Table of Contents Hide
What Is a COB LED
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.Architecture
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.Die Fabrication
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.
COB LED Packaging
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.
LED Efficiency
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.
Lumen Maintenance
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.Color Stability
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.Color Consistency
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.
Color Reproduction
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.
