A CLOSED-LOOP SYSTEM FOR THE REALTIME CONTROL OF SOLDER PASTE STENCILPRINTING

The move towards fine and ultrafine pitch components in surface mount Printed Wiring Board (PWB) assembly has increased the importance of th e solder paste printing process. The quality of the solder paste depos it 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 reasoni ng used to identify and set-up the stencil printing parameters. The in creased use of ultrafine pitch components (with lead pitch down to 0.4 mm) in PWB assembly has almost made it mandatory to inspect the solde r 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 te ndency 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 interfac es with a stencil printer and a laser based solder paste inspection sy stem to form the ''closed-loop''. This ''closed loop'' realtime system insures both proactive and reactive control of the stencil printing p rocess.