Major manufacturing processes for silicon substrate LED chips(2)

Major manufacturing processes for silicon substrate LED chips(2)

2021-11-10 16:41:52 5

2 Si substrate LED packaging technology


  2.1 Technical route


Using blue LED excitation YAG / silicate / nitrogen oxide multi-color system phosphor, emission of yellow, green, red light, synthesis of white light technology route.


Process flow: assembly of blue LED chip on metal bracket / ceramic bracket (conductive adhesive bonding process) → bonding (gold wire ball welding process) → fluorescent adhesive coating (automated graphic dispensing / automatic injection process) → Si adhesive packaging (mold potting process) → cut rib → test → packaging.


  2.2 Main packaging process


  The Si substrate power GaN-based LED package adopts the form of lumen-like holder package with the shape of Lambertian, rectangular and biplane. The fabrication process is: using the 194 alloy metal bracket with high thermal conductivity, first bonding the LED chip to the bottom of the reflector of the metal bracket, then connecting the metal leads to the LED chip and the metal bracket electrode through the bonding process to complete the electrical connection, and finally covering the chip and electrode leads with organic packaging materials (such as Si adhesive) to form the package protection and optical channel. This package is optimal for light extraction efficiency, heat dissipation performance, and increased working current density of the design. Its main features include: low thermal resistance (less than 10 ℃ / W), high reliability, the package is filled with stable flexible gel inside, in the range of -40 ~ 120 ℃, will not be generated by sudden temperature changes in the internal stress, so that the gold wire and bracket disconnected, and to prevent the organic packaging materials become yellow, the lead frame will not be stained by oxidation; optimized package structure design so that the optical efficiency, external quantum efficiency performance is excellent, its structure See Figure 2.

2.3 Key Technology and Innovation


  The thermal characteristics of power LEDs directly affect the LED operating temperature, luminous efficiency, luminous wavelength, service life, etc. The existing Si substrate power GaN-based LED chip design uses a vertical structure to improve the light extraction efficiency of the chip, improve the thermal characteristics of the chip, while increasing the chip area, increase the operating current to improve the photoelectric conversion efficiency of the device, so as to obtain a higher luminous flux The LED package design and manufacturing technology of power LEDs brings new issues. Power LED packaging is focused on the use of effective heat dissipation and non-degradable packaging materials to solve the problem of light decay. In order to achieve the packaging technology requirements, in a large number of tests and exploration, analysis to solve the relevant technical problems, the key technologies used and innovative are the following.


  (1) By designing a new ceramic package structure, the total reflection is reduced, so that the device obtains high light extraction efficiency and suitable optical space distribution.


  (2) Electro-thermally isolated package structure and optimized heat sink design are adopted to fit the packaging requirements of thin film chips.


  (3) Adopt metal support with high thermal conductivity and choose thermally conductive adhesive to bond the chip to obtain a good heat dissipation channel with low thermal resistance, so that the product light decay ≤5% (1,000 h).


  (4) Adopt high efficiency and high precision fluorescent glue ratio and spraying process to ensure the controllable and consistent light color parameters of the product.


  (5) Multi-layer composite packaging, reducing the packaging stress, implementing SSB bonding process and multi-stage curing process, improving the reliability of the product.


  (6) Assembling protection diodes to increase product ESD protection to 8 000 V.


 3 Product Test Results


  3.1 Si substrate LED chips


  By optimizing the treatment of the Si substrate surface and the buffer layer structure, the epitaxial material that can be used for high-power chips was successfully grown. The Pt electrode was used as the reflector to successfully realize the thin film transfer of the high-power chip. Adopted silver as the reflector, which greatly improved the reflection efficiency, and increased the light output power by improving the design of the reflector and introducing the coarsening technology. The cleaning process of p-type GaN surface before Ag reflector evaporation and wafer welding process were improved, the ohmic contact of Ag reflector was improved, the buffer structure was introduced before quantum well to improve the chip luminescence efficiency, the growth process of quantum well/base interface was optimized, the luminescence efficiency was further improved, the chip cracking problem during substrate transfer was reduced by improving the welding technology, the chip preparation yield was greatly improved, and the reliability is improved. Through the application and improvement of the above-mentioned technologies, blue light-emitting chips with a size of 1 mm×1 mm and a light output power of more than 380 mW at 350 mA with a wavelength of 451 nm and an operating voltage of 3.2 V were successfully prepared, fulfilling the targets specified in the project. Table 1 shows the test results of the photoelectric performance parameters of the chip.


 Note: The test conditions are 350 mA DC and Ta=25°C constant temperature.


  3.2 Si substrate LED package


  According to the optical structure of LED and the performance of chip and package materials, the optical design model and software simulation means were established to optimize the optical structure design of the package. Through the improvement of packaging process technology, the total reflection of light is reduced, and the light extraction efficiency of the product is improved. The conductive adhesive dispensing process method was improved, and the structure and precision of the mounting equipment tooling was improved. The electro-thermal isolation packaging structure and optimized heat sink design were used to reduce the device thermal resistance and improve the product heat dissipation performance. The plasma cleaning process was adopted to improve the LED package interface bonding and reliability. In response to the different requirements of lighting applications on the light color characteristics of light sources, we studied the influencing factors of warm white, daylight white, and cold white LED colors: chip parameters, phosphor performance, formulation, and dosage, and improved the control of power LED light color parameters by improving the phosphor coating process to produce products with lighting color gamut specifications on file. Blue and white LED package test results are shown in Table 2. table: φ is the luminous flux; K is the luminous efficiency; P is the optical power; R is the thermal resistance; μ is the light decay; I is the saturation current.


4 Conclusion


  Si substrate GaN-based LED manufacturing technology is the third international LED manufacturing technology route, is one of the three original LED technology, compared with the first two technology routes, has four advantages: First, with original technology property rights, the product can be sold to the international market, not subject to international patent restrictions. Second, with excellent performance, the product has good anti-static performance, long life, and can withstand high current density. Third, the device packaging process is simple, the chip is upper and lower electrode, single lead vertical structure, only single electrode lead is needed in the device packaging, which simplifies the packaging process and saves the packaging cost. Fourth, the Si substrate is much cheaper than the sapphire and SiC used in the first two technology routes, and it will be more efficient to produce in the future, so the cost is low.


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