1, Patch adhesive surface adhesive (SMA, Surface mount Adhesives) for the wave Shan and reflux Shan, mainly used to fix the components on the printed board, usually with a point of glue or stencil printing method to allocate to maintain the components on the printed circuit board (PCB) position, to ensure that the assembly line on the transmission process components will not be lost. Put on the components after the oven or reflow machine heating hardening. It is not the same as the so-called solder paste, once the heating hardening, reheat will not dissolve, that is to say, the hot hardening process of adhesive tape is irreversible. The effect of SMT patch adhesive will vary depending on the heat curing condition, the connection, the equipment used and the operating environment. The use of the production process to select patches.
2, SMD Adhesive component PCB assembly used in most of the surface patches (SMA) are epoxy resin (epoxies), although there is also polypropylene (acrylics) for special purposes. After the introduction of high-speed drip glue system and the electronics industry to grasp how to deal with the shelf life of relatively short products, epoxy resin has become the world's more mainstream adhesive technology. Epoxy resins generally provide good adhesion to a wide range of circuit boards and have very good electrical properties. The main ingredients are: the base material (that is, the main high material materials), fillers, curing agents, and other additives.
3, the use of adhesive for the purpose of a. Wave soldering to prevent the loss of components (wave soldering process) B. In the reflow soldering to prevent the other side of the components fall off (double-sided reflow soldering process) C. To prevent the displacement of components and vertical (reflow soldering process, pre-coating process) d. Marking (wave soldering, reflow soldering, pre-coating), printed boards and components of the batch changes, the use of stickers as marking.
4, the use of adhesive tape classification A. Point glue type: Through the dispensing equipment on the printed circuit board sizing. B. Squeegee type: Through the steel net or the copper net printing and scraping way to apply glue.
5, the drop gum method SMA may use the syringe drip gum method, the needle transfer method or the model printing method to apply to the PCB. The use of needle-transfer method is less than 10% of all applications, it is the use of needle array dipped in the glue of the resignation. Then the suspended plastic drops are transferred as a whole onto the board. These systems require a lower viscous adhesive and have a good resistance to moisture absorption because it is exposed to the indoor environment. Key factors in controlling needle transfer are the diameter and style of needles, the temperature of the glue, the depth of the needle immersion and the length of the gum, including the delay time of the needle before and during the PCB contact. The tank temperature should be between 25~30°c, which controls the viscosity of the adhesive and the number and form of the glue point.
Model printing is widely used in solder paste, also available with dispensing agents. Although currently less than 2% of SMA is printed with a model, interest in this approach has increased and new equipment is overcoming some of the earlier limited limits. The correct model parameter is the key to achieve good results. For example, contact printing (0 off-board height) may require a delay cycle, allowing good gum points to form. In addition, the Non-contact printing (approx. 1mm gap) of the polymer model requires optimum squeegee speed and pressure. The thickness of the metal template is generally 0.15~2.00mm, should be slightly greater than (+0.05mm) between the components and the gap between the PCB.
Finally, the temperature will affect the viscosity and the shape of the plastic point, most modern plastic machines rely on the needle nozzle or the room temperature control device to keep the glue temperature is higher than room temperature. However, if the PCB temperature from the previous process to improve, the plastic point profile may be damaged.
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