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What Is a CSP LED​

A chip scale package (CSP) LED is defined as an LED package that has a close ratio between the volume of the

How LEDs Work​

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

Conventional LED Architectures​

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

The Downsides of Overdesigned Packages​

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

CSP LED Architecture​

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.

Advantages of Chip Scale Packaging​

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

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

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.

Challenges​

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

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.

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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What Is a Horticulture LED​

Horticulture LEDs are solid state light sources that produce photosynthetically active radiation (PAR) over the spectral range of 400 to 700 nanometers (nm) to drive photosynthesis in plants. Additionally, horticulture LEDs can be used to produce electromagnetic radiation that targets the active spectrum of photomorphogenesis, photoperiodism and phototropism in plants.

Light and Plant Growth​

Light is essential for plant growth. All plants, including those flowering, fruiting, and vegetable plants, are autotrophs that have evolved to use light to drive photosynthesis. Photosynthesis is the process used by plants to convert water and carbon dioxide into complex carbohydrates (sugars) and oxygen. These carbohydrates (e.g. cellulose or glucose) provide the metabolic building blocks for the various biosynthetic pathways. The excess of carbohydrates is used for biomass formation which includes stem elongation, increase of leaf area, flowering, fruit formation, etc. The photoreceptor responsible for photosynthesis is chlorophyll, although other types of antenna photoreceptors (primarily carotenoids) also promote photosynthesis. Apart from driving photosynthesis, electromagnetic radiation at specific wavelengths is used as a source of information to drive photomorphogenesis (plant morphology changes induced by light), photoperiodism (the response to the light-dark cycle) and phototropism (direction of growth). Each type of photoreceptors is sensitive to specific wavelengths and drives a different subset of photomorphogenic changes.

Chlorophylls, the key photoreceptor in green plants, come in two major forms, A and B. Chlorophyll A is the primary phytopigment that accounts for around 75% of photosynthetic activity and has a peak absorption response at 430 nm and 680 nm. Chlorophyll B, which has absorption peaks at 460 nm and 640 nm, is an accessory pigment that collects energy and passes it on to chlorophyll A. Therefore chlorophyll B does not independently contributes to biosynthesis. Furthermore, the 3:1 ratio of chlorophyll A to B in plants indicates the dominant dependence of plants on chlorophyll A in photosynthesis. While chlorophyll levels are increased under electromagnetic radiation with a spectral composition rich in red (long wavelength) and blue (short wavelength), chlorophylls reflect a majority of the wavelengths in the green region (550 nm to 650 nm) and that's why the leaves appear green.

The carotenoid family includes beta-carotene and the principal xanthophylls (zeaxanthin, violaxanthin and lutein). These secondary metabolites absorb light most strongly in the range of 450 nm to 550 nm. Carotenoids are yellow to orange in color because they reflect or transmit light having a wavelength spectrum between about 550 and 650 nm. Carotenoids not only contribute to photosynthesis, but also protect the chlorophylls from photooxidation by dissipating excess light as heat when the photosynthetic region is overloaded with incoming energy.

There are also non-photoreceptor and non-photomorphogenic antenna pigments in plants such as anthocyanins and flavonoids. They function as sunblock and impede superoxide production in response to high intensity blue (400-500nm) or ultraviolet (300-400nm) radiation. In plants, anthocyanins, flavonoids and carotenoids are important bioactive antioxidants which quench free radicals and eliminate compounds that may cause photobleaching and growth inhibition.

Photomorphogenesis is mediated by the phytochrome, cryptochrome and phototropin photoreceptors. The phytochrome photopigment has two isoforms called Pr and Pfr which respond to 660 nm red and 735 nm infrared radiation respectively. Different photomorphogenetic responses mediated by phytochromes are sent to metabolic pathways within the plant that regulate seed germination, root development, tuber and bulb formation, leaf expansion, stem elongation, dormancy, flowering and fruit production. Cryptochromes which absorb light in the range from 340 nm to 520 nm prevent elongation of hypocotyls and mediate the entrainment of the circadian rhythms in flowering plants. The phototropins are plasma membrane-localized protein kinases that regulate phototropism, chloroplast accumulation, stomatal aperture, leaf flattening, and inhibition of leaf expansion.

