March, 2019
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Page 71
Combining Advanced Process Control with Feedback to Improve PCB Assembly
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ters often requires a high degree of expertise, because of the various environmental considerations affect- ing the process. Using AI-powered systems and M2M connectivity, man- ufacturers can link inline inspection systems with printers and mounters to overcome these challenges.
Advanced Process Control Reliable AOI methods have
become powerful, economical comple- ments to traditional test strategies. AOI can be used as a process moni- toring tool for measuring printing, placement and reflow performance for a few reasons:
l Detecting and correcting SMT
defects during process monitoring is less expensive than after final test and inspection, where repairs are typically five to ten times more expensive.
l AOI detects 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.
l AOI can identify missing, skewed
or misplaced components with incor- rect polarity earlier in the assembly process when component placement is verified before reflowing.
Yet, a single inspection system
has limitations, especially when there is little or no communication with the balance of the line. In this setup, it simply cannot optimize a PCB assem- bly process. Equipment suppliers must cooperate to achieve communica- tion for a zero-defect future. M2M con- nectivity can optimize the process by exchanging real-time SPI and AOI measurement data with other mach - ines 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 con- nected systems can auto matically define correlations between the processes. APC, however, can deliver meas-
urable and sustainable improvements in production yield. Most engineers will agree that stabilizing control loops, with underutilized or ineffective process times and strong process interactions is difficult. APC has become a standard solution for realiz- ing stable control processes — and quite simply —APC is the added-value upgrade to a process automation sys- tem. 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.
APC Printer Feedback The continued demand for
smaller, lighter and smarter elec- tronic devices has led to an increas- ing use of smaller components. These shrinking packages force smaller aperture designs and finer solder paste, which have made stencil print- ing a highly sophisticated process with a tremendous impact on produc- tion yields. In fact, 70 percent of all PCB assembly defects are primarily due to problems directly associated
with the solder printing process. An enhanced APC solution,
formed of interlinking software mod- ules, can actively optimize the print- ing process by combining real-time printing information with SPI meas- urement data. More advanced soft- ware automatically performs the design of experiment (DOE) intended to complete a detailed SPI result analysis, using advanced diagnostic algorithms and noise filtering mod- els, and then recommends the ideal print parameters. In Koh Young’s testing, its soft-
ware conducted an automatic DOE by changing the screen printing parameters. When handling two
parameters, it needed 11 prints. For three parameters (printing speed, printing pressure and separation speed), it would require 17 prints. The software triggers the SPI to
send the information to the screen printer to automatically adjust the parameters. The max and min values for printing pressure, printing speed and separation speed can be set when adjusting the printer. Once the system defined the optimum values for the printer parameters, ten boards were printed and analyzed. During testing, it was found
that the software system could pro- duce print quality comparable to that of an engineer with 25 years of print
process expertise. Other software can use multiple anomaly-detection algo- rithms to actively optimize the print process and further reduce false calls. Ideally, the software will evolve to autonomously generate optimized models and fine-tune the process parameters in real time. While the software modules
would each provide standalone process benefits, the combined power of adding an AI-powered software suite would ensure the highest process reli- ability and production flexibility.
APC Mounter Feedback What about mounters and opti-
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