LED patch glue and Dijiao basic knowledge

1, the role of plastic surface mount adhesive (SMA, surface mount adhesives) for wave soldering and reflow soldering, mainly used to fixed components in the printed circuit board, the general use of dispensing or stencil printing method to allocate To maintain the component's position on the printed circuit board (PCB), ensuring that components are not lost during transfer on the assembly line. Paste the components into the oven or reflow oven heat hardening. It is not the same as the so-called solder paste, once heated and hardened, then heat will not melt, that is, the thermal curing process of plastic patch is not reversible. The effect of the use of SMT chip adhesives will vary depending on the heat cure conditions, the materials to be connected, the equipment used, and the operating environment. Use according to the production process to select the patch glue.
2, the components of plastic patch PCB assembly used in the majority of surface mount plastic (SMA) are epoxy (epoxies), although there are polypropylene (acrylics) for special purposes. Epoxy resin has become a more mainstream glue technology in the world after the introduction of high-speed epoxy systems and the electronics industry's knowledge of how to handle products with relatively short shelf life. Epoxy resins generally provide good adhesion to a wide range of circuit boards and have very good electrical properties. The main ingredients are: base material (ie, the main high molecular materials), fillers, curing agents, and other additives.
3, the purpose of the use of plastic chip a. Wave soldering to prevent components from falling off (wave soldering process) b. Reflow to prevent the other side of the device off (double reflow process) c. To prevent displacement and components stand (Wave soldering, reflow soldering, pre-coating), printed circuit boards and components to change, with a patch of plastic for marking.
4, the use of patch glue classification a. Dispensing: through the dispensing equipment in the printed circuit board sizing. B. Scraper type: by stencil or copper mesh printing scrape way for sizing.
5, Dijiao method SMA can use syringe Dijiao, needle transfer method or template printing method applied to the PCB. Needle transfer method used less than 10% of all applications, it is the use of needle array array immersed in the plastic tray. The suspended glue drops are then transferred to the plate as a unit. These systems require a less viscous adhesive and are well resistant to moisture absorption because they are exposed to an indoor environment. Key factors in controlling the transfer of the needle include the diameter and pattern of the needle, the temperature of the glue, the depth of penetration of the needle, and the length of the cycle of the dispensing (including the delay before and during the contact of the needle with the PCB). Pool temperature should be between 25 ~ 30 ° C, which controls the viscosity of the glue and the number and form of the gel.
Template printing is widely used in solder paste, can also be used with the distribution of glue. Although less than 2% of the SMA is currently printed with templates, interest in this method has increased and new equipment is overcoming some of the earlier limitations. Correct template parameters are key to achieving good results. For example, contact printing (off-board height) may require a delay period, allowing for good dot formation. In addition, non-contact printing of polymer patterns (about 1 mm gap) requires optimum blade speed and pressure. The thickness of the metal template is generally 0.15 to 2.00 mm and should be slightly larger than (+0.05 mm) the gap between the component and the PCB.
The final temperature will affect the viscosity and the shape of the dot. Most modern dispensers rely on the temperature control device on the needle mouth or the chamber to keep the temperature of the glue above room temperature. However, if the PCB temperature is increased from the previous process, the dot profile may be damaged.