What Is an LED​

An LED (light-emitting diode) is a semiconductor device that converts electric energy into electromagnetic radiation in the visible range of the spectrum (light). Professionally speaking, an LED is a packaged device composed of a light source with mechanical supports, electrical and thermal interfaces, and an encapsulation which prevents mechanical and thermal stress shock and humidity-induced corrosion and is oftentimes phosphor-mixed to spectrally alter the light composition. Core to an LED package is a semiconductor chip (the light source, also known as a die) which is generally formed by growing epitaxial layers on a substrate. The epitaxial layers are doped with impurities to create a P-N junction. When a forward bias is applied to the diode, electrons from the positively charged (P-type) layer and holes from the negatively charged (N-type) layer flow into the P-N junction and recombine with each other. Each electron-hole recombination releases a quantum of energy in the form of a photon. A photon is a packet of electromagnetic radiation that is perceived as light.

The LED Advantages in Horticulture Lighting​

High energy efficiency and long lifespan are the iconic benefits of LED technology. In horticultural lighting, efficiency has another version of interpretation. Traditionally, horticulture lighting systems had made use of high pressure sodium (HPS), metal halide (MH) lamps, or in some cases fluorescent lamps. However, these light sources has a very low energy conversion efficiency (typically less than 20%). In contrast,

In this industry, the source or system efficiency is converted to photon efficacy which quantifies how efficient the LED is at creating photosynthetic photon flux (PPF) per joule of electrical energy used, instead of luminous efficacy which describes the ability to produce a visual sensation in the human eye. PPF refers to the total amount of photosynthetically active photons generated by a light source and measured in micromoles per second (µmol/s). The photon efficacy of horticulture LEDs is referenced in PPF/W and measured in µmol/J. In practices,

Spectral engineering has been a central theme of horticultural lighting since its beginning. As previously noted, the bandwidth of light between 400 and 700 nm is the primary part of the electromagnetic spectrum that can stimulate phytopigments for photosynthesis in plants. Even within the PAR spectrum, not all wavelengths of light are equally efficient in driving plant photosynthesis. Red and blue wavelengths stimulate photosynthesis and control plant morphology most effectively, and the wavelengths that fall within the green portion of the PAR range have a very limited effect on plant growth.

Spectral efficiency describes how well the spectral power density (SPD) of light source overlaps with the desired action spectrum for the most effective photosynthetic response. HPS, MH and fluorescent lamps have a poor spectral efficiency since their SPDs contain a considerable portion of photosynthetically inactive light such as infrared radiation (IR) and ultraviolet radiation (UV). The fixed SPDs of these broad spectrum light source means photosynthetically active radiation can be oversaturated in some wavelengths and deficient in other wavelengths.

Greater spectral control is one of fundamental advantages that LEDs maintain over traditional horticulture lighting systems. LEDs by nature are monochromatic light sources that emit in narrow spectral bands, resulting in a color light, such as red, blue or green. The narrow bandwidth spectrum emitted from LEDs can be easily tailored to correspond to the photosynthetic peaks of the PAR curve. Narrow band LEDs can be converted to a polychromatic light through phosphor conversion for a broader spectrum to support full cycle plant growth. Multiple-channel LEDs in

Unlike metal halide and high pressure sodium lamps that dissipate a large amount of infrared energy (heat) in the radiant beam of light, LEDs do not radiate thermal IR energy in its light spectrum. Absence of radiant heat allows for maximum photon irradiance at close proximity to plant canopy, which ultimately leads to better photosynthetic productivity while saving space and energy. High radiant heat flux from the HPS grow lights demands a distance between the light source and the plant and thus these light fixtures can only be used in toplighting applications. LED technology enables novel strategies such as intracanopy lighting (interlighting) to be implemented for uniform photosynthetic illuminances throughout the canopy - without unwanted heat generation.

How Are Horticulture LEDs Made​

The epitaxial layers of horticulture LEDs are made from direct bandgap semiconductors for their higher probability of radiative recombination over semiconductors with an indirect band gap. Two primary semiconductor families are the nitride diodes and the phosphide diodes. Indium gallium nitride (InGaN) of the nitride family produces electromagnetic radiation in the shorter-wavelength parts of the visible spectrum and is therefore used to build white, green, cyan, blue and royal blue diodes. Red, red-orange and amber light may be produced using LEDs formed from the phosphide semiconductors such as aluminum indium gallium phosphide (AlInGaP), the small energy band gap of which enables the diode to produce longer-wavelength radiation.

