DMAIC approach to improve the capability of SMT solder printing process

One of the major manufacturing processes of surface-mount technology (SMT) is the solder paste printing process. In this process, the thickness of deposited solder paste on printed circuit board (PCB) pads is a key quality characteristic (QCH) of main concern. In practice, large deviations of solder thickness from a nominal value result in SMT defects that may cause PCB failure. This paper implements the Define-Measure-Analyze-Improve-Control (OMAIC) approach to improve the capability of the solder paste printing process by reducing thickness variations from a nominal value. Process mapping and identifying key QCH are carried out in the "Define" phase, while mean (x) over bar and range R control charts followed by the estimates of process capability indices are adopted in the "Measure" phase. Then, the Taguchi method including L-18 orthogonal array (OA), signal-to-noise (S/N) ratio, and analysis of variance (ANOVA) for S/N ratio is implemented in the "Analyze" phase. Taguchi's two-step optimization is conducted in the "Improve phase." Finally, the T and R control charts for solder thickness are used in the "Control" phase. Adopting the DMAIC approach including the Taguchi method, the estimated standard deviation (sigma) over cap of solder thickness is reduced from 13.69 to 6.04, while the process mean is adjusted on 150.1 mu m which is very close to the target value of 150 mu m. In addition, the process capability index (C) over cap (pk) is enhanced from 0.487 to 1.432.