Home  Diodes

Oct 25 2018

High-power LED Package Types Technologies Analysis

Ⅰ Introduction

As an important part in the LED industry chain, high power LED packaging is the core manufacturing technology that promotes the practical use of semiconductor lighting and display. Through the development of LED packaging and manufacturing technologies with low thermal resistance, high light efficiency, high reliability, good mechanical and electrical protection,  high-performance LED lighting and display can be made, to promote the healthy development of the entire semiconductor lighting and display industry.

A Tutorial on the Basics of Using LEDs (Light Emitting Diodes).

Catalog

Ⅰ Introduction

Ⅱ Key Technologies for High-power LED Packaging

2.1 LED Packages Process

2.2 Low Thermal Resistance

2.3 High Brightness

2.4 LED Package Types

Ⅲ Test and Measurement

Ⅳ Technique Improvement

Ⅴ Question Related to LED Package and Going Further

5.1 Ouestion

5.2 Answer


High power LED packaging mainly involves light, heat, electricity, structure and technology. These factors are closely related to each other. Among them, lighting is the purpose of LED packaging, heat dissipation is a key task, energizing, structure and process designing are the means, and performance shows the concrete embodiment of the packaging level. In terms of process compatibility and cost reduction, LED packaging design should be carried out simultaneously with chip design, that is, packaging structure and process should be taken into account in chip design. Otherwise, the chip structure needs to be adjusted after completed due to the needs of the package, thereby prolonging the product development cycle and process cost, and sometimes even causing the product to be infeasible.

Five Key Technologies of High Power LED Packaging

Figure.1 High Power LED Packaging Technology

Ⅱ Key Technologies for High-power LED Packaging

2.1 LED Packages Process

1)Chip inspection
To check whether there is mechanical damage on the surface of the material, whether the chip size and electrode size meet the process requirements, and whether the electrode pattern is complete.

2)Chip expansion
Because the LED chips are closely spaced after dicing (about 0.1mm), it is not conducive to the operation of the subsequent process. A chip expander can be used to expand the space of the bonded chip.

3)Glue dispersion
Put silver glue or insulating glue on the corresponding position of the LED bracket. For GaAs and SiC conductive substrates, red, yellow, and yellow-green LED chips with back electrodes are used with silver glue. For blue and green LED chips with sapphire insulating substrates, insulating glue is used to fix the chip. Difficulties in the process include the amount of glue, the height of the glue dispersion, the position of the glue dispersion, etc.

4)Glue preparation
Contrary to glue dispersion, the glue preparation is to use a device to apply silver glue to the electrode on the back of the LED, and then install the LED with silver glue on the back on the LED bracket. The efficiency of glue preparation is much higher than that of dispersion, but not all LED products are suitable for this process.

5)Mounting
    a. Splint manual
First, place the expanded LED chips on the fixture of the lancing table, and the LED holder is settling under the fixture. Second, use the needle to pierce the LED chips one by one to the corresponding position under the microscope. Compared with automounter, it is easy to replace different chips at any time, and suitable for products that need to install multiple chips.
    b. Automounter
Automounter actually combines the two major steps of chip dispersion and installation. First, silver glue or insulating glue is applied to the LED bracket, and then the LED chip is sucked up to move the position with a vacuum suction nozzle, and then placed on the corresponding bracket.
The main points of automounter are equipment operation programming, equipment sticking, and the adjustment of installation accuracy. In addition, the steel nozzle will scratch the current diffusion layer on the surface of the chip, especially the blue and green chips must use bakelite.

6)Sintering or Frittage
The purpose of sintering is to solidify the silver glue. Sintering requires good temperature monitoring to prevent the occurrence of defective products. The sintering temperature of silver glue is generally controlled at 150 ℃ continued for 2 hours. According to the actual situation, it can be adjusted to 170℃ for 1 hour.
The insulating glue is generally 150℃ for 1 hour. The sintering case shall not be used for other purposes to prevent pollution.

7)Pressure welding
The purpose of pressure welding is to lead the electrode to the LED chip to connect of the inner and outer leads of the product. There are two kinds of LED welding process: gold-wire type and aluminum-wire type. What needs to be monitored are the shape of the pressure-welded arch wires and welding spot, and the tensile force, etc.

