Statistical process control for solder deposition and yield enhancement

In a generic surface mount printed circuit board assembly line, the largest segment of process-related defects can be ascribed to the solder paste deposition process. The amount of solder paste that is deposited significantly affects both process yields and product reliability. In order to monitor the solder deposition process, surface mount printed circuit board assembly incorporates vision systems with 3D paste inspection capability. But the possibility of capturing both data and the resulting knowledge that represents fluctuations in production mix and volume (to ensure effective continuous real-time monitoring) becomes infeasible because (i) inspection data is not archived and (ii) the inspection tools are not combined with active statistical process control. Having an information systems solution to access and retrieve the historical data from the inspection stations and perform statistical analysis on the available data set would aid continuous monitoring/improvement of the process. This paper provides an insight into the importance of historical data and a software model ('Cyberstat Advisor') that facilitates Statistical Process Control (SPC) and data analysis in the solder deposition process. In order to assist process improvement, data generated by the solder deposition inspection machine was converted and mapped into a relational database format. The 'Cyberstat Advisor' retrieves and utililizes the information from this database. This tool uses heuristics to search and sift through the extensive database. Based on historical data, the 'CyberStat Advisor' can (i) calculate the control limits for a site in the Printed Circuit Boards (PCBs) that had been inspected and (ii) compute the process capability of the solder paste printer with respect to a particular site. This module can also plot run charts, range charts, and bar charts for any site. These visual representations help to better understand the process, facilitating in process improvement.