1. Surface mount adhesives (SMA) are used for wave soldering and reflow soldering, mainly to fix components on printed boards. They are usually distributed by glue or steel mesh printing to maintain the position of the components on the printed circuit board (PCB) and ensure that they are not lost during transportation on the assembly line. After attaching the components, place them in an oven or reflow soldering machine to heat and harden. It is different from so-called solder paste. Once heated and hardened, it will not melt even when heated again, which means that the thermal hardening process of the patch adhesive is irreversible. The effectiveness of SMT patch adhesive may vary depending on the curing conditions, the connected object, the equipment used, and the operating environment. When using, choose the patch adhesive according to the production process.

2. Most surface mount adhesives (SMA) used in PCB assembly are epoxy resins, although polypropylene (acrylic) is also used for special purposes. After the introduction of high-speed dispensing systems and the mastery of how to handle products with relatively short shelf life in the electronics industry, epoxy resin has become a more mainstream adhesive technology worldwide. Epoxy resin generally provides good adhesion to a wide range of circuit boards and has excellent electrical performance. The main components are: base material (i.e. main high molecular weight material), filler, curing agent, other additives, etc.

3. The purpose of using patch adhesive is a Preventing component detachment during wave soldering (wave soldering process) b. Preventing component detachment on the other side during reflow soldering (double-sided reflow soldering process) c. Preventing component displacement and standing (reflow soldering process, pre coating process) d. Marking (wave soldering, reflow soldering, pre coating). When changing the batch of printed boards and components, use patch adhesive for marking.

4. Classification of the use of patch adhesive a Glue dispensing type: It applies glue to printed circuit boards through dispensing equipment. b. Scratch type: Apply glue by printing and scraping on steel or copper mesh.

5. SMA can be applied to PCBs using syringe dispensing, needle transfer, or template printing methods. The use of needle transfer method accounts for less than 10% of all applications, as it involves immersing a needle array in a tray of glue. Then the suspended adhesive droplets are transferred as a whole onto the board. These systems require a lower viscosity adhesive and good resistance to moisture absorption, as it is exposed to indoor environments. The key factors for controlling the transfer of dispensing glue from the needle include the diameter and style of the needle, the temperature of the glue, the depth of immersion of the needle, and the length of the dispensing cycle (including the delay time before and during the needle's contact with the PCB). The temperature of the tank should be between 25-30 ° C, which controls the viscosity of the adhesive and the number and form of adhesive points.

Template printing is widely used for solder paste and can also be used with dispensing adhesives. Although less than 2% of SMA is currently printed using 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 achieving good results. For example, contact printing (zero off board height) may require a delay period to allow for good adhesive dot formation. In addition, suitable scraper speed and pressure are required for non-contact printing of polymer templates (with a gap of approximately 1mm). The thickness of the metal template is generally 0.15~2.00mm, which should be slightly larger than the gap between the component and the PCB (+0.05mm).

Temperature will affect viscosity and gel point shape, and most modern droppers rely on temperature control devices on the nozzle or chamber to maintain the temperature of the gel above room temperature. However, if the PCB temperature is increased from the previous process, the contour of the adhesive dots may be damaged.

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