narrow band down-converters offers an alternative approach.
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A chip-on-board (COB) LED is a multi-die LED package in which an array of diodes are directly mounted and electrically interconnected onto a metal-core printed circuit board (MCPCB) or a ceramic substrate. The die matrix is then coated with an organic polymer containing a yellow phosphor. COB LEDs are high power packages that combine high die density, high drive current, and high temperature capability to enable breakthrough changes in form factor and emission pattern of LEDs. A large light emitting surface (LES) with high lumen density and optical uniformity delivers homogenous, powerful illumination for high lumen applications (e.g. high bay lighting street lighting ). With the ability to pump out thousands of lumens from a concentrated emitting surface, COB LEDs also fits best into spot and down lighting applications which require a high center beam punch with minimal spill outside the main beam.The COB LED is essentially a package that mounts a dense array of LED dies on a large, low thermal resistance substrate. It eliminates the intermediate substrate of a surface mounted device (SMD). Shortened thermal path enables effective thermal management and a significant reduction in package profile. COB LEDs have a single circuit and a single pair of anode (positive electrode) and cathode (negative electrode) for the entire package regardless of the number of diodes mounted on the substrate. To drive the high density array of semiconductor diodes, COB LEDs require a high forward voltage (of up to 72V). The electrical connection between the diodes is often a combination of series and parallel connections such that the circuit is protected against single LED open or short failures. The smaller the pitch (center-to-center spacing between LEDs), the more uniform and luminous the emission surface is. However, very small pitches can handicap horizontal heat extraction for diodes neighbored by other diodes in every horizontal direction. The COB LED packaging process requires both wire bonding and die bonding to provide electrical connection and thermal conduction for the LED dies. After the bonding process the die matrix is covered with a phosphor silicone mixture to produce white light and to shield the chip array from the environment.The semiconductor dies that form the die matrix of the COB array are indium gallium nitride (InGaN) LEDs. The InGaN direct bandgap semiconductor is doped with acceptor impurities and donor impurities to a positively charged (P-type) layer and negatively charged (N-type) layer, respectively. These InGaN layers are grown on a sapphire, silicon carbide (SiC), or silicon wafer. The wafer material has a significant impact on the efficiency and thermal performance of the LED. Sapphire is the dominantly used die substrate material but its density of threading dislocations to epitaxial layers is much higher than SiC. This translates to relatively low internal quantum efficiency. And SiC's high thermal conductivity of 110 - 155 W/mK allows GaN-on-SiC LEDs outperform GaN-on-Sapphire LEDs in terms of thermal conduction capacity (Sapphire has a typical thermal conductivity of 46.0 W/mK). The epitaxial layers are typically stacked with a standard chip structure found in SMD devices. Lately there has been a trend to use the flip-chip structure to make a chip-scale package (CSP) for COB applications.Depending on the light output of the COB LED package, InGaN diodes of various power ratings are used. The use of low power LED dies will inevitably increase wire bonding density and subsequently the cost and process complexity, and the use of expensive high power LED dies will compromise luminous efficacy and cause heat flux concentration. Therefore most InGaN LED dies incorporated in COB packages are usually mid-power chips in the 0.2W - 0.5W range.In COB packages, the diodes are die-bonded to the substrate using an adhesive with high electrical conductivity, high thermal conductivity and high thermal stability, which is typically a silver-based epoxy. Other die bonding materials including silver-glass pastes and liquid solder are also used. The electrical path to the diodes are made with thermosonic ball bonding using gold wires which are known for their high throughput, high strength, and resistance to surface corrosion. However, intermetallic compound formation between the gold wire and the substrate occurs at a higher temperature (>120°C). This may cause bonding failures such as the Kirkendall effect due to atomic interdiffusion between the gold wire and the aluminum bond pad. Aluminum wedge bonding allows room temperature processing and fine pitch assembly with the substrate, making it a contending option for applications where high temperature bonding is a concern.Before dispensing yellow phosphor filled silicone, a dam is drawn around the phosphor area with a viscous silicone fluid. Different phosphor packaging concepts are used in COB LEDs. Cavity encapsulation is the most commonly used phosphor packaging method which dispenses the mixture of phosphor and a silicone binder directly onto the LED chips. The challenge