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LED chip glue and Epoxy basic knowledge
- Dec 22, 2017 -

1, the role of adhesive patch Surface mount adhesive (SMA, surface mount adhesives) for wave soldering and reflow soldering, mainly used to fix the components printed on the board, the general dispensing or stencil printing method to distribute In order to maintain the position of the component on the printed circuit board (PCB), to ensure that components are not lost during transfer on the assembly line. After placing components into the oven or reflow soldering machine heated to harden. It and the so-called solder paste is not the same, once heated to harden, then the heat will not melt, that is, the glue adhesive heat-hardening process is irreversible. The use of SMT plastic patch will vary depending on the thermal curing conditions, the materials to be connected, the equipment used and the operating environment. When used according to the production process to choose the patch glue.


2, the glue composition PCB PCB assembly used in most of the surface mount adhesive (SMA) are epoxies, although there are acrylics (acrylics) for special purposes. With the introduction of high-speed dispensing systems and the electronics industry's ability to handle products with a relatively short shelf life, epoxy 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 polymer material), fillers, curing agents, other additives.


3, the purpose of the adhesive patch a. Wave soldering to prevent the components from falling off (wave soldering process) b. Reflow soldering to prevent the other side of the components off (double-sided reflow process) c. (Reflow process, pre-coating process) d. For marking (wave soldering, reflow, pre-coating), printed circuit boards and components in bulk changes, the use of plastic patch for marking.


4, the use of plastic patch classification a. Dispensing type: through the dispensing device in the printed circuit board sizing. b. Squeegee type: stencil by stencil or copper screen printing method.


5, Epoxy method SMA can use the syringe Epoxy, needle transfer method or template printing method applied to the PCB. Needle transfer method used less than 10% of all applications, it is the use of needle array immersed in plastic tray. Then the dangling drops are transferred to the board as a whole. These systems require a less viscous glue and have good resistance to moisture absorption because it is exposed to the interior environment. Key factors that control needle transfer dispensing include the needle diameter and style, the temperature of the adhesive, the depth of the needles immersed, and the length of the adhesive dispensing cycle (including the delay before and during the needle contact with the PCB). The tank temperature should be between 25 and 30 ° C, which controls the viscosity of the glue and the amount and form of glue points.


Template printing is widely used in solder paste can also be used with dispensing glue. While less than 2% of SMAs are currently printed in templates, interest in this method has increased and new devices are overcoming some of the earlier limitations. The correct model parameters are the key to good results. For example, contact printing (zero off-board height) may require a delay period, allowing for good gel point formation. In addition, non-contact printing of polymeric templates (approximately 1 mm gap) requires optimum squeegee speed and pressure. The thickness of the metal template is generally 0.15 ~ 2.00mm, it should be slightly larger than (+ 0.05mm) gap between the components and the PCB.


The final temperature will affect viscosity and gel shape, and most modern applicators rely on needle cap or chamber temperature controls to keep the gel temperature above room temperature. However, if the PCB temperature is increased from the previous process, the gel point profile may be damaged.