High-power LED packaging technology has recently been the subject of intense research due to its complex structures and processes that directly impact LED performance and lifespan, particularly regarding white high-power LED packaging solutions.
LED packaging has multiple functions, including mechanical protection to improve reliability, heat dissipation for reduced chip junction temperatures and enhanced LED performance, optical control to increase light efficiency and optimize beam distribution, power management including AC/DC conversion control, and management of power consumption.
LED packaging methods, materials, structures, and processes are largely determined by factors including chip structure, photoelectric/mechanical characteristics, specific applications, cost considerations and timelines. Over the past four decades, LED packaging has advanced through various stages like through-hole (Lamp LED), surface mount (SMD LED) and chip-on-board (COB LED) development.
Key Technologies in High-power LED Packaging
High-powered LED packaging entails many aspects, both independently and interdependently, such as light, heat, electricity, structure and processes that intertwine to form performance levels in LED packaging levels. Light serves as the purpose for LED packaging while heat serves a critical purpose while electricity, structure and processes serve as means; performance levels represent levels specific manifestation of performance levels for specific levels. To optimize compatibility and cost reduction during production phases of chips with minimal adjustments needed afterwards leading to long product development cycles and process costs.
Packaging Processes with Low Thermal Resistance Rating
Given that around 80% of input electrical energy is converted to heat by LED chips and their small areas, chip heat dissipation is of critical importance in LED packaging. It involves chip placement and selection of packaging materials such as substrate materials or thermal interface materials processes, as well as heat sink design for effective heat dissipation.
LED packaging thermal resistance includes both internal thermal resistance of materials (heat sink substrate and structure), as well as interface thermal resistance. A heat sink substrate’s purpose is to draw heat generated from chips onto its surface for transference to an external heat sink system for dissipation through exchange with its surroundings; its construction typically includes silicon, metals (such as aluminum and copper), ceramics such as Al2O3, AlN or SiC, composite and hybrid materials as heat sink substrate materials.
Studies reveal that packaging interface can have an outsized influence on thermal resistance. If handled improperly, improper heat dissipation could become increasingly challenging.
High Light Extraction Rate Packaging Structure, and Process
With LEDs, losses occur during outward emission of radiation-combined photons due to three main sources. Internal structural defects in the chip and material absorption; reflection losses at exit interface due to differences in refractive index; and total reflection caused by incident angles greater than critical angle for total reflection all contribute significantly to light lost from leaving its source into its destination environment. As such, considerable light cannot escape.
One way of mitigating losses from chips is coating their surface with an encapsulation gel having a relatively high refractive index, in order to limit losses caused by light scattering from surfaces of devices such as mobile phones. This gel layer, placed between the chip and air, effectively decreases photon losses at its interface, increasing light extraction efficiency. Furthermore, its mechanical protection, stress release capabilities and light-guiding qualities also serve to strengthen light extraction performance. Therefore, an ideal LED packaging gel must provide high transparency, high refractive index, good thermal stability, good flowability and ease of application.
In order to enhance LED packaging reliability further, moisture absorption rates, stress loads and aging resistance must also be considered factors. Silicone gel and epoxy resin are among the more frequently employed encapsulation gels, and silicone gel is especially well suited to LED packaging due to its superior transparency, refractive index, thermal stability, stress tolerance and moisture absorption properties. Silicone gel stands out due to its outstanding transparency, refractive index value, thermal stability properties as well as low stress/moisture absorption levels compared with its cheaper alternative and thus often takes precedence for high power LED packaging due to higher prices.
Studies demonstrate that increasing the refractive index of silicone gel can effectively lower photon losses caused by refractive index barriers, improving external quantum efficiency. Unfortunately, performance can be greatly influenced by environmental temperature. As temperatures increase, thermal stress within silicone increases causing internal stresses within its internal structures to rise leading to decreases in refractive index which impacts LED lighting efficiency and intensity distribution.
Phosphors play an integral part in producing white light. Their characteristics include particle size and shape as well as luminescence efficiency and conversion efficiency for maximum output and thermal and chemical stability – two essential qualities when selecting an effective phosphor material. Studies indicate that as temperatures increase, quantum efficiency of phosphor decreases leading to reduced light emission and changes in radiation wavelength.
This in turn alters color temperature and chromaticity for white LEDs as higher temperatures accelerate phosphor aging. Phosphor coatings typically consist of epoxy or silicone gel mixed with phosphor and have poor heat dissipation capabilities when exposed to blue or UV radiation, often experiencing thermal quenching, thermal aging and decreasing luminosity efficiency over time. Furthermore, there may be issues related to stability with respect to heat resistance of its encapsulation gel at higher temperatures.
As most commonly used phosphor sizes exceed one millimeter with refractive index values greater than or equal to 1.85, while silicone gel typically has a refractive index around 1.5, there exists an irreconcilable mismatch. Furthermore, particle sizes far outstripped light scattering limits (30nm), leading to excessive light scattering on their surfaces reducing efficiency by 10–20%; introduction of nano-phosphors may increase refractive index by reaching above 1.8 while also decreasing light scattering, improving LED efficiency 10-20% and improving color quality significantly.