InGaN epitaxial layers are grown on a sapphire, silicon carbide (SiC), or silicon substrate (wafer), whereas AlInGaP epitaxial layers are grown on a gallium arsenide (GaAs) or gallium phosphide (GaP) substrate. High quality epitaxial growth depends on the lattice match of the substrate material to the InGaN or AlInGaP layer. Any mismatch between the substrate and semiconductor layer results in a microcrack (threading dislocation). This type of atomic defect causes recombinations between electrons and holes to occur in a nonradiative manner and thus compromises the internal quantum efficiency (IQE) of the LED. Threading dislocations form in highest densities on silicon and sapphire based GaN LEDs. A SiC substrate creates much less dislocations and results in 5 to 10% efficacy advantage over chips with a silicon or sapphire substrate.

Horticulture LEDs are available in two primary groups: full spectrum LEDs and narrow band LEDs. Full (or broad) spectrum LEDs deliver the spectral composition of sunlight yet without thermal radiation and wavelength waste. These LEDs are formulated with emphasis on the blue and red regions while providing additional wavelengths such as far red and green to support full cycle cultivation and complete plant development. Narrow band LEDs provide monochromatic output to maximize the most needed wavelengths of light. These LEDs are available in deep blue (450 nm), hyper red (660 nm), far red (730 nm), and green (530 nm) colors. Purple LEDs, which is neither a full spectrum LED nor a narrow band LED but combines key wavelengths of red and blue into a single package, are also a standard offering in the market. Purple LEDs can also be mixed with the broad spectrum lime LEDs to increase yield (fresh weight) and antioxidant levels while producing high quality

Full spectrum LEDs and purple LEDs utilize wavelength conversion and color mixing to achieve the desired mixture of wavelengths. LED chips are coated or dispensed with a phosphor mixture which function to down-convert a portion of the short wavelengths into longer wavelengths. Hence these LEDs are called phosphor converted LED (PC-LEDs). In the PC-LED architecture, the Stokes loss due to phosphor down-conversion constitutes a considerable portion of total energy waste of the LED. Narrow band LEDs are direct emitters that do not go through phosphor down-conversion, and thus do not suffer from the Stokes loss.

Both phosphor converted LEDs and narrow band LEDs are typically encapsulated with silicone. The difference is that in PC-LEDs phosphors are mixed with silicone polymer to act as the down-converter and protective encapsulant, while in narrow band LEDs transparent silicone polymer is used to prevent ingress of contaminants and protect the chips from mechanical impacts. Silicon encapsulations boast of high thermal stability, photo stability and chemical resistance. However, extra ingress protection for the LEDs is necessary in real world applications because the high moisture and gas permeability of silicone can be a diode degradation factor in cultivation environments with high humidity.

Types of Horticulture LEDs​

PLCC type





Chip scale package (CSP) LEDs eliminate wire bonding and submount through a flip-chip architecture. The technology significantly reduces thermal resistance within the package, scales down the package size, and drives down the cost.

