Mini-LED Encapsulation Has Many Difficulties. What Is The Best Way To Solve The Printing Process Problems?

- Jul 01, 2019-

Mainstream manufacturers in the world have basically completed the research and development process of Mini-LED backlight, and entered the stage of small batch sample or large batch supply.            In recent years, with the rapid development of panel industry, after the crazy expansion of domestic manufacturers, the relationship between supply and demand of products has been basically eased, and manufacturers began to pay attention to the improvement of product quality. Display panel industry will develop towards high-resolution picture, surface, ultra-thin plane, thinning, flexibility, high dynamic HDR, high contrast and wide gamut. Mini-LED emerges as the times require.            According to the forecast, Mini-LED will officially break out in 2019 and enter the stage of commercialization. At present, the mainstream manufacturers in the world have basically completed the research and development process of Mini-LED backlight, and entered the stage of small batch sample or mass supply. Major domestic enterprises are also carrying out intensive process research to speed up the process of putting into the market.            Mini-LED uses LED chips of micron size. Each Mini-LED circuit board usually has thousands of chips and tens of thousands of solder joints to connect RGB trichrome chips. Such a huge number of solder joints has brought great difficulty to chip packaging. Today, let's learn about the printing process.            Compared with the traditional SMT printing process, the requirement of Mini-LED has reached the extreme. According to statistics, more than 60% of the welding defects are caused by the printing process. For the precision printing of Mini-LED, higher requirements are put forward for equipment (printing press), accessories (steel mesh) and materials (solder paste), which are indispensable.            Equipment - High Precision Printer            Mini-LED has smaller pads and thinner steel mesh, which requires higher precision and function of the equipment. Common problems in printing at present include:            Questions 1 and 2: Mini-LED process uses steel mesh thickness between 0.03-0.05 mm, steel mesh is thin and easily deformed, requiring equipment to have steel mesh adsorption function, so that the steel mesh and welding pad can be closely fitted.            Question 3: Mini-LED pad size is generally less than 120um, which requires higher printing accuracy and repetitive alignment accuracy of the equipment.            Question 4: Because the size of the opening is smaller, the adjustment range and precision of the angle, pressure and speed of the scraper are more demanding.            Effect of Three Elements on Solder Paste Filling            1) The pressure of scraper is high, which is good for filling solder paste and adhering to PAD, but the wear of steel mesh must be considered.            2) When there is less solder paste on steel mesh, the angle of scraper can be adjusted to increase filling ability of solder paste; when there is more solder paste on steel mesh, the angle of scraper can be adjusted to reduce filling ability of solder paste; the smaller the angle of scraper, the larger the volume of solder paste.            3) The scraper speed can affect the filling quantity of solder paste, but the volume change of solder paste is small, mainly considering the effect of scraper angle.            Precision printing equipment can be based on the characteristics of printing steel mesh and products, the above three elements should have a wide adjustment space. At present, the existing equipment can achieve stable mass production of Mini-LED.            Accessories - Steel mesh + nano coating            Good demoulding performance is an important condition to ensure welding performance. For small openings, nano-coating can be applied on the surface of the steel mesh to make the steel mesh have high hydrophobicity and oil-thinning, eliminate burrs, make the hole wall smoother and neater, improve the demoulding property of solder paste, reduce the wear of printing scraper and steel mesh, and prolong the service life of the steel mesh.            Material - 6 # Powder Solder Paste            EM-6001 is a solder paste developed by Shenzhen Chenri Science and Technology Co., Ltd. for Mini LED printing process. It is made of Sn96.5Ag3Cu0.5 tin powder with superfine, narrow particle size distribution, high sphericity and low oxygen content and ROL0 solder paste. It has good printability and demoulding under fine spacing pad size and is reflow soldered. After welding, there are few residues, no corrosiveness, full and bright solder joints, no collapse and short-circuit phenomenon of welding bridges, which meet the requirements of high precision and high reliability of electrical parameters, and the poor rate of SPI on-line detection is low, which can effectively improve the productivity of products.            Characteristic:            The stable physical properties and oxidation resistance lasted for more than 8 hours.            Good printability and film removal, tin uniformity, no tail, wire drawing and solder joint adhesion.            Narrow particle size and ultra-fine solder powder make the tin content more stable under demoulding.            Better anti-collapse performance to prevent Liantin tin;            Excellent welding reliability, very small void rate, no small tin beads, chip will not occur tilt, offset and other phenomena.