1、 Preface: The following is a solder paste composed of a solder lead ratio of 63/37, assembled on a FR-4 board with a foot spacing of 0.6mm or more. And it is a high-quality, small quantity, and high mixing SMT assembly process. The purpose of this article is to describe the three main steps in the SMT process: solder paste printing, component placement, and the first thing that needs to be done after reflow soldering. Visual inspection "can also be used as a way to determine the quality of the manufacturing process after the process step.

2、 When printing solder paste, first check whether the parameter settings of the solder paste printing machine are correct. The solder paste on the board should be placed on the solder pads, and the height of the solder paste should be consistent or in a "trapezoidal" shape. The edges of the solder paste should not have rounded corners or collapsed into a pile shape, but some peak shaped shapes caused by pulling up some solder paste when the steel plate is detached are allowed. If the solder paste is not evenly distributed, check whether the solder paste on the scraper is insufficient or unevenly distributed, and also check the printing steel plate and other parameters. Finally, under the microscope, the solder paste should appear shiny or damp rather than dry.

3. Before placing components on the first board that has been soldered, it is necessary to confirm whether the material rack is properly placed, whether the components are correct, and whether the machine's placement position is correct. After completing the first board, a detailed inspection should be conducted to ensure that each component is correctly placed and lightly pressed in the center of the solder paste, rather than just "placed" on top of the solder paste. If you can see a slight indentation in the solder paste under the microscope, it indicates that the placement is correct. This can avoid the phenomenon of "sliding" of components during reflow soldering. Need to confirm again whether the surface of the solder paste is still damp? If the board has been printed with solder paste for a long time, the solder paste will appear to have a dry and cracked surface. This type of solder paste can cause "rosin solder joints" (RSJs), which cannot be detected unless they have passed through the reflow oven. This type of rosin solder joint is usually found during the assembly process of through holes, which creates a thin and transparent layer of rosin between the components and solder pads, and blocks any electrical transmission. Is the last second check that all components on the Bill of Materials (BOM) are consistent with the components on the board? Are all components sensitive to positive and negative electrodes, such as diodes, tantalum capacitors, and IC components, placed in the correct direction?

4、 Once the reflow temperature curve of the reflow oven is set (which means that many boards have been measured with thermocouples beforehand and confirmed to have no defects), the reflow curve will only be adjusted when there is a significant change or major deficiency in quantity. The so-called "perfect" solder joint refers to a shiny and smooth appearance with a complete solder coating around the pins. Near the solder joints, some residual oxides mixed with rosin can also be seen, indicating that the flux has a cleaning function. This type of oxide is normal and usually detached from the PCB, but it is also more likely to detach from the pins on the component due to the cleaning effect of the flux, indicating that the component may have been stored for a long time, even longer than the PCB. Old or incompletely mixed solder paste may produce small solder balls due to poor wetting with solder pads or component pins (note: small solder balls may also be caused by process defects such as moisture in the solder paste or defects in the green solder mask). However, poor welding conditions may also be due to poor management, resulting in some boards being touched by workers' hands and causing defects due to residual grease on the solder pads. Of course, this phenomenon may also be caused by thin tin plating on the solder pads or component pins. Finally, for an inspector, the slightly gray solder joints may be caused by the solder paste being too old, the reflow temperature being too low, the reflow time being too short, or the reflow curve being set incorrectly, or else it may be a reflow furnace malfunction. And the small tin beads may be caused by the board not being baked or baked for too long, or by the components being too hot or placed. Someone adjusted the components before entering the reflow oven, causing the solder paste to be squeezed out of the solder pads.

5、 The technique of visual inspection can be used to inspect boards that have just passed through the reflow oven. Firstly, scan the entire board with your eyes, and then inspect the defective areas with a microscope. If there is a lack of tin, short circuit, or bent pins, it can be easily detected when adjusting the line of sight by tilting the board. Using the eyes to inspect irregular areas is usually much easier and more time-saving than using a microscope to examine them bit by bit. After a problem is discovered, use a microscope for a more detailed examination. Continuously practicing and memorizing the position and appearance of components on the board can effectively improve inspection speed and defect detection rate. For small components, this method can also be used. Most small components will be at a 90 degree angle or parallel to the edge of the board, so as long as the welding is incomplete, most small components will not be parallel or perpendicular to the edge of the board, but will show off solder pads, tombstones, or short circuits. One of the reasons for this phenomenon is that one solder pad is heated and melts with the pins faster than another solder pad, causing tin flow and melting to this solder pad. Additionally, due to the cohesive force of tin itself, the entire part is pulled towards this solder pad, causing the tin on the other solder pad to not be able to melt with the pins in time, resulting in the phenomenon of the part "standing". Another reason is due to defects in the layout of solder pads and the pins of the components themselves. In addition, the bending of the pins of IC components almost invariably causes either empty soldering or poor soldering. The color discrimination quality microscope should only be used at the first site to inspect defects. From the color or the brightness (or particles) of the hardened rosin and solder paste surface, it can be determined whether the highest temperature during reflow is sufficient, too low or too high. The rosin dots that cause insulation between the solder pads and contacts were also identified by the microscope. The ideal color of rosin should be transparent or transparent with white. If it appears slightly yellow, it indicates that the welding temperature is too high. If the welding of the joints between the component pins and solder pads on the board appears wrinkled rather than smooth and shiny, it indicates that the reflow oven temperature is too high or the cooling is too fast. The reflow curve during board assembly must be carefully measured by thermocouples to obtain a good reflow curve before it can be applied to formal products.