Chip-on-Board (COB) LED packaging refers to placing the LED die directly on a PCB substrate where it is fixed by silvered epoxy or solder. The intended circuit is then completed with wire bonds between the LED chips and the PCB. Generally, an encapsulation, hard of soft, is used to protect the wire bonds and LEDs. As the LED chip is the smallest light building block available, the light engine itself can be a highly customized, application specific device with many LEDs with almost any spacing or array dimension. Multiple colors and driving circuits can be designed. Optics can be mounted directly to the PCB. The electronics if desired can be on or off the PCB (or light). The LED chips ranging in size generally from ~300 microns to 1-mm per side can be considered Lambertian emitters and have no integrated optics like common through-hole or surface mount devices. This allows the optical performance to be customized for the application starting with the most basic element. The chips are directly mounted to PCB substrate so the heat is removed from the LED more efficiently than through several intermediate junctions in a surface mount LED or through the leads in a through-hole LED package. In short Chip-on-Board is ideal for the highest performance or highly constrained applications.
HumenChem contains other products and information you need, so please check it out.
MTBF is not generally used in the LED industry because LEDs when used properly, that is with good heat sinking and without excessive current, don’t generally catastrophically fail. Their light output will gradually decrease from the initial maximum. LED lifetime is generally defined by the time it takes for the maximum optical power to decrease to 70% of its original. This is termed “L70” by the larger LED industry.
We rely on the specification of the LEDs run at rated current by our vendors to estimate lifetime. The lifetime will be at least 50,000 hours and could be 100,000. Our lights feature better thermal management than is typically assumed so even when run continuously at higher currents we expect to achieve 50,000 hours at minimum. Strobing or intermittent operation generally increases the effective lifetime because heat is not built up and the light is off more often than on. MTBFs will be 100,000 hours or more.
Yes.
Strobing is generally done to stop motion in a machine vision application or produce a very brief pulse of light much brighter than at the rated current. Remember an overdriven pulse that might be 10 times brighter will put out the same amount of light as the light would if left on at the original current for 10 times longer. Intensity in a camera exposure or process is directly proportional to the length of the time the light is on and the amount of light emitted per unit time. The main function of strobing is to stop motion with fast well timed pulses. Overdriving allows for either adequate exposure levels or shorter pulses and therefore faster processes. Strobing or simple on/off control also reduces the affects of heat on the lights.
Strobing or frequent on/off operation does not damage LED lights so long as it is done properly. However, it is very easy to damage LED lights by strobing them if it is done improperly. All strobing is done at your risk.
The LEDs themselves have response times of the order of nanoseconds and track the power supplied to them extremely closely. However, the relation between light output and current is not linear at high currents. The efficiency of the LED decreases and effectually rolls over. If you are pulsing in this regime you have a high risk of damaging the light. For support in determining the maximum safe strobe current for your application contact us.
We offer strobe controllers that can strobe as fast as a microsecond and offer programmable deterministic currents, pulse widths, delay, and duty cycle limits. With these or your own current controller it is often possible to overdrive our lights by as much as 10-15 times safely. Note that high pulse currents are only safe for short pulse widths, generally a millisecond or less, and low duty cycles, generally 10% or less.
The COBRA Slim linescan light features optional strobing functionality.
The SpecBright product family, when configured for current control, can be safely overdriven many times. The exactly amount depends on the wavelength, thermal management, pulse width, and duty cycle. SpecBright 24VDC configured lights should not be overdriven though they can be strobed for very short pulses without risk.
Unfortunately we cannot guarantee pulsed output intensities and overdriven pulsing is done at the users risk.
While many high brightness LEDs available from major manufacturers are described as products as 1W, 3W, or 5W devices this is not a good description of how an LED light will perform, just as it is not a good description of how a fluorescent or halogen light will perform. This is a description of the electrical power draw, only a fraction of which is converted to optical power and a fraction of that reaches where it is needed.
We specify our products or try to define specifications for custom designs in Watts per square meter (W/m2) as projected over a given area at specific distance from the source. That being said our lights or designs produce under a Watt of optical power to several 100 Watts of optical power. Direct comparison with legacy lighting technologies is difficult and requires very controlled testing. However, our customers have confirmed we make lights that can be brighter than halogen and other lighting technologies.