8)Packaging
There are three types of LED packaging: glue, potting, and molding.
Glue dispersion : Basically, the difficulties in process control are bubbles, and black spots. The design is mainly about the selection of materials, and the epoxy and brackets with good combination are selected. In addition, epoxy resin adhesive will thicken during gluing process. White LEDs also has the problem of chromatic aberration due to the precipitation of fluorescent powder.
Pouring: First inject liquid epoxy into the LED molding cavity, then insert the pressure-welded LED bracket, and put it in an oven to allow the epoxy to cure. Lamp-LED packages mostly use this method.
Molding: First, put the pressure-welded LED bracket into the mold, connect the upper and lower two molds with a hydraulic machine and evacuate it. Second, put the solid epoxy into the entrance of the injection channel with the hydraulic jack, then the epoxy resin adhesive enters each LED molding tank along the glue channel and solidifies.

9)Curing and post curing
As for the curing of epoxy resin, the general curing conditions are 135 ℃, 1 hour. Molding packages are generally at 150°C for 4 minutes. Post-curing is to allow the epoxy to cure fully, while thermally aging the LED. Post-curing is very important to improve the bonding strength between epoxy and bracket (PCB), generally at 120°C for 4 hours.

10)Cutting and scribing
Since the LEDs are connected together on board in production, Lamp-LED uses the method of cutting the LED bracket. SMD-LED is on a PCB board, which needs a dicing machine to complete the separation work.

11)Test
Test the photoelectric parameters of the LED, check the external dimensions, and sort the LED products.

Dual In-line Package

Figure 2. Dual In-line Package

2.2 Low Thermal Resistance

For the existing LED luminescence efficiency level, because about 80% of the input electric energy is converted to heat, and the area of LED chip is small, the heat dissipation of the chip in LED packaging is the key task that must be solved. The process of low thermal resistance packaging mainly includes chip layout, packaging material selection (substrate material, thermal interface material) and process, heat sink design and so on.

 LED package thermal resistance mainly includes material(heat-dissipation substrate and heat sink structure) internal thermal resistance and interface thermal resistance. The function of the heat-dissipation substrate is to absorb the heat generated by the chip and transmit it to the heat sink to realize the heat exchange with the outside. Common heat-dissipation substrate materials include silicon, metals (such as aluminum, copper), ceramics (such as AlN and SiC) and composites, etc.

 For example, in the third generation LED, Nichia Company uses CuW as the substrate and inverted the 1mm chip on it, which reduced the thermal resistance of the package, and improved the luminous power and efficiency; As shown in figure 3 (a), Lamina Ceramics Company has developed the low temperature co-fired ceramic metal substrate and corresponding LED packaging technology. The technique first prepares a high-power LED chip suitable for eutectic soldering and a corresponding ceramic substrate, and then directly solders the LED chip to the substrate. Since the eutectic solder layer, the electrostatic protection circuit, the driving circuit and the control compensation circuit are integrated on the substrate, the substrate presents a solution for high power LED array packaging as it has advantages of simple structure, high thermal conductivity, small thermal interface and improved heat dissipation performance; 

The high thermal conductivity copper-clad ceramic plate, developed by Curmilk Company in Germany, is made of ceramic substrate (AlN) and conductive layer (Cu) that sintered under high temperature and high pressure without the use of binder. So it has good thermal conductivity, high strength and strong insulation. As shown in Figure 3 (b), the thermal conductivity of AlN is 160 W / mk, and the thermal expansion coefficient is equal to that of silicon, thus reducing the Package thermal stress.

Five Key Technologies of High Power LED Packaging

Figure 3. LED Structure

The results show that the package interface attaches a great impact on thermal resistance, and if the interface is not handled correctly, it is difficult to dissipate heat. For example, the interfacial gap may exist at high temperature when the interface is in good contact at room temperature, and the warping of the substrate may also affect bonding and local heat dissipation. Reducing interfaces and interface-contact thermal resistance, and enhancing heat-dissipation are the key to improve LED packaging. Therefore, the choice of thermal interface material (TIM) between chip and heat dissipation substrate is very important. Conductive adhesive and thermal conductive adhesive are the common TIM in LED packaging. Low thermal conductivity of 0.5-2.5 W / mK results in high interface thermal resistance. However, using conductive adhesive that has nano-particles inside as a thermal interface material can greatly reduce the interface thermal resistance.

 

2.3 High Brightness

When applied LED, the loss of photons, which produced by radiation recombination, in outward emission mainly includes three aspects: defective chip internal structure and low materials utilization; the reflection loss of photons at the exit interface due to refractive index difference; and the total reflection loss caused by the incident angle larger than the critical angle of total reflection.

