1, the role of patch glue
Surface Seal adhesives (SMA, surface mA potential adhesives) for wave soldering and reflow soldering, mainly used to fixed components on the printed circuit board, the general use of dispensing or stencil printing method to allocate to keep the components in the printed circuit Board (PCB) to ensure that the components are not lost during transmission on the assembly line. Paste the components into the oven or reflow machine after heating hardening. It is not the same with the so-called solder paste, once heated and hardened, and then heating will not melt, that is, the hot film curing process is irreversible. The effect of the SMT patch will vary depending on the heat curing conditions, the connectors, the equipment used, and the operating environment. When used in accordance with the production process to choose patch glue.
2, the composition of patch adhesive
Most of the surface mount adhesives (SMA) used in PCB assembly are epoxies, although there are acrylics for special applications. In the introduction of high-speed gluing system and the electronics industry to master how to deal with relatively short shelf life of the product, the epoxy resin has become the world's more mainstream glue technology. 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 polymer materials), fillers, curing agents, and other additives.
3, the use of patch glue purposes
a. wave soldering to prevent components off (wave soldering process)
b. reflow to prevent the other side of the components off (double-sided reflow process)
c. to prevent component displacement and stand (reflow process, pre-coating process)
d. for marking (wave soldering, reflow soldering, pre-coating), printed circuit boards and components to change the volume, with a patch for the mark.
4, the use of patch glue classification
a. Dispensing type: through the dispensing equipment in the printed circuit board sizing.
b. Scraping type: sizing by stencil or copper screen printing.
5, gluing method
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 the total application, and it is used to place the array of needles in a tray of glue. And then hang the drops as a whole to the plate. These systems require a lower tacky glue and have a good resistance to moisture absorption because it is exposed to the indoor environment. Key factors in the control of needle transfer diets include needle diameter and pattern, the temperature of the gel, the depth of the needle immersion, and the length of the duration of the dispenser (including the delay time before and during contact of the needle). The tank temperature should be between 25 and 30 ° C, which controls the viscosity and the number and form of glue.
Template printing is widely used in solder paste, also available with the distribution of glue. Although less than 2% of the SMA is currently printed with the template, but the interest in this method has increased, the new equipment is to overcome some of the earlier limitations. The correct template parameter is the key to achieving good results. For example, contact printing (zero plate height) may require a delay period, allowing good glue to form. In addition, non-contact printing (approximately 1 mm gap) for polymer templates requires optimum scraper speed and pressure. The thickness of the metal template is generally 0.15 to 2.00 mm and should be slightly larger than (+0.05 mm) between the component and the PCB.
The final temperature will affect the viscosity and the shape of the dot, and most modern dispensers rely on the temperature control device on the mouth of the mouth or the chamber to keep the gel temperature higher than room temperature. However, if the PCB temperature from the front of the process to be improved, the plastic dot contour may be damaged.
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