Horticulture LEDs are solid state light sources that produce photosynthetically active radiation (PAR) over the spectral range of 400 to 700 nanometers (nm) to drive photosynthesis in plants. Additionally, horticulture LEDs can be used to produce electromagnetic radiation that targets the active spectrum of photomorphogenesis, photoperiodism and phototropism in plants. Horticulture lighting systems have been developed to provide supplemental photoperiodic light in greenhouse environments or to provide a sole-source of photosynthetic light in an indoor controlled environment. The use of energy- and spectrally-efficient LED technology in plant grow lights kick-started a revolution in horticultural lighting.Light is essential for plant growth. All plants, including those flowering, fruiting, and vegetable plants, are autotrophs that have evolved to use light to drive photosynthesis. Photosynthesis is the process used by plants to convert water and carbon dioxide into complex carbohydrates (sugars) and oxygen. These carbohydrates (e.g. cellulose or glucose) provide the metabolic building blocks for the various biosynthetic pathways. The excess of carbohydrates is used for biomass formation which includes stem elongation, increase of leaf area, flowering, fruit formation, etc. The photoreceptor responsible for photosynthesis is chlorophyll, although other types of antenna photoreceptors (primarily carotenoids) also promote photosynthesis. Apart from driving photosynthesis, electromagnetic radiation at specific wavelengths is used as a source of information to drive photomorphogenesis (plant morphology changes induced by light), photoperiodism (the response to the light-dark cycle) and phototropism (direction of growth). Each type of photoreceptors is sensitive to specific wavelengths and drives a different subset of photomorphogenic changes.Chlorophylls, the key photoreceptor in green plants, come in two major forms, A and B. Chlorophyll A is the primary phytopigment that accounts for around 75% of photosynthetic activity and has a peak absorption response at 430 nm and 680 nm. Chlorophyll B, which has absorption peaks at 460 nm and 640 nm, is an accessory pigment that collects energy and passes it on to chlorophyll A. Therefore chlorophyll B does not independently contributes to biosynthesis. Furthermore, the 3:1 ratio of chlorophyll A to B in plants indicates the dominant dependence of plants on chlorophyll A in photosynthesis. While chlorophyll levels are increased under electromagnetic radiation with a spectral composition rich in red (long wavelength) and blue (short wavelength), chlorophylls reflect a majority of the wavelengths in the green region (550 nm to 650 nm) and that's why the leaves appear green.The carotenoid family includes beta-carotene and the principal xanthophylls (zeaxanthin, violaxanthin and lutein). These secondary metabolites absorb light most strongly in the range of 450 nm to 550 nm. Carotenoids are yellow to orange in color because they reflect or transmit light having a wavelength spectrum between about 550 and 650 nm. Carotenoids not only contribute to photosynthesis, but also protect the chlorophylls from photooxidation by dissipating excess light as heat when the photosynthetic region is overloaded with incoming energy.There are also non-photoreceptor and non-photomorphogenic antenna pigments in plants such as anthocyanins and flavonoids. They function as sunblock and impede superoxide production in response to high intensity blue (400-500nm) or ultraviolet (300-400nm) radiation. In plants, anthocyanins, flavonoids and carotenoids are important bioactive antioxidants which quench free radicals and eliminate compounds that may cause photobleaching and growth inhibition.Photomorphogenesis is mediated by the phytochrome, cryptochrome and phototropin photoreceptors. The phytochrome photopigment has two isoforms called Pr and Pfr which respond to 660 nm red and 735 nm infrared radiation respectively. Different photomorphogenetic responses mediated by phytochromes are sent to metabolic pathways within the plant that regulate seed germination, root development, tuber and bulb formation, leaf expansion, stem elongation, dormancy, flowering and fruit production. Cryptochromes which absorb light in the range from 340 nm to 520 nm prevent elongation of hypocotyls and mediate the entrainment of the circadian rhythms in flowering plants. The phototropins are plasma membrane-localized protein kinases that regulate phototropism, chloroplast accumulation, stomatal aperture, leaf flattening, and inhibition of leaf expansion.An LED (light-emitting diode) is a semiconductor device that converts electric energy into electromagnetic radiation in the visible range of the spectrum (light). Professionally speaking, an LED is a packaged device composed of a light source with mechanical supports, electrical and thermal interfaces, and an encapsulation which prevents mechanical and thermal stress shock and humidity-induced corrosion and is oftentimes phosphor-mixed to spectrally alter the light composition. Core to an LED package is a semiconductor chip (the light source, also known as a die) which is generally formed by growing epitaxial layers on a substrate. The epitaxial layers are doped with impurities to create a P-N junction. When a forward bias is applied to the diode, electrons from the positively charged (P-type) layer and holes from the negatively charged (N-type) layer flow into the P-N junction and recombine with each other. Each electron-hole recombination releases