1, the role of patch adhesive surface mount adhesive (SMA, surface mount adhesives) for wave soldering and reflow soldering, mainly used to fix the components on the printed board, usually by dispensing or stencil printing method to distribute In order to keep the components on the printed circuit board (PCB) so that they are not lost during transfer on the assembly line. After affixing the components and placing them in an oven or reflow machine, heat and harden them. It is not the same as the so-called solder paste. Once it is hardened by heating, it will not melt after reheating, that is, the thermal hardening process of the patch glue is irreversible. The effect of using SMT adhesives varies depending on the thermal curing conditions, the materials to be joined, the equipment used, and the operating environment. When using, choose the patch glue according to the production process.
2. Composition of SMT Adhesives Most surface mount adhesives (SMAs) used in PCB assembly are epoxies, although there are also acrylics for special applications. After the introduction of high-speed dispensing systems and the electronics industry's understanding of how to handle products with relatively short shelf lives, epoxy resins have 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 (ie, the main high molecular material), filler, curing agent, and other additives.
3. The purpose of the use of patch glue a. Prevent the component from falling off during wave soldering (wave soldering process) b. Prevent the other component from falling off during reflow (double-sided reflow process) c. Prevent component displacement and damage Location (reflow process, pre-coating process) d. Marking (wave soldering, reflow, pre-coating). When printed boards and components are changed in bulk, they are marked with a piece of adhesive.
4, the use of patch glue classification a. dispensing type: dispensing equipment in the printed circuit board sizing. b. Squeegee type: sizing by stencil or copper screen printing.
5. Epoxy Method The SMA can be applied to the PCB using a syringe dispensing method, a needle transfer method, or a template printing method. The use of the needle transfer method is less than 10% of all applications. It uses a needle array to dip in a plastic tray. The hanging glue droplets are then transferred to the board as a whole. These systems require a less viscous glue and are very resistant to moisture absorption because it is exposed to the indoor environment. The key factors that control needle dispensing are the diameter and style of the needle, the temperature of the glue, the depth of penetration of the needle, and the length of the dispensing cycle (including the delay time before and during the needle contacting the PCB). The bath temperature should be between 25~30°C. It controls the viscosity of the glue and the quantity and form of the glue spots.
Template printing is widely used for solder pastes and can also be used with dispensing glues. 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 limitations of the earlier. 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, non-contact printing of polymer templates (approximately 1 mm gap) requires optimum squeegee speed and pressure. The thickness of the metal template is generally 0.15~2.00mm and should be slightly larger than the (+0.05mm) gap between the component and the PCB.
The final temperature will affect the viscosity and dot shape. Most modern dispensers rely on the temperature control device on the needle or chamber to keep the temperature of the adhesive above room temperature. However, if the PCB temperature is increased from the previous process, the dot profile may be damaged.