Packaging technology for high-power lighting-class LEDs

From the perspective of practical application, high-power LED devices that are easy to install and use and relatively small in size will inevitably replace traditional low-power LED devices in most lighting applications. In order to meet the needs of lighting, lighting fixtures composed of low-power LEDs must concentrate the light energy of many LEDs to meet the design requirements, but the disadvantages are that the circuit is extremely complex and the heat dissipation is not smooth. In order to balance the current and voltage relationship between each LED, a complex power supply circuit must be designed. In contrast, the power of a high-power single LED is much greater than the sum of the powers of several low-power LEDs, the power supply circuit is relatively simple, the heat dissipation structure is perfect, and the physical properties are stable. Therefore, the packaging method and packaging material of high-power LED devices cannot simply apply the packaging method and packaging material of traditional low-power LED devices. Large dissipated power, large heat generation and high light extraction efficiency have put forward new and higher requirements for LED packaging technology, packaging equipment and packaging materials.

1. High-power LED chips

In order to obtain high-power LED devices, it is necessary to prepare suitable high-power LED chips. The common methods for manufacturing high-power LED chips internationally are as follows:

① Increasing the size. By increasing the effective light-emitting area and size of the single LED, the current flowing through the TCL layer is evenly distributed to achieve the expected luminous flux. However, simply increasing the light-emitting area cannot solve the heat dissipation and light output problems, and cannot achieve the expected luminous flux and practical application effect.

② Silicon backplane flip-chip method. First, prepare a large-size LED chip suitable for eutectic welding, and at the same time prepare a silicon backplane of the corresponding size, and make a gold conductive layer and a lead conductive layer (ultrasonic gold wire ball solder joint) for eutectic welding on the silicon backplane, and then use eutectic welding equipment to weld the large-size LED chip and the silicon backplane together. This structure is more reasonable, taking into account both the light output problem and the heat dissipation problem. This is the current mainstream production method of high-power LEDs.

In 2001, Lumileds, an American company, developed an AlGaInN power flip chip (FCLED) structure. Its manufacturing process is as follows: first, a NiAu layer with a thickness greater than 500A is deposited on the P-type GaN on the top of the epitaxial wafer for ohmic contact and back reflection; then a mask is used to selectively etch away the P-type layer and the multi-quantum well active layer to expose the N-type layer; an N-type ohmic contact layer is formed by deposition and etching. The chip size is 1mm×1mm, the P-type ohmic contact is square, and the N-type ohmic contact is inserted in a comb shape, which can shorten the current expansion distance and minimize the expansion resistance; then the AlGaInN chip with metallized bumps is flip-chip soldered on a silicon carrier with an anti-static protection diode (ESD).

③ Ceramic base flip-chip method. First, use LED chip general equipment to prepare LED chips with large light-emitting areas suitable for eutectic welding electrode structures and corresponding ceramic bases, and make eutectic welding conductive layers and lead-out conductive layers on the ceramic bases, and then use eutectic welding equipment to weld large-size LED chips and ceramic bases together. This structure takes into account both light emission and heat dissipation issues, and the ceramic bases used are high thermal conductivity ceramic plates, which have ideal heat dissipation effects and are relatively cheap, so it is currently a more suitable base material and can reserve space for future integrated circuit packaging.

④ Sapphire substrate transition method. After growing a PN junction on a sapphire substrate according to the traditional InGaN chip manufacturing method, the sapphire substrate is cut off and then connected with traditional quaternary materials to manufacture a large-size blue light LED chip with upper and lower electrode structures.

⑤ AlGaInN silicon carbide (SiC) back-lighting method. Cree, an American company, is the only manufacturer in the world that uses SiC substrate to manufacture AlGaInN ultra-high brightness LEDs. In recent years, the structure of its AlGaInN/SiCa chips has been continuously improved, and the brightness has been continuously improved. Since the P-type and N-type electrodes are located at the bottom and top of the chip respectively, single wire bonding is used, which has good compatibility and is easy to use, thus becoming another mainstream product in the development of AlGaInN LEDs.