of using this method is to ensure uniform mixing and dispersion of the binder and phosphor so that color quality is not adversely affected. Conformal phosphor coating refers to spraying phosphor with minimal binder on die surface for a very consistent coating thicknesses around the entire die. CSP-based COB LEDs typically use this method to deposit phosphors to all five facets of the die except for the one with contact pads. A more delicate COB packaging method is to apply the phosphor mix to an optical cup inside which the LED die resides. The optical cup acts as a reflector to extract more light from the die while reducing the use of phosphor material as well as improving heat dissipation. Remote phosphor solutions, which place the phosphor layer at a distanced from the die, are also an option to provide a uniform phosphor conversion layer and lower the probability of light to be scattered back on the substrate surface.The COB substrate is designed to facilitate assembly and handling of the LED package and also to ensure an efficient thermal path between the LED package and the heat sink . COB LED arrays are typically fabricated on metal core printed circuit board (MCPCB) or ceramic substrate. Ceramic substrates are noted for their high chemical and thermal stability. They are preferred in environmentally demanding applications. However, the thermal conductivity of common ceramics is low (20-30 W/mK for alumina). The aluminum nitride (AlN) ceramic has exceptional thermal conductivity, but is expensive. Compared with ceramic substrates, MCPCBs, which are designed to provide high through-board thermal conductivity, have advantages of lower costs and better mechanical strength. The most common MCPCB construction consists of a base plate made of aluminum or copper, a dielectric layer, and a top copper layer. Thermal resistance of an MCPCB depends on the chemistry of the organic dielectric layer which is sandwiched between two metal layers.The luminous efficiency of COB LEDs is inherently lower than that of mid-power LEDs which have highly reflective cavities to facilitate efficient light extraction. The internal quantum efficiency (IQE) of InGaN LEDs largely depends on the wafer material. The large mismatch (13%) between the crystal lattice structure of sapphire and that of InGaN creates a high density of threading dislocations. Recombination of electronic carriers (electrons and holes) that occurs at such sites are primarily nonradiative. SiC substrates have a substantially low mismatch to GaN (3.4%). As such, the probability of photon generation in GaN-on-SiC LEDs is intrinsically higher than that in GaN-on-Sapphire LEDs. Nevertheless, growing GaN or InGaN on foreign substrates inevitably yields epitaxial defects and dislocations which are all compromising the IQE. LEDs fabricated on homoepitaxially grown GaN substrates are a superior approach to improving internal quantum efficiency. GaN-on-GaN LEDs have no lattice mismatch and CTE mismatch between the substrate and the n-type GaN layer, and therefore induce no non-radiative recombinations due to threading dislocations.The package-level efficiency loss of LEDs occurs at the phosphor layer. Wide emission linewidths of the red and green phosphor bands cause the conversion of a part of the shorter wavelengths to longer wavelengths to take place at a poor spectral efficiency. Typically, about 15–25% of the blue light absorbed by the wide band phosphor is converted to Stokes heat. The solution is to formulate phosphors with a narrow FWHM (full width half maximum) for the red and green bands or to use quantum dots (QDs) as narrow band down-converters. Light scattering and total internal reflection (TIR) are two other major contributors to package inefficiency in the powder-in-polymer approach. Maintaining a close refractive index match between the polymer matrix and phosphor particles will reduce the scattering and TIR related light loss. An anti-reflection coating (ARC) may be applied to the encapsulant to further mitigate the total internal reflection. The remote phosphor concept is developed to produce significant gains in package efficiencies while providing a spectrally optimized output from a uniform, pixilation-free LES.The removal of the intermediate substrate in COB packages allows heat generated at the LED junction to be transferred to a heat sink via the shortest possible thermal path. The absence of lead frames and plastic housings means COB LEDs do not have to struggle with lumen depreciation factors such as discoloration and yellowing. Lumen maintenance failures in COB LEDs occur typically due to inefficient or inadequate system-level thermal management. The high die density COB LEDs produce a substantial amount of heat which, if allowed to accumulate, will accelerate the phosphor degradation process and result in a permanent reduction in light output. This degradation can be exacerbated by the presence of moisture and contaminants as the gas/humidity permeability of silicone polymers increases at higher temperature. Entrapped moisture and volatile organic compound (VOC) in the encapsulation significantly decreases the conversion efficiency of silicone/YAG phosphor