As a result, a lot of light cannot be sent out of the chip. Since the adhesive layer wraps the chip, it can effectively reduces the loss of photons at the interface by coating a transparent adhesive layer(pouring sealant), which has relatively high refractive index on the chip surface, thereby improving extraction efficiency. In addition, pouring sealant plays the role of mechanical protecting the chip, stress releasing, and being a photoconductive structure.

Therefore, the adhesive layer is required to have high light transmittance, high refractive index, good thermal stability, good fluidity, and to be easy spraying. In order to improve the reliability of the LED package, the pouring sealant is also required to have low hygroscopicity, low stress, and aging resistance.

The commonly used sealants include epoxy resin and silica gel. Silica gel is better than epoxy resin due to its advantages of high light transmittance, large refractive index, good thermal stability, low stress and low hygroscopicity. And it is widely used in high power LED packaging, but its cost is high. In addition, it is shown that increasing the refractive index of silica gel can effectively reduce the photon loss caused by the refractive index and improve the external quantum efficiency. However, the properties of silica gel are greatly affected by ambient temperature. With the increase of temperature, thermal stress inside the silica gel increases, which results in the decrease of the refractive index, affecting the luminous efficiency and light intensity distribution of the LED.

LED Modules

The function of fluorescent powder lies in the combination of light and color to form white light. Its characteristics include particle size, shape,luminous efficiency, conversion efficiency, thermal and chemical stability, etc. Luminous efficiency and conversion efficiency are the key. It is shown that with the increase of temperature, the quantum efficiency of fluorescent powders decreases, the emission decreases, and the radiation wavelength changes, which results in the change of colour temperature and chromaticity of white LED, thereby accelerating the aging of fluorescent powders. The reason is that the fluorescent powder coating is from fluorescent powder mixing with epoxy or silica gel, and has poor heat-dissipation property. When exposed to violet or ultraviolet radiation, temperature quenching and aging are liable to occur, resulting in reduced luminous efficiency.

In addition, the thermal stability problem of potting and fluorescent powders exsists at high temperature. The size of common fluorescent powders is above 1um, the refractive index is no less than 1.85. The refractive index mismatch as that of silica gel is about 1.5, and the size of the fluorescent powder particle is much larger than that of the light scattering limit (30nm), so fluorescent powder has light scattering on its surface, which reduces the output efficiency. By adding nano-fluorescent powders into silica gel, the refractive index can be increased to more than 1.8, the light scattering can be reduced, the light efficiency of LED can be improved (10- 20%) and the quality of light and color can be improved.

The traditional fluorescent powder coating method is to mix the fluorescent powder with the coating glue and then apply it on the chip. Because the coating thickness and shape of fluorescent powders can not be controlled accurately, the emitted color of the fluorescent powders is inconsistent, and there may be blue or yellow light. The conformal coating technology developed by Lumileds Company can realize the uniform coating of fluorescent powders and ensure the uniformity of light and color, as shown in figure 4 (b). However, when the fluorescent powder is directly coated on the chip surface, the light efficiency is low due to the existence of light scattering. In view of this, the Rensselaer Institute of the United States proposed a Scattered Photon Extraction method(SPE). By placing a focusing lens on the surface of the chip and placing the glass containing fluorescent powder in a certain position from the chip, it not only improved the reliability of the device but greatly improved the light efficiency to 60%, as shown in Fig. 4 (c).

Five Key Technologies of High Power LED Packaging

Figure 4. LED Package Strucutres

In general, to improve the luminous efficiency and reliability of LED, the encapsulated adhesive is gradually replaced by high refractive index transparent glass or glass-ceramics. Doping phosphors or coating it on the glass surface can not only improves the uniformity of phosphors, but also improves the packaging efficiency. In addition, reducing the number of optical interfaces of LED is also an effective measure to improve the light efficiency.

 

2.4 LED Package Types

After more than 40 years of development, LED packaging technology and structure have gone through four stages, as shown in figure 5.

Five Key Technologies of High Power LED Packaging

Figure 5. Development of Packaging Technology and Structure

        ① Lamp LED 

 Lamp LED packaging is a commonly encapsulation structure of 3-5mm. It is generally used in LED packaging with low current (20-30mA) and low power (less than 0.1W). Mainly used for instrument display or indication, large-scale integration can also be used as a display screen. It has disadvantages of large thermal resistance (generally higher than 100K/W) and short life.

