Compared with traditional electronic processing industries, surface mount technology (SMT) processing plants are better able to meet customer processing needs. In SMT processing, we must operate according to quality inspection standards. So what are the quality inspection standards in SMT processing? What are the reasons for the occurrence of wicking phenomenon in SMT processing?

　　 I. Quality Inspection Standards in Surface Mount Technology (SMT) Processing

　　1. Development of Inspection Standards

　　Corresponding inspection standards should be developed for each quality control point in SMT processing. The content includes inspection objectives and inspection content. Quality inspectors on the SMT production line should strictly follow the inspection standards. If there are no inspection standards or the content is incomplete,

　　it will bring considerable trouble to the quality control of the entire PCBA processing process. For example, when determining whether the components are misaligned, how much offset is considered unqualified? Inspectors often rely on their own experience to judge, which is not conducive to the uniformity and stability of product quality.

　　The development of quality inspection standards for each process should be based on its specific situation, and all defects should be listed as much as possible, using graphical methods to facilitate inspectors' judgment.

　 　2. Statistics of Quality Defects

　　In the SMT processing process, the statistics of quality defects are very necessary. It will help technicians and managers understand the quality of the company's products. Then take corresponding measures to solve and improve the product quality.

　　Among them, PM quality control, that is, the defect statistics method of parts per million (PPM), is commonly used in defect statistics. The calculation formula is as follows:

　　Defect rate [PPM] = Total number of defects / Total number of solder joints * 10 to the power of 6.

　　Total number of solder joints = Number of inspected circuit boards * Solder joints per board

　　Total number of defects = Total number of defects on the inspected circuit boards

　　For example, there are 1000 solder joints on a printed circuit board, and the number of inspected printed circuit boards is 500. The total number of defects detected is 20. According to the above formula, it can be calculated that

　　Defect rate [PPM] = 20 / (1000 * 500) * 10 to the power of 6 = 40 PPM

　　Compared with the traditional method of calculating the first-pass yield of printed circuit boards, the PPM quality control system can more intuitively reflect the control of product quality. For example, some printed circuit boards have more components and are double-sided.

　　The process is more complex, while some printed circuit boards have simple installation and fewer components. Similarly, calculating the single-board first-pass yield is obviously unfair to the former, while the PPM quality control system makes up for this deficiency.

　 　3. Implementation of Quality Management

　　In order to carry out effective quality management, in addition to strictly controlling the production quality process, we also adopt the following management measures:

　　(1) When purchasing components or outsourced parts, they must be inspected by inspectors before entering the warehouse (or full inspection). If the pass rate does not meet the requirements, they should be returned, and the inspection results should be recorded in writing and filed.

　　(2) Quality technicians should formulate necessary quality regulations and responsibilities. Through regulations, human-caused avoidable quality accidents can be avoided.

　　(3) Establish a comprehensive quality organization network within the enterprise to ensure timely and accurate quality feedback. Select personnel with good quality as quality inspectors on the production line, while administratively still under the management of the quality department, to avoid interference from other factors in quality judgment work.

　　(4) Ensure the accuracy of inspection and maintenance instruments and equipment. Product inspection and maintenance are implemented through necessary equipment and instruments, such as multimeters, anti-static wrist straps, soldering irons, and ICT. Therefore, the quality of the instruments themselves will directly affect the production quality. They should be sent for inspection and measurement according to regulations to ensure the reliability of the instruments.

　　(5) SMT processing plants should hold regular quality analysis meetings to discuss quality problems and determine solutions.

　　II. Causes and Countermeasures of Wicking Phenomenon in SMT Processing

　　The main reason is that the thermal conductivity of the component leads is high, so the temperature rises rapidly, causing the solder to wet the leads first. The wetting force between the solder and the leads is much greater than the wetting force between the solder and the pads. In addition, the upward bending of the leads will further aggravate the occurrence of wicking.

　 　Solutions:

　　1. For vapor phase reflow soldering, the SMA should be preheated sufficiently before being placed in the vapor phase furnace;

　　2. Carefully check the solderability of the PCB pads. PCBs with poor solderability cannot be used for production;

　　3. Pay full attention to the coplanarity of the components. Components with poor coplanarity cannot be used for production.

　　In infrared reflow soldering, the PCB substrate and the organic solder flux in the solder are good absorbers of infrared rays, while the leads can partially reflect infrared rays. Therefore, the solder melts first, and the wetting force between the solder and the pads will be greater than the wetting force between the solder and the leads.

　　Therefore, the solder will not rise along the leads, and the probability of wicking is much smaller.