Application example of 6A method in SMT product quality management
A manufacturing company decided to use the 6a DMAIC method for analysis and research in view of the high rate of SMT product assembly defects.
(1) Problems and goals. Problem: After the reflow process, it is found that there is a higher rate of assembly defects and a larger amount of additional rework. Goal: find out the main reason, make improvements, eliminate or reduce the defect rate.
(2) Team building. The company has established a 6a project team for this purpose.
(3) Definition stage. In order to determine the main issues and research projects, a breakdown of all defects in production was drawn up first. Through the analysis of this table, it is preliminarily determined that the main issues affecting the quality of the process are the quality of component placement and welding.
(4) Measurement phase. In order to further clarify the cause of the defect problem, a hierarchical Pareto chart classified by different production lines was drawn. It was confirmed that patch and soldering defects were the primary quality problems of each production line, and the severity of the problem was clarified. Then, in order to obtain more detailed analysis results, a typical production line was selected for research.
(5) Analysis and improvement stage
Through analysis, one of the focus of the problem is on the misalignment of the patch positioning. In this case, the placement machine is equipped with two rotating heads, each with 12 nozzles. In order to analyze the root causes of component misalignment, a multivariate study was carried out to detect changes in the rotating head, nozzle, and placement angle. Then draw the multivariate chart and the main influence chart, showing that the biggest positioning difference comes from the changes of different rotating heads and nozzles. Through analysis, specific suggestions for improvement are proposed as follows.
Problem: The nozzle design is not suitable for the shape of the components. Solution: Design special nozzles for these types of components.
Problem: The vacuum nozzle is worn and contaminated. Solution: Use a new method to clean the nozzles, arrange regular inspections and exchange nozzles.
Problem: The Nirich operating procedures of the feeder are inappropriate, the operator’s questions are not answered correctly, and the maintenance quality is not controlled. Solution: Standardize the feeder maintenance content, time interval, checklist and operation instructions to control the quality of the feeder.
Problem: The change of the rotating head during maintenance is not monitored. Solution: Update the maintenance checklist and method to control the maintenance quality of the rotating head.
The second focus of the problem is the solder paste printing process. In this case, the printing process is analyzed to correct solder bridging and open circuit defects. Since the amount of solder paste is one of the key output quantities reflecting soldering defects, the planning of experiment method (DOE) is used to optimize the process settings to obtain the expected solder paste volume specifications. In this experiment, the output includes the measurement of the printing volume reflecting the quality of soldering, and the measurement of the solder joint tension of the components after the reflow process reflecting the reliability of the process. Therefore, the expected input setting process window is obtained through analysis, and the process parameters such as printing speed and squeegee pressure are determined, and the solder paste printing process is realized that not only meets the specification requirements of solder paste volume, but also meets the requirements of tensile specifications.
(6) Control phase
According to the test and analysis results, the process control plan and process parameter settings are finally determined. The daily operation control is implemented in accordance with the process control plan, the checklist is updated daily, and the standardized process is implemented and effectively monitored. After implementing the improvement process for a period of time, update and submit the defect Pareto chart to compare the results before and after. According to the comparative data, it is estimated that the chip and soldering defects decrease after adopting the new process. Analyze the results, stabilize the results, and discuss the need for further improvement.