Ref. Lawal, O., Pagan, J. Hansen, M. . When Will UV-C LEDs be Suit-able for Municipal Treatment? Conference Presentation. IUVA World Congress. 18 September .
UV-C LEDs are a relatively new technology. As can be seen from the chart above, UVC LED efficiency low, below 10%. However, UVC LED development is following a similar rapid improvement profile that has occurred for other LED technologies such as red, blue and UV-A wavelengths.
Low efficiency also means that the lifetime of these LEDs is less than more mature LED technology. However, for some wavelengths, 10,000 hrs have been reported.
As applications are still being developed and LED manufacturers improve their processes, LED unit costs continue to reduce. However, the pricing is still not at the level of more mature LED wavelengths and commercially viable applications are limited for the next 3-5 years. As a result of the COVID-19 pandemic, there has been increased interest in UV-C LEDs which may increase this timeline.
The presence of particles can protect microorganisms from UV, for example, UV is not effective in highly turbid water due to low transmissivity.
UV products cannot penetrate particles like dust or oils, so dirty surfaces will cause effectiveness to drop.
Some microorganisms have built up some immunity to UV. Therefore, a higher dose of UV must be deployed to kill these microorganisms. Indeed, the type of microorganism targeted can impact results. For example, 265nm is considered the optimum wavelength to use for disinfection but 275nm has been shown to disinfect E. Coli.
Microorganisms cannot be affected if they are not exposed to the light. Any area where a shadow occurs from an obstacle not allowing the UV light to shine will have no effect on the organism.
Ref. Li X, Cai M, Wang L, Niu F, Yang D, Zhang G. Evaluation survey of microbial disinfection methods in UV-LED water treatment systems. Sci Total Environ. ;659:-. doi:10./j.scitotenv..12.344
Wavelength, Irradiance and time are the most important factors.
Wavelength: UVC wavelength range is between 100nm to 300nm. Generally, UV-C germicidal wavelength at 265nm is the most effective to kill harmful microorganisms via molecular lesions of DNA and RNA in the air, water and on surfaces as this is the absorption peak of DNA and RNA. Disinfection can occur more efficiently at different wavelengths via different disinfection mechanisms such as cell wall degradation which will cause death. Sometimes the optimum peak wavelength may not be the best choice of wavelength. If an alternative LED wavelength delivers higher intensity and dose at a more efficient cost, it may be a better choice that the “optimum wavelength”.
Irradiance is a measure of how much UV energy falls on a particular surface point. Importantly, it is not the same as how strong a particular UV lamp is. A 10W lamp that emits UV in all directions, for example, may provide lower UV irradiance at a particular spot than a 5W lamp that focuses its UV output in a narrow beam.
Time: The effective UV energy density, or dose, is the amount of UV energy that falls on a surface for a particular time. It refers to the necessary UV energy required for micro-organisms (bacteria, viruses, algae in suspension…) to absorb to the point of being neutered or killed.
A wavelength of 265nm is considered the optimum as it is the peak of the DNA absorption curve. UVC has been shown to be effective in the disinfection of water, air, and surfaces on various pathogens such as E. coli, L. innocua, and COVID-19. However, disinfection and sterilisation occur over a range of wavelengths and in certain applications alternative wavelengths should not be dismissed. UVC LEDs can be manufactured to target very specific and narrow wavelengths. This allows solutions to be tailored to the particular application need.
Inside the semiconductor material of the LED, the electrons and holes are contained within energy bands. The separation of the bands determines the energy of the photons (light particles) that are emitted by the LED. The photon energy determines the wavelength of the emitted light, and hence its color. Different semiconductor materials with different bandgaps produce different colors of light. One the advantages of LEDs over Hg lamps is the precise wavelength (color) can be tuned by altering the composition of the light-emitting, or active, region.
At present, the commonly used encapsulation materials for LED electronic components mainly include epoxy resin, silicone, and other materials with high transparency. Epoxy resin has become the mainstream material of low-power LED packaging because of its excellent bonding performance, electrical insulation, dielectric performance, low cost and easy molding. But for power LED and printed circuit boards, congenital defects such as strong moisture absorption, easy aging, poor heat resistance and impact resistance of epoxy resin directly affect the service life of LED, and the color change under high temperature and short wave light directly affects the luminous efficiency. It is far from meeting the requirements of packaging materials in terms of high refractive index, low stress and high temperature aging resistance, so it is not suitable for power LED packaging materials.
