• PCB assembly technology core and key points

PCB assembly factory is aim to produce qualified solder joints, which depends on the appropriate pad design , solder paste use volume , and the appropriate reflow soldering temperature curve.  Using the same equipment, some PCB assembly manufacturers have higher soldering pass rates the others have lower soldering rates. The difference lies in the different processes. It is reflected in the "scientific, refined, standardized" curve settings, and the interval once entering the furnace and the tooling during assembly Equipment, etc., which require companies to take a long time to explore, accumulate and standardize. And these proven and solidified SMT process methods, technical documents, and tooling design are "processes", which are the core of PCB assembly technology. According to the business division, the SMT process can be generally divided into process design, process trial production and process control. Its core goal is to reduce the problems of soldering, bridging, less printing and displacement through the design of appropriate solder paste volume and consistent printing deposition. In each business, there is a set of process control points, including process control key points about pad design, Stencil design, solder paste printing and PCB support.

With the continuous reduction of the size of the pads and the space of the SMT processing components, the area ratio of the template openings and the space between the template and the PCB are more and more important during printing. The former is related to the solder paste transfer rate, while the latter is related to the consistency of the solder paste printing volume and the printing yield, in order to obtain a solder paste transfer rate of more than 75%. According to experience, the area ratio of the template opening to the sidewall is generally required to be ≥ 0.66: To obtain a stable amount of solder paste that meets the design expectations, the smaller the gap between the stencil and the PCB when printing, the better. It is not a difficult task to achieve an area ratio ≥ 0.66, but it is a very difficult task to eliminate the gap between the template and the PCB. This is because the gap between the template and the PCB is related to many factors such as the design of the PCB, the warpage of the PCB, and the support of the PCB during printing. Sometimes, the equipment subject to product design and use is uncontrollable, and this is assembly key of precisely the fine-pitch components . Almost 100% of soldering defects like CSP with 0.4mm lead pitch, multi-row pins QFN, LGA, and SGA are related to this. Therefore, in the advanced professional SMT processing factory, many very effective PCB support tools have been invented to correct PCB bridge curvature and ensure zero-gap printing.