Analysis of various forms of packaging structures of LED light-emitting diodes

LED is a type of light-emitting device that can directly convert electrical energy into visible light and radiant energy. It has low operating voltage, low power consumption, high luminous efficiency, extremely short luminous response time, pure light color, solid structure, impact resistance, and vibration resistance. , with a series of characteristics such as stable and reliable performance, light weight, small size, and low cost, it has developed rapidly and can now mass-produce high-brightness and high-performance products in various colors throughout the visible spectrum. Domestic production of red, green, orange and yellow LEDs accounts for about 12% of the world's total. The industrial goal during the "Tenth Five-Year Plan" period is to achieve an annual production capacity of 30 billion units and achieve the production of ultra-high brightness AiGslnP LED epitaxial wafers and chips. Large-scale production, with an annual output of more than 1 billion red, orange, and yellow ultra-high-brightness LED dies, breakthroughs in key technologies of GaN materials, and realization of medium-volume production of blue, green, and white LEDs.

In the LED industry link, the upstream is LED substrate wafer and substrate production, the midstream industrialization is LED chip design and manufacturing, and the downstream is LED packaging and testing. The development of packaging technology with low thermal resistance, excellent optical properties, and high reliability is The only way for new LEDs to become practical and marketable is through industrialization. In a sense, they are the link between industry and market. Only when they are packaged can they become end products, be put into practical applications, and can provide services to customers and make them more practical. The industrial chain is interlocked and seamless.

2 Specialties of LED packaging

LED packaging technology is mostly developed and evolved based on discrete device packaging technology, but it has great particularities. Generally, the die of a discrete device is sealed in a package. The main function of the package is to protect the die and complete electrical interconnection. LED packaging completes the output of electrical signals, protects the normal operation of the tube core, and outputs: visible light functions. It has both electrical parameters and optical parameters. It has 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 from the pn junction area are non-directional, that is, they have the same probability of being emitted in all directions. Therefore, not all the light generated by the tube core can be released, which mainly depends on the quality of the semiconductor material, the tube core structure and geometry. , the internal structure and packaging materials of the package, and 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 tube with a side length of 0.25mm on the lead frame. The positive electrode of the tube is bonded to the gold wire through a spherical contact point and bonded to an inner lead connected to a pin, and the negative electrode is connected to a pin through a reflection The cup is connected to another pin of the lead frame, and the top is then encapsulated with epoxy. The function of the reflective cup is to collect the light emitted from the side and interface of the tube core and emit it in the desired direction angle. The top-encapsulated epoxy resin is made into a certain shape and has several functions: protecting the tube core from external erosion; using different shapes and material properties (with or without dispersion agent) to act as a lens or diffusion lens Function, control the divergence angle of light; the refractive index of the tube core is too closely related to the refractive index of air, causing the critical angle of total reflection inside the tube core to be very small. Only a small part of the light generated by the active layer is taken out, and most of it is easily absorbed in the tube. The inside of the core is absorbed after multiple reflections, and total reflection is prone to occur, resulting in excessive light loss. An epoxy resin with a corresponding refractive index is used 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 for the light emitted by the tube core. Selecting packaging materials with different refractive indexes, the impact of packaging geometry on photon escape efficiency is different. The angular distribution of luminous intensity is also related to the die 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 will be smaller; if the top resin lens is round or flat, the corresponding viewing angle will increase.

Under normal circumstances, the luminous wavelength of LED changes with temperature by 0.2-0.3nm/℃, and the spectrum width increases accordingly, affecting the brightness of the color. In addition, when the forward current flows through the pn junction, the heat loss causes a temperature rise in the junction area. Near room temperature, for every 1°C increase in temperature, the luminous intensity of the LED will correspondingly decrease by about 1%, and the package heat dissipation will maintain the color. Purity and luminous intensity are very important. In the past, methods were often used to reduce the driving current and lower the junction temperature. The driving current of most LEDs is limited to about 20mA. However, the light output of LED will increase with the increase of current. At present, the driving current of many power LEDs can reach 70mA, 100mA or even 1A. It is necessary to improve the packaging structure, new LED packaging design concept and low thermal resistance packaging structure and technology to improve thermal characteristics. For example, a large-area chip flip-chip structure is adopted, 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 continued to develop and innovate in terms of high efficiency, ultra-high brightness, and full color. The luminous efficiency of red and orange LEDs has reached 100Im/W, and that of green LEDs is 501m/W. The luminous flux of a single LED has also reached dozens. Im. LED chips and packages no longer follow traditional design concepts and manufacturing production models. In terms of increasing the light output of the chip, research and development is not limited to changing the number of impurities, lattice defects and dislocations in the material to improve internal efficiency. At the same time, how to improve The internal structure of the tube core and package enhances the probability of photon emission inside the LED, improves light efficiency, solves heat dissipation, optimizes the design of light extraction and heat sink, improves optical performance, and accelerates the process of surface mount SMD, which is the mainstream research and development direction in the industry. .