composites.As with the lumen depreciation factors, the color maintenance failures of COB LEDs are primarily caused by high thermal stress, and diffusion or reaction of moisture and contaminants at the phosphor layer. The color shift behaviors of COB LEDs usually fall between the blue and green directions. A drop in phosphor quantum efficiency due to thermal degradation or chemical reaction will develop a chromaticity shift in the blue direction. Overdriving of the LEDs will cause the phosphor to operate above the saturation flux level, and the chromaticity shift can move toward the blue direction. The presence of moisture may lead to a decrease in peak wavelength in red phosphors, which causes a blue shift in the initial stage. Blue shift will also occur when there's settling and precipitation of the phosphor as a result of improper mixing and dispersion of the binder and phosphor. Photo-oxidation of the phosphor matrix will introduce a shift in the green direction. Additionally, the degradation in reflective white coating on the MCPCB can be a contributor to a spectral shift of the LED emission and a decrease in efficacy.COB LEDs are binned according to color coordinates (chromaticity), lumen output, and forward voltage to minimize differences in color and output that might be visible from fixture to fixture. Compared with discrete mid-power LEDs, the flux binning of COB LEDs is more important because COB lighting systems often incorporate single- LED modules . As always, keeping chromaticity coordinates under tight control is a critical detail in architectural lighting . To counter chromaticity variability that is inherent in the manufacturing process, COB LEDs are sorted into bins based on the Standard Deviation Color Matching (SDCM) MacAdam ellipses or the American National Standards Institute (ANSI) parallelograms.A MacAdam ellipse is an elliptical zone established around a chromaticity coordinate in the CIE 1931 (x,y) color space. The smaller the ellipse, the less color variation. The ANSI color binning system uses parallelograms to quantify the perceptual difference between LEDs. The parallelograms used by ANSI to define the color bins in the C78.377-2008 standard encloses a 7-step MacAdam ellipse and are centered on the black body line. To this day, many lighting professionals use "MacAdam ellipses" to define the level of color consistency. High end architectural lighting typically uses COB LEDs with 2- or 3-step MacAdam ellipse color tolerance. Chromaticity deviations at a 3-step MacAdam ellipse are considered barely perceptible. In general lighting applications a 5-step ellipse is still sufficient, and a 7-step ellipse can be tolerated for entry-level applications.COB LEDs are a popular type of light source for architectural-grade downlights and spotlights, despite the fact that these large-LES light sources require a very large optical assembly (such as a TIR lens and reflector combo) to achieve a directional light output and a controlled beam angle. In lighting design for retail and hospitality environments, or museums and galleries, high fidelity color rendering is a must-have feature. At present, color quality is measured by a metric called color rendering index (CRI). However, the CRI value Ra does not take into consideration the ability of the light source to faithfully reproduce highly saturated colors. Thus R9, the index for a saturated red, is sometimes listed individually as a supplement to the CRI general index. A minimum Ra of 90 and R9 of 60 is generally required to reveal the true character and quality of merchandise, to create a visually appealing environment, or to accentuate the texture, color and shape of exhibits in aforementioned environments.In order for a phosphor-converted LED to render colors accurately, the phosphor emission has to cover as broad a wavelength range as possible. However, in current phosphor-conversion systems there's an intrinsic trade-off between the CRI of an LED and its luminous efficacy. This is because wavelength conversion at the wide FWHM red and green phosphor bands causes a significant amount of Stokes energy loss. Two strategies are being researched to overcome this limitation: using a narrow-band red phosphor and down-converting the blue light using quantum dots. The use of quantum dots to generate spectrally narrow primaries has emerged as the preferred choice.COB LEDs can be either mounted directly to a heat sink and discrete wires are used to deliver power to the LED, or attached a heat sink via a COB connector (holder) which offers solderless electrical connection to LED and poke-in wire termination. In direct attachment designs, a thermal interface material (TIM) must be applied between the COB substrate and heat sink. The TIM ensures the thermal impedance between the LED and heat sink is reduced to a minimum by completely filling interfacial air gaps and voids. The use of a COB connector simplifies assembly and can facilitate aligning the secondary optic with the LES. The Zhaga Consortium has standardized a family of COB form factors to enable the interchange of COB modules and COB holders made by different manufacturers.
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