High-performance silicone potting compounds and encapsulation materials have a series of excellent properties, such as high light transmittance, low internal stress, excellent high and low-temperature resistance, moisture insulation, UV and ozone aging resistance, good hydrophobicity and electrical insulation, and have widely become an ideal choice for LED potting and encapsulation applications materials.
Different encapsulation forms require different materials. In recent years, LED has been developing in the direction of high power, high energy efficiency, high brightness and high reliability. This puts forward higher requirements for the optical properties, mechanical properties, aging resistance and construction properties of silicone potting and encapsulation materials. Good optical properties can ensure that LED light can be output more effectively, and good mechanical and adhesive properties can make LED chip-sensitive components better protected. Good aging resistance can prolong the service life of LED, and good construction performance can improve encapsulation compound efficiency.
Silicone materials are a kind of polymers with Si-O-Si bond as the main chain and side chains connected with various organic groups through silicon atoms.
Silicone materials have the dual characteristics of both organic polymers and inorganic materials, with high bond energy (452KJ/mol), large bond length (0.165nm), bond angle (109) of Si-O bond, low rotational hindrance of Si-O bond and good flexibility of chain segment, good thermal conductivity.
These inherent properties make silicone rubber materials have a series of excellent chemical resistance properties for protecting electrical components: excellent high and low-temperature resistance, which can work in the temperature range of-50 ℃-250 ℃; excellent aging resistance (heat resistance, ultraviolet resistance, ozone aging resistance), with several decades of service life in the natural environment; good hydrophobicity, surface energy as low as 21-22mN/m2; high light transmittance and very low internal stress; excellent electrical insulation for radiator leak sealing applications, good flexibility for vibration isolation applications, etc.
Silicone encapsulant materials can be divided into two categories: low refractive coefficient and high refractive coefficient according to their use. in some special applications, there will be medium refractive coefficient products between them.
The refractive index of encapsulation materials with low refractive coefficient is 1.40 ~ 1.45, most of them are silicone methyl encapsulant materials, and the refractive index of high refractive coefficient silicone packaging materials is 1.50 ~ 1.55, most of which are phenyl silicone packaging materials.
However, due to the large difference between the refractive index of the silicone packaging adhesive with a low refractive coefficient and the refractive index of the chip, part of the light is fully reflected back into the chip, affecting the light output.
At present, the preparation of phenyl-containing vinyl polysiloxane and hydrogen-containing polysiloxane to obtain high refractive index encapsulation materials is one of the more mature and widely used methods. Silicone high refractive encapsulation materials generally use a two-component addition vulcanization system.
Generally, the curing principle of silicone encapsulant material is to use vinyl silicone resin or silicone oil as the base glue, Si-H containing silane oligomer as the crosslinking agent, and platinum complex as the catalyst to form the encapsulant elastomer resists weathering, high or low temperatures, thermal shock and so on. The addition crosslinking room temperature curing reaction occurs between Si-CH=CH2 and Si-H of silicone polymer under the action of the catalyst.
Addition reaction equation
Silicone potting and encapsulation raw materials are two-component colorless and transparent liquid substances. Component a generally includes base glue, catalyst and tackifier, and component B generally includes base glue, crosslinking agent and inhibitor. A small amount of reinforcing filler and pigment can be added according to the use situation.
It is mainly an R-vinyl silicone resin / R-vinyl silicone oil, which provides vinyl groups that can participate in the reaction. R groups can be methyl, phenyl, tetrachlorophenyl and aminopropyl, etc., which are endowed with different packaging characteristics by connecting different groups.
Common synthesis methods mainly include functional siloxane hydrolysis condensation method, cyclosiloxane anionic ring-opening polymerization method, siloxane catalytic equilibrium method, etc. vinyl silicone oil or silicone resin can also be prepared by cationic polymerization with acid catalysts such as trifluoromethane sulfonic acid and cation exchange resin.
It is generally a low-viscosity hydrogen-containing silicone oil or silicone resin, by controlling the molecular weight of hydrogen-containing silicone oil and functional group distribution and active hydrogen content and distribution, which can improve the performance of silicone encapsulation materials. The commonly used synthesis methods of hydrogen-containing silicone oil are generally hydrolysis condensation or cationic ring-opening polymerization under acidic conditions.