2. Power type packaging

Power LEDs first emerged when HP introduced the "Piranha" package structure LED in the early 1990s. The company introduced the improved "Snap LED" in 1994, which has two operating currents, 70mA and 150mA, and an input power of up to 0.3W. The input power of power LEDs is several times higher than that of the original bracket-packaged LEDs, and the thermal resistance is reduced to a fraction of the original. Watt-level power LEDs are the core of future lighting devices, so major companies around the world have invested a lot of effort in researching and developing packaging technology for watt-level power LEDs.

LED chips and packages are developing towards high power. Under high current, they can produce luminous flux 10 to 20 times greater than that of φ5mm LEDs. Effective heat dissipation and non-degrading packaging materials must be used to solve the light decay problem. Therefore, tube shells and packaging are key technologies. Currently, LED packages that can withstand several watts of power have appeared. The 5W series of white, green, cyan, and blue power LEDs have been launched on the market since the beginning of 2003. The light output of white light LEDs reaches 187lm, and the light efficiency is 44.3lm/W. Currently, LEDs that can withstand 10W power are being developed. They use large-area tube cores with a size of 2.5mm×2.5mm. They can work under 5A current and have a light output of 200lm.

Luxeon series power LED is a power type AlGaInN flip chip die flipped on a silicon carrier with solder bumps, and then the flip chip silicon carrier is placed in the heat liner and tube shell, and the lead wires are bonded for packaging. This package has the best light collection efficiency, heat dissipation performance and design to increase the working current density.

In application, the packaged product can be assembled on a metal core PCB with an aluminum interlayer to form a power density LED. The PCB is used as the wiring for connecting the device electrodes, and the aluminum core interlayer can be used as a thermal lining to obtain higher luminous flux and photoelectric conversion efficiency. In addition, the packaged SMD-LED is very small and can be flexibly combined to form a variety of lighting sources such as module type, light guide plate type, focusing type, and reflective type.

When ultra-high brightness LEDs are used as signal lights and other auxiliary lighting sources, multiple Φ5mm packaged monochrome and white light LEDs are generally assembled on a lamp panel or standard lamp holder, with a service life of up to 100,000 hours. In 2000, a study pointed out that after Φ5mm white light LEDs worked for 6000 hours, their light intensity had dropped to half of the original. In fact, the life of a light-emitting device using a Φ5mm white light LED array may be only 5000 hours. The light decay speed of LEDs of different colors is different, with red being the slowest, blue and green being in the middle, and white being the fastest. Since Φ5mm packaged LEDs were originally used only for indicator lights, their package thermal resistance is as high as 300℃/W, which cannot fully dissipate heat, causing the temperature of the LED chip to rise, resulting in accelerated light decay of the device. In addition, the yellowing of epoxy resin will also reduce light output. High-power LEDs produce 10 to 20 times more luminous flux than Φ5mm white light LEDs at high currents, so the light decay problem must be solved through effective heat dissipation design and the use of non-degrading packaging materials. Tube shells and packaging have become one of the key technologies for the development of high-power LEDs. The new LED power package design concept can be divided into two categories: single-chip power package and multi-chip power package.

(1) Single chip packaging of power LEDs

In 1998, Lumileds of the United States developed the Luxeon series of high-power LED single-chip packaging structure. This power single-chip LED packaging structure is completely different from the conventional Φ5mm LED packaging structure. It directly solders the LED chip with front light emission to the thermal liner, or flips the LED chip with back light emission on a silicon carrier with solder bumps and then solders it to the thermal liner, so that the thermal characteristics of large-area chips working under high current are improved. This packaging is the best design for light collection efficiency, heat dissipation performance and current density, and its main features are:

① Low thermal resistance. Traditional epoxy packaging has very high thermal resistance, while the thermal resistance of this new packaging structure is generally only 14℃/W, which can be reduced to 1/20 of conventional LEDs.