Traditional applications of phosphor involve mixing it with an encapsulation gel before dotting onto a chip, but inaccuracy in controlling thickness and shape leads to inconsistent emission colors that vary bluish or yellowish in hue. Lumileds has developed Conformal Coating technology, enabling uniform coating of phosphor to ensure color uniformity. However, research indicates that when directly coated on chip surface surface phosphor will scatter light reducing light efficiency significantly and leading to reduced light efficiency and light efficiency.
Rensselaer Polytechnic Institute in the US has come up with an Scattered Photon Extraction (SPE) method, comprising of placing a focusing lens onto a chip’s surface and placing a glass plate filled with phosphor particles a certain distance away – not only improving device reliability, but also significantly increasing efficiency (60%).
LED chips have recently taken center stage within the display industry. Thanks to their superior brightness, self-luminosity and full color capabilities, these lights have spread into diverse display environments worldwide. SMD and COB packaging technologies have played a pivotal role in driving this expansion.
Surface Mount Device (SMD) LEDs began their meteoric rise to prominence beginning in 2002 and have gradually taken control of LED display device markets ever since. SMD packaging involves attaching individual or multiple LED chips onto a metal bracket fitted with a plastic “cup-shaped” frame whose pins connect directly with their P and N poles on LED chips; liquid encapsulation gel is then poured in via high temperature baking followed by cutting.
Thanks to surface Mount Technology (SMT), SMDs achieve high levels of automation compared to pin type packaging technology’s counterpart; SMD LEDs offer better performances regarding brightness, consistency reliability viewing angle viewing angles as well as appearance compared with their pin counterparts compared with their pin counterparts!
Technical Advantages of SMD Packaging:
Maturing technology featuring well-established production equipment and processes as well as an efficient supply system.
Widely applied with mature display control that guarantees maximum stability.
Achieve high lamp bead pass rates while maintaining consistent quality levels and providing for increased reliability.
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SMD Packaging LED Display:
SMD-packaged LED displays quickly gained market acceptance for their advanced and stable technology, efficient heat dissipation, easy maintenance procedures, seamless splicing capabilities and low brightness and grayscale capabilities, quickly seizing an impressive portion of the market for LED applications.
Today, LED displays are used widely in command rooms, control rooms, meeting rooms and other applications. SMD-packed LED displays have found wide use across government emergency commands, intelligent command control systems, public security and traffic police commands, general meeting decision making processes, energy dispatch centers and military/police operations – with great success across these different domains. Their influence only continues to expand.
COB (Chip-on-Board) packaging technology provides an integrated packaging system without brackets that directly encases multiple LED chips on a PCB board, eliminating surface mounting processes and bracket soldering in favor of tightly sealing individual pixels’ LED chips with epoxy resin for protection, thus mitigating potential external damages to pixels points, thereby offering exceptionally stable micropitch LED displays with minimal maintenance requirements.
Micro-pitch LED high-definition display systems were first widely employed at the forefront of information technology when first released for sale in 2013. Subsequent research and innovation efforts focused on Micro LED technology combined with COB packaging processes have seen continuous research conducted, effectively supporting entry of COB micro-pitch LED displays into information technology in 2017.
Voury specializes in SMD and COB technologies used for micro-pitch LED display units as its core business. Utilizing its diverse processing system as control center, the company provides intelligent splicing large screen solutions to IT clients.
COB packaging uses surface emitting technology, giving Voury COB-packaged LED display a 170 degree viewing angle in both dimensions ensuring consistent colors and brightness from any viewing position. Furthermore, its superior optical diffusion and uniform color dispersion with greater display coverage area allows seamless and color accurate viewing from every perspective for perfect image presentations at all times.
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MiP Vs. COB LED Display Technology Market
Additionally, COB packaging technology seals all pixel points on PCB boards to achieve comprehensive sealing of circuitry components such as crystal particles, solder joints and leads. With high stability and ease of maintenance in mind, daily cleaning of surfaces using damp cloth wipers provides IP65 full protection, offering properties such as anti-scratch protection as well as shock resistance, compression resistance water resistance moisture resistance dust resistance oil resistance anti oxidation resistance anti static capabilities along with full protection properties such as anti scratch anti impact shock compression resistance anti rust proof properties along with full IP65 full protection with properties including anti scratch anti – anti scratch anti impact resistance shock compression resistance water resistant properties; properties like anti – with high stability and ease of maintenance being handled daily by wiping surfaces using damp cloth wiper wiper.
Simultaneously, COB-packaged micro-pitch LED displays utilize ultra-black panels with over 97% black coverage across their screens to produce an unprecedented contrast ratio of 10,000:1, creating an qualitative leap and assuring peak color performance, producing exquisite images that stand the test of time.