A "CLOSED LOOP" SYSTEM FOR THE REALTIME CONTROL OF SOLDER PASTE STENCIL PRINTING

The move towards fine and ultrafine pitch components in surface mount Printed Wiring Board (PWB) assembly has increased the importance of the solder paste printing process. The quality of the solder paste deposit is influenced by factors such as the characteristics of the board, the solder paste selected, the printer used, etc. These inter-related process and material factors complicate the procedures and the reasoning used to identify and set-up the stencil printing parameters. The increased use of ultrafine pitch components (with lead pitch down to 0.4 mm) in PWB assembly has almost made it mandatory to inspect the solder paste deposit prior to component placement. This inspection process provides the necessary information to correct the printing process in a realtime fashion if printing is out of control or is exhibiting a tendency to go out of control.This research designed and developed a "closed-loop" system for realtime control of the solder paste stencil printing process. This knowledge based process control system interfaces with a stencil printer and a laser based solder paste inspection system to form the "closed-loop". This "closed loop" realtime system insures both proactive and reactive control of the stencil printing process.