② High reliability. The inside is filled with a stable flexible gel. When the temperature is between 40 and 120°C, the internal stress generated by the sudden temperature change will not cause the gold wire and the frame lead to break. When this silicone rubber is used as a sealing material for optical coupling, it will not turn yellow like ordinary optical epoxy resin, and the metal lead frame will not become dirty due to oxidation.

③ The optimal design of the reflector cup and lens makes the radiation controllable and the optical efficiency the highest. In the application, they can be assembled on a circuit board with an aluminum interlayer (aluminum core PCB board). The circuit board is used as the wiring for the device electrode connection, and the aluminum core interlayer can be used as a thermal lining for the power LED. In this way, not only a higher luminous flux can be obtained, but also a higher photoelectric conversion efficiency.

The first single-chip watt-level power LED was Luxeon LED launched by Lumileds in 1998. The packaging structure is characterized by the use of thermal and electrical separation, the flip chip is directly welded on the thermal liner with a silicon carrier, and new structures and new materials such as reflective cups, optical lenses and flexible transparent adhesives are used. Single-chip 1W, 3W and 5W high-power LED products are now available. OSRAM launched the single-chip Golden Dragon series LED in 2003. Its structural feature is that the thermal liner is in direct contact with the metal circuit board, which has good heat dissipation performance and an input power of up to 1W.

(2) Multi-chip combination packaging of power LEDs

The diameter of the hexagonal aluminum substrate is 3.175cm (1.25 inches), and the light-emitting area is located in the center of the substrate, with a diameter of about 0.9525cm (0.375 inches), which can accommodate 40 LED chips. The aluminum plate is used as a heat liner, and the bonding wires of the chip are connected to the positive and negative electrodes through two contact points made on the substrate. The number of dies arranged on the substrate is determined according to the required output light power. The combined packaged ultra-high brightness chips include AlGaInN and AlGaInP, and their emitted light can be monochrome, color (RGB), and white (synthesized by RGB three primary colors or by blue and yellow binary synthesis). Finally, high refractive index materials are used for packaging according to the optical design shape, which not only has high light extraction efficiency, but also can protect the chip and bonding wires. The lumen efficiency of the LED packaged by 40 AlGaInP (AS) chips is 20lm/W. The combined package module using RGB three primary colors to synthesize white light, when the color mixing ratio is 0:43 (R) 0:48 (G): 0.009 (B), the typical value of luminous flux is 100lm, the CCT standard color temperature is 4420K, the color coordinates x is 0.3612, and y is 0.3529. It can be seen that this power LED using conventional chips for high-density combined packaging can achieve a higher brightness level, with low thermal resistance, can work under high current and high light output power.

There are many structures and packaging forms for high-power LEDs with multi-chip combination packages. In 2001, UOE of the United States launched the Norlux series of LEDs with multi-chip combination packages, which use hexagonal aluminum plates as substrates. In 2003, Lanina Ceramics launched a high-power LED array packaged using the company's unique low-temperature sintered ceramic on metal substrate (LTCC-M) technology. In 2003, Panasonic launched a high-power white light LED packaged with 64 chips. In 2003, Nichia launched an ultra-high brightness white light LED with a luminous flux of up to 600lm. When the output beam is 1000lm, the power consumption is 30W, the maximum input power is 50W, and the luminous efficiency of the white light LED module is 33lm/W. The MB series of high-power LEDs packaged with metal bonding technology by UEC (Guolian) of Taiwan, my country are characterized by using Si instead of GaAs substrate, which has good heat dissipation effect, and using the metal bonding layer as the light reflection layer to improve the light output.

The thermal characteristics of power LEDs directly affect the operating temperature, luminous efficiency, luminous wavelength, service life, etc. Therefore, the packaging design and manufacturing technology of power LED chips are particularly important. The main issues to be considered in high-power LED packaging are:

① Heat dissipation. Heat dissipation is crucial for power LED devices. If the heat generated by the current cannot be dissipated in time and the junction temperature of the PN junction is kept within the allowable range, it will be impossible to obtain stable light output and maintain normal device life.