        ② SMD LED 

Surface mount technology (SMT) is a kind of packaging technology which can directly weld the encapsulated device to the designated position of the PCB surface. Specifically, using a specific tool or device to point the chip pin at the pads precoated with adhesive and paste, and then directly attach it to the surface of the PCB that has not been drilled, after wave soldering or reflow welding, a reliable mechanical and electrical connection between the device and the circuit is established. SMT technology, the most popular packaging technology in electronic industry, has the advantages of high reliability, high frequency characteristic, easy automation and so on.

Table 1. Main SMD LEDs

SMD LED

Power(W)

Lumen(LM)

Luminous Efficiency(LM/W)

LED 2835

0.2

20~22, 22~24, 24~26

130

LED 3014

0.1

11~12, 12~13

130

LED 3528

0.06

6~7, 7~8, 8~9

150

LED 5050

0.2

20~22, 22~24, 24~26

130

LED 5630

0.5

50~55, 55~60

130


        ③ COB LED 

Chip On Board (COB) is a packaging technology that directly attach the LED chip to the PCB through glue or solder, and the electrical interconnection between chip and PCB is realized by wire bonding. The PCB can not only be low-cost FR-4 material (Glass fiber reinforced epoxy resin), but metal matrix or ceramic matrix composites with high thermal conductivity (such as aluminum substrates or copper clad ceramic substrates, etc.). Wire bonding can adopt hot ultrasonic bonding (gold wire ball welding) at high temperature and ultrasonic bonding at room temperature. COB technology is mainly used in LED packaging of high power multi-chip array. Compared with SMT, COB technology can not only greatly improves the power density of the package, but also reduces the thermal resistance of the package (generally 6-12W/mK).

For the cost and application, COB will become the mainstream direction of future luminaire design. COB packages multiple LED chips on the bottom plate to improve the brightness, and reduce the amount of input current of a single LED chip to ensure high efficiency. COB can greatly expand the heat dissipation area of the package, making the heat more easily transmitted to the housing. With the advantages of low thermal resistance, good light quality, solder-free and low cost, COB will be widely used in the future.

        ④ SiP LED  

        Based on the System on Chip (SOC), System in Package (SiP) is a new packaging and integration method to meet the requirements of portable development and miniaturization of the whole system. For SiP-LED, it is not only to assemble multiple luminous chips in a single package, but to integrate various types of devices (such as power supply, control circuits, optical microstructures, sensors, etc.) into a more complex and complete system. Compared with other packaging structures, SiP has better process compatibility (SiP can use existing packaging material and process). Including easy to block test, SiP has the advantages of high integration, low cost, more new functions, short development cycle and so on. According to the type of technology, SiP can be divided into four types: chip stacking, module, MCM and three dimensional (3D) encapsulation.

        At present, in order to replace incandescent and high pressure mercury lamps, the high brightness LED devices must improve the total or the available luminous flux. The increase of luminous flux can be achieved by increasing integration, increasing current density or using large size chips. But all of these will increase the power density of LED, such as poor heat-dissipation, which will increase the junction temperature of LED chip and thereby directly affect the performance of LED devices (such as decreased luminous efficiency, red shift of outgoing light, lower lifetime, etc.). 

        Multi-chip array packaging is one of the most feasible methods to obtain high luminous flux at present, but the density of LED array packaging is limited by price, available space, electrical connection, especially heat-dissipation and so on. Due to the high density integration of the luminous chip, the temperature on the heat-dissipation substrate is very high, so it is necessary to adopt effective heat sink structure and proper packaging technology. The commonly used heat sink structure is divided into passively and actively heat dissipation. The passively heat dissipation usually uses the fin with high rib coefficient and dissipates the heat into the environment through the natural convection between the fin and the air. The scheme is simple in structure and high in reliability, but due to the low heat transfer coefficient of natural convection, it is only suitable for packaging with low power density and low integration. For high-power LED packaging, actively heat dissipation must be used, such as fin + fan, heat pipe, liquid forced convection, microchannel cooling, phase change cooling, etc.

 

  • COB LED vs SMD LED

LED

SMD LED

COB LED

Definition

SMD stands for Surface Mounted Devices, a special type of high power LED lights.

