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 (DMAIC) 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 macr and range R control charts followed by the estimates of process capability indices are adopted in the "measure" phase. Then, the Taguchi method including L₁₈ 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 x macr 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 circ of solder thickness is reduced from 13.69 to 6.04, while the process mean is adjusted on 150.1 mum which is very close to the target value of 150 mum. In addition, the process capability index C circ_pk is enhanced from 0.487 to 1.432.