Among the commonly used heat dissipation materials, silver has the highest thermal conductivity, but the cost of silver is high and it is not suitable for general-purpose heat sinks. The thermal conductivity of copper is close to that of silver, and its cost is lower than that of silver. Although the thermal conductivity of aluminum is lower than that of copper, its overall cost is the lowest, which is conducive to large-scale manufacturing.

After experimental comparison, it is found that the more appropriate approach is: use copper-based or silver-based heat linings to connect the chip, and then connect the heat linings to the aluminum-based heat sink, use a stepped thermal conductive structure, and use the high thermal conductivity of copper or silver to efficiently transfer the heat generated by the chip to the aluminum-based heat sink, and then dissipate the heat through the aluminum-based heat sink (through air cooling or heat conduction). The advantages of this approach are: fully consider the cost-effectiveness of the heat sink, combine heat sinks with different characteristics, achieve efficient heat dissipation and rationalize cost control.

It should be noted that the choice of material for connecting the copper-based thermal liner and the chip is very important. The commonly used chip connection material in the LED industry is silver glue. However, after research, it was found that the thermal resistance of silver glue is 10~25W/(m·K). If silver glue is used as a connection material, it is equivalent to artificially adding a thermal resistance between the chip and the thermal liner. In addition, the internal basic structure of the silver glue after curing is an epoxy resin skeleton + silver powder filled thermal conductive structure. This structure has extremely high thermal resistance and a low TG point, which is extremely unfavorable for the heat dissipation and stability of the physical properties of the device. The solution to this problem is to use tin sheet solder as the connection material between the grain and the thermal liner [the thermal conductivity of tin is 67W/(m·K)], which can obtain a more ideal thermal conductivity effect (thermal resistance is about 16℃/W). The thermal conductivity and physical properties of tin are far better than those of silver glue.

② Light extraction. The traditional LED device packaging method can only utilize about 50% of the light energy emitted by the chip. Due to the large difference in the refractive index between the semiconductor and the enclosed epoxy resin, the critical angle of total reflection inside is very small. Only a small part of the light generated by the active layer is extracted, and most of the light is absorbed after multiple reflections inside the chip. This is the fundamental reason why the light extraction efficiency of ultra-high brightness LED chips is very low. How to utilize the 50% of the light energy consumed by refraction and reflection between different internal materials is the key to designing the light extraction coefficient.

The flip chip technology can get more effective light output than the traditional LED chip packaging technology. However, if a reflective layer is not added under the chip's light-emitting layer and the electrode to reflect the wasted light energy, it will cause about 8% light loss, so a reflective layer must be added to the base material. The light on the side of the chip must also be reflected by the mirror processing method of the thermal liner to increase the light output rate of the device. In addition, a layer of silicone material should be added to the sapphire substrate part of the flip chip and the epoxy resin light guide bonding surface to improve the refractive index of the chip light.

The improvement of the above optical packaging technology can greatly improve the light output rate (luminous flux) of high-power LED devices. The optical design of the top lens of high-power LED devices is also very important. The usual practice is: when designing the optical lens, the optical design requirements of the final lighting fixture should be fully considered, and the design should be carried out as far as possible in accordance with the optical requirements of the lighting fixture.

Common lens shapes include: convex lens, concave cone lens, spherical lens, Fresnel lens and combined lens. The ideal assembly method for lens and high-power LED device is to adopt airtight packaging. If limited by the lens shape, semi-airtight packaging can also be adopted. The lens material should be selected from synthetic materials such as glass or acrylic with high light transmittance. Traditional epoxy resin module packaging can also be used. Adding secondary heat dissipation design can basically achieve the effect of improving light output.