Chip-on-Board (COB) LEDs are high-power white LED arrays.

Thermal Resistance

High

Low

Light Quality

Point light and glare exist in the combination of discrete devices.

The angle of view is large and easy to adjust, and the refraction loss is small.

Characteristic

1) 60~70 lm per watt

2) Because it is multiple patches, even if one led lamp is damaged, it will not affect other light source product.

3) For RGB mode, with three primary colors LED lights, it can be developed up to 16 million colors.

4) High color rendering

5) The illumination angle can reach 360º.

6) Good shock resistance.

7) Unidirectional light source, it is suitable for home appliances.

1) 120 lm per watt

2) Illumination angle up to 160 degrees.

3) High light intensity , there is no need to reduce the light angle at all.

4) No special circuit requirements for stability.

5) Multi-directional light source reduces the possibility of glare.

6) Can withstand power fluctuations without affecting the quality of the light source.

Application

During the process, it needs to be pasted first and then fixed on the PCB by reflow soldering.

COB light source is directly applied to lamps without mounting and reflow soldering.

Production Efficiency

Low

High

Cost

High

Low


 

Ⅲ Test and Measurement

        The failure modes of LED devices mainly include electrical failure (such as short circuit or open circuit), optical failure (such as high temperature yellowing of sealant, deterioration of optical properties, etc.) and mechanical failure (such as lead breakage, desoldering, etc.), and all these factors are related to the packaging structure and process. The service life of LED is defined by the mean time to failure (MTTF), and it generally refers to the LED output flux attenuation to the initial 70% (for display purposes generally defined as 50% of the initial value) of the service time. Due to the long life of LED, the method of accelerated environmental test is usually used to test and estimate reliability. Test content mainly includes high temperature storage (100 ℃, 1000h), low temperature storage (-55 ℃, 1000h), high temperature and high humidity (85 ℃ / 85 ~ 1000h), high and low temperature cycle (85 ℃ ~ -55 ℃), thermal shock, corrosion resistance, resistance to solubility, mechanical impact and so on. However, accelerated environmental testing is only one aspect of the problem, and the research on the prediction mechanism and method of LED life is still a difficult problem to be studied.

LEDs

Ⅳ Technique Improvement


1)New phosphor and coating process development
Phosphor quality and coating process are the elements to ensure LED light efficiency. The development of phosphor technology includes the development of nano-crystal phosphor technology, surface-coated phosphor technology, etc., as well as the mixing of phosphors with packaging materials.

2)New package materials
Develop new materials with high thermal conductivity installed on the bottom plate of the LED chip, and then increase the operating current density of the LED chip by about 5 to 10 times. Due to better features such as high-temperature resistance, UV resistance, and low water absorption, thermosetting materials EMC, thermoplastic PCT, modified PPA and ceramic-like plastics will be widely used.

3)Wafer Bonding

Wafer bonding technology refers to making chip structures and packaging circuits on wafer, and then cutting wafer to form a single chip. Die bonding refers to after the chip structure and circuit completed on wafer, cutting the wafer to form the die, and then package single die (similar to the current LED packaging process).

Obviously, the wafer bonding is more efficient and of higher quality. Because the cost of packaging accounts for a large proportion of the manufacturing cost of LED devices, changing the existing LED packaging form (from die bonding to wafer bonding) will greatly reduce the packaging and manufacturing cost. In addition, wafer bonding can also prevent the damage of the device structure caused by scribing and slicing before bonding, improve the cleanliness of LED device production, the package yield and reliability. Therefore, wafer bonding is an effective mean to reduce packaging cost.

In addition, for high power LED packaging, it is necessary to adopt few packaging steps as far as possible in the process of chip design and package design. It can simplify the package structure and minimize the number of thermal and optical interfaces so as to reduce the thermal resistance of the package, and improve the efficiency of light production.


4)Multi-chip integrated package
High-power LEDs using common chips for high-density combination packaging can achieve higher luminous flux.

 

5.1 Ouestion

What is an LED package?

5.2 Answer

In electronics, a light-emitting diode (LED) is a semiconductor light source that emits light when current flows through it. While an LED package is a plastic casing that carries an LED chip and phosphor. The LED chip is the semiconductor material that emits light and the phosphor material converts some of this light into green and red wavelengths. This mixture results in the white light that is emitted by the LED package.

Related Articles

0 comment

Leave a Reply

Your email address will not be published.

 
 
   
Rating: