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Surface mount technology core and key points

 

The goal of SMT (surface mount technology) process is to manufacture qualified solder joints. To obtain good solder joints, it depends on the proper pad design、solder paste and  reflow soldering temperature profile, all these are process conditions. Using the same equipment, some manufacturers have higher straight-through rates for welding and some are lower. The difference is that the process is different. It is reflected in “scientific, refined and standardized”. For example, the solder paste used, the thickness and opening design of the template, the printing support and parameter adjustment, the placement program setting, the temperature curve setting, and the interval between the furnace and the tooling during assembly, etc., often require long-term expore, accumulated and standardize by the enterprise. These proven and cured process methods, technical documents, and tooling design are the “processes” that are at the heart of SMT. According to the business division, the SMT process can be generally divided into process design, process trial production and process control. The core goal is to reduce the problems of open welding, bridging, less printing and shifting by designing the appropriate solder paste amount and consistent printing deposition.

here is a set of surface mount technology process control points in each business, where pad design, stencil design, solder paste printing and PCB support are key points of process control. As the component pads and spacer dimensions continue to shrink, the area ratio of the template opening and the gap between the template and the PCB during printing become more and more important. The former is related to the transfer rate of the solder paste, while the latter is related to the uniformity of the solder paste and printing yield.

 

In order to obtain a transfer rate of solder paste of more than 75%, according to experience, the area ratio of the template opening to the sidewall is generally required to be 0.66 or more. To obtain a stable amount of solder paste that meets the design expectations, the gap between the template and the PCB during printing the smaller the better. To achieve an area ratio of 0.66 or more is not a difficult task, but it is very difficult to eliminate the gap between the template and the PCB. This is because the gap between the template and the PCB and the design of the PCB, the warpage of the PCB, There are many factors related to PCB support during printing, and sometimes the equipment that is limited to product design and use is uncontrollable, and this is precisely the key to surface mount technology of fine pitch components, such as CSP with 0.4mm lead pitch and multiple leads. The welding defects of the feet QFN, LGA, and SGA are almost 100% related. Therefore, in the advanced professional foundry, many very effective PCB supporting tools have been invented to correct the warpage of the PCB and ensure zero-gap printing.

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