3. Progress of power LEDs

The development of power LEDs began with GaAs infrared light sources in the mid-1960s. Due to its high reliability, small size, light weight, and ability to operate at low voltage, it was first used in military night vision devices to replace the original incandescent lamps. In the 1980s, InGaAsP/InP double heterojunction infrared light sources were used in some special test instruments to replace the original large and short-lived xenon lamps. The DC operating current of this infrared light source can reach 1A, and the pulse operating current can reach 24A. Although infrared light sources are early power LEDs, they have been developed to this day, with products constantly being updated and more widely used, and have become the technical foundation that can be inherited by the development of today's power LEDs.

In 1991, the practical application of red, orange and yellow AlGaInP power LEDs made the application of LEDs move from indoors to outdoors, and they were successfully used in various traffic lights, car taillights, turn signals and outdoor information display screens. The successive development of blue and green AlGaInN ultra-high brightness LEDs has realized the ultra-high brightness full color of LEDs. However, the use of ultra-high brightness LEDs for lighting is another new field for the expansion of ultra-high brightness LEDs. Replacing traditional glass-shell lighting sources such as incandescent lamps and fluorescent lamps with LED solid lamps has become the development goal of LEDs. Therefore, the research and development and industrialization of power LEDs will become another important direction for future development. The key technology is to continuously improve the luminous efficiency and the luminous flux of each device (component). The epitaxial material used for power LEDs adopts MOCVD epitaxial growth technology and multi-quantum well structure. Although its internal quantum efficiency needs to be further improved, the biggest obstacle to obtaining high luminous flux is still the low light extraction efficiency of the chip. At present, due to the use of the traditional indicator light type LED packaging structure, the operating current is generally limited to 20mA. Power LEDs designed and manufactured according to this conventional concept cannot meet the requirements of high efficiency and high luminous flux at all. In order to improve the luminous efficiency and luminous flux of visible light power LEDs, new design concepts must be adopted. On the one hand, the light collection efficiency can be improved by designing a new chip structure, and on the other hand, the photoelectric conversion efficiency of the device can be improved by increasing the chip area, increasing the working current, and adopting a low thermal resistance packaging structure. Therefore, designing and manufacturing new chips and packaging structures, and continuously improving the light collection efficiency and photoelectric conversion efficiency of the device have always been crucial issues in the development of power LEDs.

Power LEDs have greatly expanded the application of LEDs in various signal display and lighting source fields, mainly in automobile interior and exterior lights and various traffic lights, including urban transportation, railways, highways, airports, harbor lighthouses, safety warning lights, etc. Power white light LEDs have begun to be used as reading lights in cars and airplanes as special lighting sources, and are also increasingly used in portable lighting sources (such as key lights, flashlights), backlight sources and miner's lights. In addition to being synthesized by three primary colors, white light can also be formed by applying a special phosphor to GaN blue or ultraviolet wavelength power LED chips. Power LEDs show their unique characteristics compared with similar products in building decorative light sources, stage lighting, shopping mall window lighting, advertising light box lighting, courtyard lawn lighting, city night scenes, etc. Using power RGB three-primary color LEDs, a digital color-modulated light source with a compact structure and higher luminous efficiency than traditional incandescent light sources can be made. With computer control technology, extremely colorful luminous effects can be obtained. Power LEDs have the advantages of low voltage, low power consumption, small size, light weight, long life and high reliability, making them suitable for military use as special solid light sources required for field warfare, diving, aerospace and aviation.

The improvement of power LED structure, the optimization of light collection and heat lining design make its luminous efficiency and luminous flux continue to improve. The lamp plate and lamp head assembled by multiple 5mm LEDs will be replaced by the wick assembled by power LEDs. In the past 30 years from 1970 to 2000, the luminous flux has doubled every 18 to 24 months. Since the advent of Norlux series power LED in 1998, the luminous flux has increased faster.

With the improvement of power LED performance, LED lighting sources have attracted greater attention in the lighting field. The demand for general lighting market is huge, and power LED white light technology will be more suitable for general lighting applications. As long as the LED industry can continue this development direction, LED solid lighting will achieve a major market breakthrough in the next 5 to 10 years.