Excerpt: The Printed Circuit Assembler’s Guide to... SMT Inspection: Today, Tomorrow, and Beyond, Chapter 5

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Chapter 5: Advanced Process Control and Inspection
Reliable AOI methods have become powerful, economical complements to traditional test strategies. AOI can be used successfully as a process monitoring tool for measuring printing, placement, and reflow performance. Some advantages include: 

--Detecting and correcting SMT defects during process monitoring is less expensive than after final test and inspection, where repairs are typically 5 to 10 times more costly.

--Detecting trends in process behavior, such as placement drift or incorrect mounting, earlier in the overall process. Without early inspection, more boards with the same defect could be rejected during functional test and final inspection.

--Identifying missing, skewed, or misplaced components with incorrect polarity earlier in the assembly process when component placement is verified before reflowing.

Yet, a single inspection system has limitations, especially when there is limited or no communication with the balance of the line. In this setup, it simply cannot optimize a printed circuit board assembly process. Equipment suppliers must cooperate to achieve communication for a zero-defect future. M2M connectivity can optimize the process by exchanging real-time SPI and AOI measurement data with other machines in the production line. This real-time feedback includes measurement data such as offset, volume, height, area, and warnings to other systems, while analyzing trends to optimize the process and identify trends. The connected systems can automatically define correlations between the processes.

For instance, the electronics manufacturing industry has created numerous studies and reports detailing how the solder reflow process can help position surface mount components normally on the pads, even if component placement is off pad. However, the trend to shrink components to 0.3 mm bumps or 0201M microchips is opening doors to explore how process controls can improve yields in high-density placements.

Enter advanced process control (APC), a proven control and optimization technology delivering measurable and sustainable improvements in production yield. Most engineers will agree that stabilizing control loops, with underutilized or ineffective process time and strong process interactions, is exceedingly difficult. APC has become a standard solution for realizing stable control processes. Quite simply, APC is the added value upgrade to a process automation system. APC collects and analyzes solder and component location data from an inspection system, and then sends the recommendations across the line to printers or mounters for automatic implementation.

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