Various forms of LED packaging structures and technologies

Various forms of LED packaging structures and technologies

In the LED industry chain, the upstream is LED substrate wafers and substrate production, the midstream industrialization is LED chip design and manufacturing production, and the downstream is LED packaging and testing. The research and development of low thermal resistance, excellent optical properties, and high-reliability packaging technology is the only way for new LEDs to become practical and market-oriented industrialization. In a sense, it is the link between the industry and the market. Only well-packaged ones can become terminal products, be put into practical applications, and provide services to customers, making the industrial chain interconnected and seamless.

The particularity of LED packaging

LED packaging technology is mostly developed and evolved on the basis of discrete device packaging technology, but it has great particularity. Generally speaking, the die of discrete devices is sealed in the package body, and the main function of the package is to protect the die and complete the electrical interconnection. LED packaging is to complete the output of electrical signals, protect the normal operation of the die, and output: visible light. It has both electrical parameters and optical parameters design and technical requirements. It is impossible to simply use the packaging of discrete devices for LEDs.

The core light-emitting part of the LED is a pn junction die composed of p-type and n-type semiconductors. When the minority carriers injected into the pn junction recombine with the majority carriers, visible light, ultraviolet light or near-infrared light will be emitted. However, the photons emitted by the pn junction area are non-directional, that is, there is an equal probability of emission in all directions. Therefore, not all the light generated by the die can be released. This mainly depends on the quality of the semiconductor material, the structure and geometry of the die, the internal structure of the package and the encapsulation material. The application requires improving the internal and external quantum efficiency of the LED. The conventional Φ5mm LED package is to bond or sinter a square die with a side length of 0.25mm on the lead frame. The positive electrode of the die is connected to a pin through a spherical contact point and a gold wire, and the negative electrode is connected to another pin of the lead frame through a reflective cup, and then the top is encapsulated with epoxy resin. The function of the reflective cup is to collect the light emitted from the side and interface of the die and emit it in the desired direction angle. The epoxy resin encapsulated on the top is made into a certain shape, which has the following functions: protecting the tube core from external erosion; using different shapes and material properties (with or without dispersion agent) to play the role of lens or diffuse lens to control the divergence angle of light; the refractive index of the tube core is too related to the refractive index of air, resulting in a very small critical angle of total reflection inside the tube core, and only a small part of the light generated by its active layer is taken out, and most of it is easily absorbed after multiple reflections inside the tube core, which is prone to total reflection and excessive light loss. Epoxy resin with corresponding refractive index is selected as a transition to improve the light emission efficiency of the tube core. The epoxy resin used to form the tube shell must have moisture resistance, insulation, mechanical strength, and high refractive index and transmittance of the light emitted by the tube core. When selecting packaging materials with different refractive indices, the impact of packaging geometry on photon escape efficiency is different, and the angular distribution of luminous intensity is also related to the tube core structure, light output mode, and the material and shape of the packaging lens. If a pointed resin lens is used, the light can be concentrated in the axial direction of the LED, and the corresponding viewing angle is smaller; if the top resin lens is circular or flat, the corresponding viewing angle will be increased.

In general, the wavelength of LED light changes with temperature by 0.2-0.3nm/℃, and the spectral width increases accordingly, affecting the color vividness. In addition, when the forward current flows through the pn junction, the heat loss causes the junction area to rise in temperature. Near room temperature, the LED's luminous intensity will decrease by about 1% for every 1℃ increase in temperature. The package heat dissipation; it is very important to maintain color purity and luminous intensity. In the past, the method of reducing its driving current was mostly used to reduce the junction temperature. The driving current of most LEDs is limited to about 20mA. However, the light output of LEDs will increase with the increase of current. At present, the driving current of many power LEDs can reach 70mA, 100mA or even 1A level. It is necessary to improve the packaging structure, a new LED packaging design concept and a low thermal resistance packaging structure and technology to improve thermal characteristics. For example, a large-area chip flip-chip structure is used, silver glue with good thermal conductivity is selected, the surface area of ​​the metal bracket is increased, and the silicon carrier of the solder bump is directly mounted on the heat sink. In addition, in application design, the thermal design and thermal conductivity of PCB circuit boards are also very important.

