Over the last several years there has been a great deal of advancement in post-print inspection technology. The capability to inspect printed solder paste deposits immediately after the printing process and immediately before the component placement process has become more accurate, more repeatable and much faster. Post print solder paste inspection systems which are an integral feature of the printing machine itself or a standalone automated optical inspection (AOI) system are now able to provide much more reliable inspection capability at or near the speed of the product cycle times. However, regardless of how sophisticated, no post print inspection system can actually correct the defects it detects. The existing post print inspection systems can certainly identify the defects and notify the operator or process engineer of a problem. The identified defect can be fixed before it becomes a more costly defect than if it were discovered later in the manufacturing process. A process engineer or technician must evaluate the solder paste printing performance data generated by the post print inspection and/or statistical process control (SPC) systems to determine the "root cause" of the defect and what action is required to actually eliminate or minimize the problem from reoccurring. No post print inspection system has ever eliminated any defect from reoccurring without a process engineer implementing permanent corrective action. The c'oncept of a closed-loop process control solder paste printing process involves a system that not only detects the defects but also has the intelligence to make adjustment to the process (primarily adjustments to the printing equipment's operating parameters) to prevent them from reoccurring. This paper will discuss various concepts of closed-loop process control involving the solder paste printing process. The discussion includes the current status of our research in this field, benefits, limitations, the technology required for implementation and future innovations.
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