LED SMT and Epoxy Basics

- May 07, 2019-

1. The role of patch adhesives (SMA, surface mount adhesives) is used for wave soldering and reflow soldering. It is mainly used to fix components on printed boards, usually by dispensing or stencil printing. To maintain the position of the component on the printed circuit board (PCB), to ensure that components are not lost during transmission on the assembly line. After attaching the components, put them in an oven or reflow soldering machine to heat harden. It is different from the so-called solder paste. Once it is hardened by heat, it will not melt even if it is heated. That is to say, the thermal hardening process of the patch glue is irreversible. The use of SMT patch adhesives may vary depending on the conditions of the heat curing, the materials to be joined, the equipment used, and the operating environment. When using, choose the patch glue according to the production process.

2. Components of SMD Adhesive Most surface mount adhesives (SMA) used in PCB assembly are epoxy (epoxies), although polypropylene is also used 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 life, epoxy resins have become the more mainstream glue technology worldwide. Epoxy resins generally provide good adhesion to a wide range of boards and have very good electrical properties. The main ingredients are: base material (ie, main body high molecular material), filler, curing agent, other additives.

3, the purpose of the use of patch glue a. Prevent the components from falling off during wave soldering (wave soldering process) b. Prevent the other components from falling off during reflow soldering (double-sided reflow soldering process) c. Prevent component displacement and standing (Reflow soldering process, pre-coating process) d. Marking (wave soldering, reflow soldering, pre-coating), when the printed boards and components are changed in batches, they are marked with patch glue.

4, the use of patch glue classification a. Dispensing type: sizing on the printed circuit board through the dispensing equipment. b. Squeegee type: sizing by stencil or copper mesh printing.

5, the dispensing method SMA can be applied to the PCB using the syringe dispensing method, the needle transfer method or the template printing method. The use of the needle transfer method is less than 10% of the total application, and it is immersed in a plastic tray using a needle array. The suspended glue drops are then transferred to the board as a whole. These systems require a less viscous glue and are well resistant to moisture absorption because it is exposed to indoor environments. Key factors in controlling needle transfer dispensing include needle diameter and pattern, temperature of the glue, depth of needle immersion, and length of the dispensing cycle (including delay time before and during contact of the needle with the PCB). The bath temperature should be between 25 and 30 ° C, which controls the viscosity of the glue and the number and form of glue points.

Template printing is widely used in solder pastes, and can also be used to dispense glue. Although less than 2% of SMA is currently printed with templates, interest in this approach has increased and new devices are overcoming some of the earlier limitations. The correct template parameter is the key to achieving good results. For example, contact printing (zero off-plate height) may require a delay period that allows for 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, 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 dots. Most modern dispensers rely on temperature control devices on the nozzle or chamber to keep the temperature above the room temperature. However, if the PCB temperature is increased from the previous process, the glue dot profile may be damaged.