In this type of reaction, the catalyst is mainly used to catalyze the addition reaction of vinyl and active hydrogen to fast curing, and complexes such as platinum and palladium are generally used as catalysts. Among them, platinum-based catalysts with higher catalytic efficiency have a high catalytic effect at a concentration of 10-100 ppm.
However, silicone materials for LED encapsulation have high requirements for transparency and catalytic activity. At present, there are kinds of literature showing that the Karstedt catalyst and Speier catalyst are due to the use of tetramethyltetravinylcyclotetrasiloxane, tetramethyldivinyl Ethyl disiloxane, methyl vinyl phenyl cyclopolysiloxane or isopropanol solution are used as coordination solvents, which increase the compatibility with silicone resins and improve the overall performance, and have become the current silicone packaging materials. The most commonly used synthesis catalyst.
If you are looking for more details, kindly visit Led Encapsulation Solutions.
Although the amount used in the reaction system is very small, it plays a crucial role in improving the storage stability of the reaction system, inhibiting the hydrosilylation reaction, and preventing excessive crosslinking. Commonly used inhibitors include ethynyl cyclohexanol, triphenyl propargyl alcohol, heavy metal ion compounds, 6-12-membered cyclic siloxane oligomers containing olefin groups, and benzotriazoles.
Ethynyl cyclohexanol
Triphenyl propargyl alcohol
Because there are no reactive groups on the surface of the substrate, ordinary addition-type silicone materials have little adhesion to the substrate. In order to maintain good sealing between the packaging material and the LED device, the packaging material must have good adhesion to the LED bracket and the bottom plate material. A common solution is to introduce tackifiers into LED packaging materials. Commonly used tackifiers include silane coupling agents and silane coupling agents modified (containing epoxy, acryloxy and vinyl) polysiloxanes. A large number of experiments have confirmed that the silicone tackifier modified by the silane coupling agent can greatly improve the bonding and sealing performance with the substrate without affecting its transparency and mechanical properties.
Polysiloxane tackifier modified by silane coupling agent
There are mechanical property issues with traditional two-component addition silicone materials, thus it requires reinforcing filler to meet the mechanical properties of packaging materials.
Commonly used reinforcing fillers mainly include fumed silica and MQ silicone resin, etc. For the special requirements of LED packaging materials for transparency, currently, the most commonly used silicone packaging material reinforcement filler is MQ silicone resin with good compatibility with the system. MQ silicone resin is a kind of silicone resin containing both monofunctional Si-O chain unit (M) and tetrafunctional Si-O chain unit (Q). MQ silicone resins such as vinyl MQ silicone resin and hydrogen-containing MQ silicone resin have good compatibility with the silicone packaging material system for LEDs and the reinforcement effect is obvious.
With the continuous improvement of power and brightness and the rapid development of white LEDs, silicone encapsulation materials have advantageous over EP encapsulation materials. Among them, high-refractive-index silicone packaging materials will be ideal matrix resin for high-power LED packaging. With the continuous breakthrough of technical barriers, the application scope of high-refractive-index silicone packaging materials will continue to expand, and many domestic LED lighting manufacturers will promote and apply it.
LED (light-emitting diode) has the characteristics of energy saving and environmental protection, long life, low use voltage, short switching time, etc., and is widely used in lighting, display, backlight and many other fields. At present, it is developing in the mature technology direction of higher brightness, high color, high weather resistance, and high luminous uniformity.
The LED industry chain can be divided into upstream, middle, and downstream, which are LED chips, LED packaging and LED lighting applications. As an LED package that connects the upper and lower levels in the LED industry chain, it plays an important role in the entire industry chain.
LED is composed of the led chip, bonding wire, led bracket, conductive adhesives, packaging materials, etc., among which packaging materials are one of the key factors affecting LED performance and service life.
At present, due to its special requirements for light transmission, the main materials currently used on the market are epoxy resin, silicone, polycarbonate, glass, polymethacrylate and other high-transparency materials. However, since most of these materials are hardened and inconvenient to process, they are basically used for outer lens materials.
Traditional LED epoxy resin encapsulation materials have defects such as large internal stress, poor heat resistance, and easy aging, which cannot meet the increasingly developing needs of LED packaging materials, and are gradually being replaced by silicone materials or silicone-modified materials.
