1. The role of SMD adhesives. 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. They are generally distributed by dispensing or stencil printing. , In order to maintain the position of the component on the printed circuit board (PCB) to ensure that the component will not be lost during the transfer process on the assembly line. After the components are pasted, they are placed in an oven or reflow soldering machine to heat and harden. It is not the same as the so-called solder paste. Once heated and hardened, it will not melt when heated again, that is to say, the thermal hardening process of the patch adhesive is irreversible. The use effect of SMT patch adhesive will vary depending on the thermal curing conditions, the connected object, the equipment used, and the operating environment. When using, choose the patch glue according to the production process.

　　2. Components of SMD Adhesives Most of the surface mount adhesives (SMA) used in PCB assembly are epoxy resins (epoxies), although there are acrylics used for special purposes. After the introduction of high-speed glue dispensing systems and the electronics industry mastered how to deal with products with relatively short shelf life, epoxy resin has become a more mainstream glue technology worldwide. Epoxy resin generally provides good adhesion to a wide range of circuit boards and has very good electrical properties. The main ingredients are: base material (that is, the main high-molecular material), filler, curing agent, other additives, etc.

　　3. Purpose of use of SMD adhesive a. Prevent components from falling off during wave soldering (wave soldering process) b. Prevent components on the other side from falling off during reflow soldering (double-sided reflow soldering process) c. Prevent component displacement and erection Location (reflow soldering process, pre-coating process) d. Marking (wave soldering, reflow soldering, pre-coating), when printed boards and components are changed in batches, use patch glue for marking.

　　4. Classification of the use of patch glue a. Dispensing type: Glue is applied on the printed circuit board through a dispensing equipment. b. Squeegee type: Glue is applied by means of printing and scraping on steel or copper screen.

　　5. Glue dropping method SMA can be applied to PCB using syringe dropping method, needle transfer method or template printing method. The needle transfer method uses less than 10% of all applications. It uses an array of needles immersed in a plastic tray. Then the hanging glue drops are transferred to the board as a whole. These systems require a lower viscosity glue and good resistance to moisture absorption because it is exposed to indoor environments. The key factors that control the transfer of glue by the needle include the diameter and style of the needle, the temperature of the glue, the depth of the needle immersion and the length of the glue cycle (including the delay time before and during the needle contact with the PCB). The tank temperature should be between 25~30°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 for dispensing glue. Although less than 2% of SMAs are currently printed in templates, interest in this method has increased, and new equipment is overcoming some of the earlier limitations. Correct template parameters are the key to achieving good results. For example, contact printing (zero height off the board) may require a delay period to allow good glue dot formation. In addition, non-contact printing of polymer templates (approximately 1mm gap) requires the best 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.

　　Finally, the temperature will affect the viscosity and the shape of the glue point. Most modern glue dispensers rely on the temperature control device on the nozzle or chamber to keep the temperature of the glue above room temperature. However, if the PCB temperature is increased from the previous process, the glue dot profile may be damaged.