Basic knowledge of LED patch glue and glue
Basic knowledge of LED patch glue and glue
1. The role of surface mount adhesives (SMA, surface mount adhesives) are used for wave soldering and reflow soldering. They are mainly used to fix components on the printed board, and are generally distributed by dispensing or steel screen printing In order to maintain the position of the component on the printed circuit board (PCB), ensure that the component will not be lost during the transfer process on the assembly line. Put the components in the oven or reflow soldering machine for heating and hardening. It is different from the so-called solder paste. Once it is hardened by heating, it will not melt when heated again, that is to say, the heat curing process of the patch adhesive is irreversible. The use effect of SMT patch adhesive will vary depending on the thermal curing conditions, the connected objects, the equipment used, and the operating environment. When using it, the patch glue should be selected according to the production process.
2. The composition of the patch adhesive Most surface mount adhesives (SMA) used in PCB assembly are epoxy resins (epoxies), although there are acrylics for special purposes. After the introduction of high-speed glue dispensing systems and the electronics industry mastering how to handle products with relatively short shelf life, epoxy resin has become the more mainstream glue technology worldwide. 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 (that is, the main high molecular material), filler, curing agent, other additives, etc.
3. The purpose of using the patch glue a. To prevent the components from falling off during wave soldering (wave soldering process) b. To prevent the other components from falling off during reflow soldering (double-sided reflow soldering process) c. To prevent the components from moving and standing Location (reflow soldering process, pre-coating process) d. Marking (wave soldering, reflow soldering, pre-coating), when the printed board and components are changed in batches, use patch adhesive for marking.
4. Classification of the use of patch glue a. Dispensing type: the glue is applied on the printed circuit board through the dispensing equipment. b. Squeegee type: apply sizing by steel screen or copper screen printing.
5. Dispensing method SMA can be applied to PCB using syringe dispensing method, needle transfer method or template printing method. The needle transfer method uses less than 10% of all applications. It uses a needle array to be dipped in a plastic tray. The suspended glue drops are then transferred to the board as a whole. These systems require a lower viscosity glue and have good resistance to moisture absorption because it is exposed to the indoor environment. The key factors that control the transfer of needle dispensing glue include the diameter and style of the needle, the temperature of the glue, the depth of the needle immersion and the length of the dispensing cycle (including the delay time before and during the needle touches the PCB). The tank temperature should be between 25 and 30°C, which controls the viscosity of the glue and the number and form of glue spots.
Template printing is widely used in solder paste, and can also be used with dispensing adhesives. Although less than 2% of SMAs are currently printed with templates, interest in this method has increased, and new equipment is overcoming some of the earlier limitations. The correct template parameters are the key to good results. For example, contact printing (zero off-board height) may require a delay period, allowing good dot formation. In addition, the non-contact printing of polymer templates (approximately 1mm gap) requires optimal squeegee speed and pressure. The thickness of the metal template is generally 0.15~2.00mm, which should be slightly larger than (+0.05mm) the gap between the component and the PCB.
The final temperature will affect the viscosity and the shape of the glue dot. Most modern glue dispensers rely on the temperature control device on the needle mouth or the chamber to keep the glue temperature above room temperature. However, if the PCB temperature is increased from the previous process, the dot contour may be damaged.