Silicone materials are low-stress materials with high UV resistance and aging resistance, making them ideal for LED encapsulation materials. The light transmittance of silicone resin is proportional to the luminous intensity and efficiency of LED devices, and the higher the light transmittance, it is conducive to increasing the luminous intensity and efficiency of LED devices. Since gallium nitride chips have a high refractive index (about 2.2), the refractive index of general silicone materials is only 1.4, so improving the refractive index of silicone materials can reduce the difference with the refractive index of the chip, reduce the light loss caused by interface reflection and refraction, and enhance the light efficiency of LED devices.
Epoxy resin has a high refractive index and light transmission, and the mechanical and adhesive properties are quite good, so there are still some products in the market. By introducing silicone functional groups to modify epoxy resins, the high-temperature performance and impact resistance of epoxy resins can be improved, the shrinkage and thermal expansion of products can be reduced, and the application range of products can be improved, even in the harsh environments. According to the reaction mechanism, silicone-modified epoxy resins can be divided into two methods: physical blending and chemical copolymerization.
The chemical copolymerization method uses the active groups on the silicone polymer, such as hydroxyl and alkoxy groups, to react with the epoxy groups on the epoxy resin to produce a copolymer for the purpose of modification. As early as , this method was used to carry out research on the use of silicone copolymer-modified epoxy encapsulation materials for LED products, and their experiments proved that this encapsulation process method could significantly improve the impact resistance and high and low-temperature resistance of the encapsulation materials, and significantly reduce the shrinkage and thermal expansion coefficient.
Silicone epoxy resin has recently been more widely used as it can reflect the advantages of both epoxy resin and silicone resin. It has shown excellent performance in mechanical properties, bonding, aging resistance, UV resistance, refractive index, light output, etc. It is a future research direction for LED packaging materials and will definitely make significant progress.
1 Characteristic of silicone materials
Silicone polymers have Si—O bonds as the main chain, and organic groups are connected to the silicon atoms, for example, R3 SiO1 /2( M) 、R3 SiO2 /2( D) 、R3 SiO3 /2( T) 、R3 SiO2( Q) and other chain links are combined in a certain proportion; the Si-O bond energy is high, which makes it have better high-temperature resistance or radiation performance, and the Si-O bond angle is large, which can make the molecular chain of the material soft. Silicone materials have excellent properties in terms of heat resistance and anti-yellowing, so as to can be used in outdoor environments. Silicone materials are easy to modify, and functional groups with improved refractive index, such as sulfur, benzene, phenol, and epoxy groups, can be introduced into the side chain to improve the refractive index of packaging materials and improve the luminous efficiency of the LED device.
2 Synthesis of silicone resin
According to its refractive index, it can be divided into two types: low refractive index (1.4) and high refractive index (1.5). Since the higher the refractive index of the silicone material, the higher the light extraction efficiency, so the refractive index of the silicone material should be increased as much as possible.
Silicone resins are generally prepared from organosilanes by hydrolysis and polycondensation in the presence of solvents. The raw material for synthesis is generally chlorosilane or alkoxysilane. The specific process is as follows: firstly, silane is hydrolyzed into silanol under certain conditions, secondly, silanol itself undergoes a polycondensation reaction, and thirdly, it is neutralized by water washing and concentrated to remove small molecules to obtain silicone resin.
Silicone Rubber Encapsulation Materials
Silicone LED encapsulation materials can also be made from molded liquid silicone rubber. Liquid silicone rubber encapsulation material is made of vinyl-containing linear polysiloxane as the base polymer, vinyl silicone resin as reinforcing filler, and hydrogen-containing silicone oil as cross-linking agent. The molding silicone rubber does not produce by-products in the vulcanization process, has a very small shrinkage rate, and also has high cross-linking density, which is widely used in many fields. The use of molded silicone polymers as packaging materials for white LED devices has the effect of reducing costs and improving the service life of LEDs. Low-viscosity liquid silicone rubber has excellent penetration.