a quantum of energy in the form of a photon. A photon is a packet of electromagnetic radiation that is perceived as light.High energy efficiency and long lifespan are the iconic benefits of LED technology. In horticultural lighting, efficiency has another version of interpretation. Traditionally, horticulture lighting systems had made use of high pressure sodium (HPS), metal halide (MH) lamps, or in some cases fluorescent lamps. However, these light sources has a very low energy conversion efficiency (typically less than 20%). In contrast, LED chips have a wall plug efficiency of up to 66% and the phosphor-converted LEDs have a radiant efficiency well over 40%.In this industry, the source or system efficiency is converted to photon efficacy which quantifies how efficient the LED is at creating photosynthetic photon flux (PPF) per joule of electrical energy used, instead of luminous efficacy which describes the ability to produce a visual sensation in the human eye. PPF refers to the total amount of photosynthetically active photons generated by a light source and measured in micromoles per second (µmol/s). The photon efficacy of horticulture LEDs is referenced in PPF/W and measured in µmol/J. In practices, LED grow lights can hit a photon efficacy of 3.2 PPF/Watt, whereas a typical HPS grow light can reach only up to 1.7 PPF/Watt.Spectral engineering has been a central theme of horticultural lighting since its beginning. As previously noted, the bandwidth of light between 400 and 700 nm is the primary part of the electromagnetic spectrum that can stimulate phytopigments for photosynthesis in plants. Even within the PAR spectrum, not all wavelengths of light are equally efficient in driving plant photosynthesis. Red and blue wavelengths stimulate photosynthesis and control plant morphology most effectively, and the wavelengths that fall within the green portion of the PAR range have a very limited effect on plant growth.Spectral efficiency describes how well the spectral power density (SPD) of light source overlaps with the desired action spectrum for the most effective photosynthetic response. HPS, MH and fluorescent lamps have a poor spectral efficiency since their SPDs contain a considerable portion of photosynthetically inactive light such as infrared radiation (IR) and ultraviolet radiation (UV). The fixed SPDs of these broad spectrum light source means photosynthetically active radiation can be oversaturated in some wavelengths and deficient in other wavelengths.Greater spectral control is one of fundamental advantages that LEDs maintain over traditional horticulture lighting systems. LEDs by nature are monochromatic light sources that emit in narrow spectral bands, resulting in a color light, such as red, blue or green. The narrow bandwidth spectrum emitted from LEDs can be easily tailored to correspond to the photosynthetic peaks of the PAR curve. Narrow band LEDs can be converted to a polychromatic light through phosphor conversion for a broader spectrum to support full cycle plant growth. Multiple-channel LEDs in RGB , RGBA or RGBW combinations can additively mix out any colors within the constituent LEDs, enabling unprecedented spectral flexibility and efficiency.Unlike metal halide and high pressure sodium lamps that dissipate a large amount of infrared energy (heat) in the radiant beam of light, LEDs do not radiate thermal IR energy in its light spectrum. Absence of radiant heat allows for maximum photon irradiance at close proximity to plant canopy, which ultimately leads to better photosynthetic productivity while saving space and energy. High radiant heat flux from the HPS grow lights demands a distance between the light source and the plant and thus these light fixtures can only be used in toplighting applications. LED technology enables novel strategies such as intracanopy lighting (interlighting) to be implemented for uniform photosynthetic illuminances throughout the canopy - without unwanted heat generation.The epitaxial layers of horticulture LEDs are made from direct bandgap semiconductors for their higher probability of radiative recombination over semiconductors with an indirect band gap. Two primary semiconductor families are the nitride diodes and the phosphide diodes. Indium gallium nitride (InGaN) of the nitride family produces electromagnetic radiation in the shorter-wavelength parts of the visible spectrum and is therefore used to build white, green, cyan, blue and royal blue diodes. Red, red-orange and amber light may be produced using LEDs formed from the phosphide semiconductors such as aluminum indium gallium phosphide (AlInGaP), the small energy band gap of which enables the diode to produce longer-wavelength radiation.InGaN epitaxial layers are grown on a sapphire, silicon carbide (SiC), or silicon substrate (wafer), whereas AlInGaP epitaxial layers are grown on a gallium arsenide (GaAs) or gallium phosphide (GaP) substrate. High quality epitaxial growth depends on the lattice match of the substrate material to the InGaN or AlInGaP layer. Any mismatch between the substrate and semiconductor layer results in a microcrack (threading dislocation). This type of atomic defect causes recombinations between electrons and holes to occur in a nonradiative manner and thus compromises the internal quantum efficiency (IQE) of the LED. Threading dislocations form in highest densities on silicon and sapphire based GaN LEDs. A SiC substrate creates much less dislocations and results in 5 to 10% efficacy advantage over chips with a silicon or sapphire