After entering the 21st century, LEDs have been continuously developing and innovating in terms of high efficiency, ultra-high brightness, and full color. The luminous efficiency of red and orange LEDs has reached 100Im/W, that of green LEDs is 501m/W, and the luminous flux of a single LED has reached tens of Im. LED chips and packages no longer follow the traditional design concept and manufacturing production model. In terms of increasing the light output of chips, research and development is not limited to changing the amount of impurities in the material, lattice defects and dislocations to improve internal efficiency. At the same time, how to improve the internal structure of the tube core and package, increase the probability of photon emission inside the LED, improve light efficiency, solve heat dissipation, optimize the design of light extraction and heat sink, improve optical performance, and accelerate the process of surface mounting SMD is the mainstream direction of research and development in the industry.

Product packaging structure type

Since the 1990s, the research and development of LED chip and material manufacturing technology has achieved many breakthroughs, transparent substrate trapezoidal structure, textured surface structure, chip flip-chip structure, commercial ultra-high brightness (above 1cd) red, orange, yellow, green, blue LED products have been launched one after another, as shown in Table 1, and have been used in special lighting with low and medium light flux since 2000. The upstream and midstream industries of LED have received unprecedented attention, further promoting the development of downstream packaging technology and industry, using different packaging structure forms and sizes, tube cores with different luminous colors and their two-color or three-color combination methods, and can produce a variety of series, varieties and specifications of products.

The types of LED product packaging structures are shown in Table 2. They are also classified according to the characteristics of luminous color, chip material, luminous brightness, size, etc. A single tube core generally constitutes a point light source, and multiple tube cores can generally constitute a surface light source and a line light source for information, status indication and display. The light-emitting display also uses multiple tube cores, which are combined with appropriate connections (including series and parallel connections) of the tube cores and suitable optical structures to form the light-emitting segments and light-emitting points of the light-emitting display. Surface mount LEDs can gradually replace pin-type LEDs, and the application design is more flexible. They have occupied a certain share in the LED display market and have an accelerating development trend. Some solid lighting sources have been launched on the market, becoming the medium- and long-term development direction of LEDs in the future.

Leaded package

LED foot-type package uses lead frame as pins of various package shapes. It is the first package structure successfully developed and put on the market. There are many varieties and the technology is mature. The structure and reflective layer of the package are still being improved. Standard LED is considered by most customers to be the most convenient and economical solution in the display industry. The typical traditional LED is placed in a package that can withstand 0.1W input power. 90% of its heat is dissipated from the negative lead frame to the PCB board and then to the air. How to reduce the temperature rise of the pn junction during operation is a must for packaging and application. The encapsulation material is mostly high-temperature curing epoxy resin, which has excellent optical properties, good process adaptability, and high product reliability. It can be made into colored transparent or colorless transparent and colored scattering or colorless scattering lens packages. Different lens shapes constitute a variety of shapes and sizes. For example, the circle is divided into several types according to the diameter, such as Φ2mm, Φ3mm, Φ4.4mm, Φ5mm, Φ7mm, etc. Different components of epoxy resin can produce different luminous effects. There are many different packaging structures for colorful point light sources: ceramic base epoxy resin packaging has good operating temperature performance, the pins can be bent into the required shape, and the volume is small; metal base plastic reflective cover packaging is an energy-saving indicator light, suitable for power indication; the flashing type combines the CMOS oscillation circuit chip with the LED tube core, which can generate flashing light with strong visual impact; the two-color type is composed of two tube cores with different luminous colors, which are packaged in the same epoxy resin lens. In addition to the two colors, a third mixed color can be obtained. It is widely used in large-screen display systems and can be packaged to form a two-color display device; the voltage type combines the constant current source chip with the LED tube core, which can directly replace various voltage indicator lights of 5-24V. The surface light source is formed by bonding multiple LED tube cores to the specified position of the micro PCB board, using a plastic reflective frame cover and potting epoxy resin. The different designs of the PCB board determine the arrangement and connection of the external leads, and there are structural forms such as dual-row in-line and single-row in-line. Hundreds of package shapes and sizes have been developed for point and surface light sources for market and customer application.