GaN-based power type white LED is the focus of current development. It has the characteristics of high heat, short light wavelength, etc., and has stricter requirements on the performance of packaging materials. At the same time, the use of packaging materials with high refractive index, UV resistance, heat aging and low stress can significantly improve the light output power of LED, and can also extend the pot life of the product. At the same time, the development of high-power LED devices also requires silicone packaging materials to develop products with high transparency, refractive index, UV aging and heat aging resistance as early as possible. In view of the problems of silicone materials, such as low refractive index, poor adhesion and low mechanical strength, the epoxy modification method is used to combine the advantages of the two or hybrid with inorganic materials to improve the refractive index of silicone materials.
When working together, the packager and silicone provider should take into account several details. One major consideration is the methodology used when packaging an LED, such as surface mount LED, compression molding or injection molding. Besides that, the end use of the LED and the longevity of the LED must be addressed. As we often find, producing an LED is a game of compromise – durability versus reliability. We will explore some of the basics of silicone to get a better understanding of the material. Also, we will delve into a few questions to be asked when packaging an LED, such as what type of material to use or what RI should be targeted. In all cases, the necessity of a relationship between the silicone provider and the LED manufacturer will become clearer.
Silicone Basics
Understanding the basic structure of the silicone polymer is the first step in deciding what silicone to choose for a particular LED application. First, we have the basic structure of the silicone polymer (Figure 1). Silicone polymers are chains (backbones) comprised of repeating Si-O units, termed siloxane, with organic groups occupying any of the remaining bonding sites (R) on the silicon atom not already occupied by oxygen atoms. Because of this combination, these polymers are often referred to as polyorganosiloxanes.
During the polymerization process, there are two main factors that are controlled: the substituent groups and the degree of polymerization. Although many combinations are possible, the main pendant groups (R) attached to the silicon atom of the polymer backbone can be methyl, phenyl or trifluoropropyl. Generally, a material is referred to as a “polymer” if a molecule contains only one type of organic group, while it is called a “copolymer” if a molecule contains a combination of substituent groups. Altering these substituent groups is how one is able to control many factors about the polymer important to the LED packager such as permeability or Refractive Index (RI).
Furthermore, all silicone polymers can be synthesized to a desired degree of polymerization. The degree of polymerization dictates the average molecular weight, which in turn governs the viscosity. A silicone polymer may possess a viscosity close to that of water (20 cP) or be so large as to be a solid (millions of cP). This aspect plays a crucial role in how the silicone will be used for the LED packager. Whether one needs gels of varying durometers or elastomers of varying durometers, altering the degree of polymerization can help optimize a formulation for a specific application.
Gel or Elastomer
The packaging process of LEDs is always evolving. Historically, with surface-mount-type LEDs, silicone gels were used as an encapsulant to fill the void between the diode of an LED and a lens (Figure 2). Lenses were made of various materials, like glass or polycarbonate, and adhered onto the housing of the LED in a separate process made of various materials such as glass or plastic. This design left a gap between the diode and the lens. Consequently, a material was needed to fill this gap and, for several reasons, silicone was used.
One main reason was silicone gel’s low modulus, which protected wire bonds and left them unaltered when the silicone was cured. Also, silicone was used to increase the efficiency of light transmittance from the diode to the lens. Ultimately, this increases the amount of light that reaches out into the environment. By increasing the silicone gel’s RI to be closer to the diode emitting the light, the amount of light and power can be better transported to the lens. Other reasons silicone gels have been utilized to fill the gap between the die and lens over epoxies are their ability to adhere to multiple substrates; optical stability; low modulus; and, again, the ability to alter the RI.
While this conventional method worked, LED packaging has undergone several optimizations.1
The next evolutionary step was to begin using a silicone material for the lens and the encapsulant. Previously, the CTE differences between the silicone encapsulant and the lens made of a different material would cause problems such as voids or bubbles. By switching to a higher durometer silicone elastomer for the lens, the CTE of the lens and the encapsulant were very similar. This reduced the amount of voids due to the silicone’s much larger CTE compared to thermoplastics and more importantly decreased cure time, as the gel fill and silicone lens could be rapidly heat cured together without voids. Increased adhesion of the encapsulant to the lens was another benefit because silicone adheres to silicone well.
This alteration was short lived, however, as manufacturers quickly realized that if silicone can be used in two separate joining parts, why couldn’t they combine them and drastically reduce their work in progress? The need to combine the encapsulant and lens also required a new packaging process to achieve this goal. With processes such as overmolding or using injection or compression molding, packagers combined the encapsulant and lens into one silicone material and one step (Figure 3). The material of choice is one that is compatible with the equipment for compression or liquid injection. This silicone must also protect the diode and transmit the light generated efficiently. Molding processes allow for lower-cost and higher-volume LED manufacturing in a fraction of the time. Also, by eliminating multiple separate components, molded LEDs can be made smaller, allowing for more LEDs to be produced per shot in the same surface area.