substrate.Horticulture LEDs are available in two primary groups: full spectrum LEDs and narrow band LEDs. Full (or broad) spectrum LEDs deliver the spectral composition of sunlight yet without thermal radiation and wavelength waste. These LEDs are formulated with emphasis on the blue and red regions while providing additional wavelengths such as far red and green to support full cycle cultivation and complete plant development. Narrow band LEDs provide monochromatic output to maximize the most needed wavelengths of light. These LEDs are available in deep blue (450 nm), hyper red (660 nm), far red (730 nm), and green (530 nm) colors. Purple LEDs, which is neither a full spectrum LED nor a narrow band LED but combines key wavelengths of red and blue into a single package, are also a standard offering in the market. Purple LEDs can also be mixed with the broad spectrum lime LEDs to increase yield (fresh weight) and antioxidant levels while producing high quality white light to aid in visual inspection and plant harvesting.Full spectrum LEDs and purple LEDs utilize wavelength conversion and color mixing to achieve the desired mixture of wavelengths. LED chips are coated or dispensed with a phosphor mixture which function to down-convert a portion of the short wavelengths into longer wavelengths. Hence these LEDs are called phosphor converted LED (PC-LEDs). In the PC-LED architecture, the Stokes loss due to phosphor down-conversion constitutes a considerable portion of total energy waste of the LED. Narrow band LEDs are direct emitters that do not go through phosphor down-conversion, and thus do not suffer from the Stokes loss.Both phosphor converted LEDs and narrow band LEDs are typically encapsulated with silicone. The difference is that in PC-LEDs phosphors are mixed with silicone polymer to act as the down-converter and protective encapsulant, while in narrow band LEDs transparent silicone polymer is used to prevent ingress of contaminants and protect the chips from mechanical impacts. Silicon encapsulations boast of high thermal stability, photo stability and chemical resistance. However, extra ingress protection for the LEDs is necessary in real world applications because the high moisture and gas permeability of silicone can be a diode degradation factor in cultivation environments with high humidity.PLCC type mid-power LEDs (surface-mount devices that consume less than 1 watt of power) are the most popular light source for both general illumination and horticulture lighting simply because they have a relatively higher efficacy and lower cost than other types of packages. Nevertheless, this type of LEDs is highly prone to accelerated performance degradation and premature failure. As such, a very competitive initial cost often does not translate to a good return on investment (ROI), a long payback period, and peace of mind. PLCC is short for plastic leaded chip carrier. A mid-power LED using this architecture has its chip mounted on a silver (Ag)-coated metal lead frame molded in a plastic housing wherein a reflective cavity is formed to improve light extraction. The cavity is filled with a clear or phosphor mixed silicone polymer to encapsulate the chip. Electrical connection and thermal path between the LED chip and lead frame is made through wire bonding. The cavity or plastic housing of cheap products is made of polyphthalamide (PPA) or polycyclohexylenedimethylene terephthalate (PCT) which has poor resistance to photo-oxidation and thermal degradation. The silver lead frame plating is susceptible to corrosion due to interaction with sulfur-containing contaminants, which can penetrate into LEDs through the silicone encapsulation. Wire bonding used in PLCC packages can break. The inefficient thermal conduction path may cause heat flux concentrations which introduce a high thermal stress to the LED. High power LEDs fabricated on ceramic substrates have a robust thermal conduction path that enables a high photosynthetic photon flux density (PPFD) to be delivered to plant canopies. High power LEDs may be driven at current from hundreds of mA to more than an ampere and produce over 10 µmol/s of photosynthetic photon flux from a single package. A large chip or a multi-die array is mounted onto a ceramic substrate which is metalized and comes with thermal vias to provide efficient heat dissipation. Excellent PPF maintenance and wavelength stability justify the higher cost of these ceramic based horticulture LEDs. Chip-on-board (COB) LEDs provide a large light emitting surface (LES) that enables delivery of high and uniform PPFD values across an entire canopy. A COB LED package consists of a dense array of LED chips which are die-bonded onto a metal-core printed circuit board (MCPCB) or a ceramic substrate. This large, low thermal resistance substrate enables better thermal contact with a flat, clean heat sink . The removal of the intermediate substrate reduces the thermal resistance of the package. Efficient thermal design allows the COB package to operate at high current density and deliver a PPF of up to hundreds of micromoles per second.Chip scale package (CSP) LEDs eliminate wire bonding and submount through a flip-chip architecture. The technology significantly reduces thermal resistance within the package, scales down the package size, and drives down the cost. CSP LEDs fundamentally address the performance degradation factors in PLCC type mid-power LEDs, making them a very attractive solution for the horticulture lighting industry.

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