Automatic Solder Paste Printer Positional Feedback Control

Stencil printing is a critical first step in surface mount assembly. It is often cited that the solder paste printing operation causes about 50%-80% of the defects found in the assembly of PCBs. Printing is widely recognized as a complex process whose optimal performance depends on the adjustment of a substantial number of parameters. It is not uncommon to hear that stencil printing is more of an “art” than “science”. In fact, the process is so complex that sub-optimal print parameters usually end up being used. In addition, stencil printing produce relatively noisy data, which makes the print process extremely difficult to control. Ideally, minimizing the variance of the deposited location and volumes will improve the quality of the process and produce more reliable solder joints.
In general, there are two critical aspects to a printing process. The goal of solder paste printing is to put down the right “volume” of paste on the right “spot”. In another words, we not only have to monitor the amount of paste volume we also need to monitor X, Y and Ө registration of the board. This issue is compounded when dealing with miniature components such as 0201, 01005, 0.4mm and 0.3mm CSP and lead free paste. Lead-free paste is known to have less spread, or wet-ability, and adds to the challenge. In this paper we present the results of tests that show registration control, using a closed loop control scheme, is viable. Using registration feedback from a solder paste inspection system to the solder paste screen printer, the solder paste can be repeatedly put in the right “spot” board after board. We will also discuss best practices regarding how to set up the closed loop between the solder paste inspection system and the screen printer. To develop a robust feedback mechanism, the control process must work over a wide range of board layouts and configurations. To this end, there are best practices to follow when setting up the control loop that optimize the feedback process.