We begin to see how important the relationship is between the packager and the silicone material provider. As dynamic as the LED market is, changes in formulations need to become available as packaging procedures and demands evolve. However, silicone formulary changes are not only dictated by processing needs but by performance and demand.
Higher RI
Increasingly, the demand for High Brightness LEDs (HBLEDs) is growing. From back lighting for a laptop computer to illuminating a room, the demand on silicone to transmit this increase in power is rising. To accomplish this, the industry has had to address two separate, but related, issues concerning silicone: a) what phosphor to add and how to add it to produce brighter LED light, especially white light, and b) how to increase the RI of the material housing the diode to allow a greater transmittance of the brightness?
Typically, in white High Brightness LEDs (HBLEDs), it has been found that using a 405nm blue gallium nitride LED covered by a yellowish cerium doped yttrium aluminum garnet (YAG) phosphor coating produces the desired light. This brought about a challenge in itself when considering the most efficient way to introduce the phosphor to the light path of the diode to the environment. HBLED packagers initially added phosphor by mixing it into the silicone gel to get the desired effect. However, many factors — such as phosphor concentrate and silicone temperature or viscosity — affected the efficiency of this process.2 After silicone packagers moved from surface-mount LED processing to more advanced methods, the phosphor added could be introduced into the equation separately, on top of the diode and before the silicone is molded. This effectively allowed for the increased brightness and increased efficiency of phosphor addition for the LED packager.
Next, to address the second issue, silicone manufacturers needed to assist the LED packager and develop a material with a higher RI to transmit this increased brightness. By increasing the refractive index, the material is able to reduce internal reflections improving light extraction. Previously, a dimethyl silicone was used and had heritage in several industries. As described above, altering the R groups attached to the siloxane backbone can alter the silicone polymer in many ways. For example, adding trifluoropropyl groups to the siloxane backbone lowers the refractive index to around 1.38. More important however, by altering the substituent R groups on the backbone of the silicone polymer chain with phenyl groups, a higher RI can be achieved compared to a traditional methyl system. Paramount to achieving a higher RI is the addition of phenyl. Depending on the amount of phenyl added, the RI can range from 1.43 to greater than 1.55 (Figure 4). In addition, adding phenyl provides a secondary benefit of lowering permeability of moisture and other various gases.3
However, LED applications that use silicones with increased RI should also take into account the increased thermal energy associated with increased brightness. While the physical properties of silicone can withstand a wide temperature range, typically -100°C to +200°C, studies have shown that phenyl containing silicones exposed to higher temperatures over time may begin to discolor, ultimately decreasing the transmittance over time.4
Here is where the manufacturer must make a decision. If a need for higher RI silicones is required, proper thermal management must be considered such as thermally conductive interfaces and heat sinks attached to the LED assembly. The more heat generated, the greater the need and size of the heat sink. This need for thermal management and higher phenyl content increases the cost of the LED. Applications that will exist in harsher environments and have a need for a longer operating life would benefit from using a dimethyl silicone such as the headlight of a car. On the other hand, lighting inside the car that is in a controlled environment could benefit from a higher IR, phenyl-containing silicone.
Partnership Reaffirmed
Obviously, the relationship between the LED packager and the material provider is crucial. Both entities are responsible in different ways for ensuring the end product is high quality. Because LEDs are a relatively new lighting source, industry standards are continuously refined as more is understood about LED performance in specific markets.
On the material level, Accelerated Life Testing (ALT) and other accelerated aging tests have set test conditions and industry standards for various applications. While it is important for the silicone material provider to supply initial ALT data, the LED packager has the most control over design, materials and processing. The packager must, ultimately, develop rigorous ALT testing regimes that ensure the chosen materials not only give expected light output over time, but also high yields once in production. A close relationship between the material supplier and LED packager can ensure the best candidate is chosen for the specific design and process, which will accelerate time to market and keep high yields.
For more High Refractive Index Encapsulation Materials For Ledinformation, please contact us. We will provide professional answers.