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28 SMT007 MAGAZINE I NOVEMBER 2020 light, green light" approach, test and inspection is an end-of-the- line process. D'Amico: While I understand and agree that most manufactur- ers rely on a single post-reflow AOI system, the problem is that it becomes more difficult to diagnose and resolve where an issue may have occurred within the manufacturing line because you're only looking at a snapshot of the final result of all the other assembly equipment within the line. From there, you're kind of guessing where the issue occurred. An SPI system, for example, would provide immediate feedback on issues such as insuf- ficient or excessive solder, solder bridging, or solder offset directly after solder deposition. These issues are typically related to process parameters, such as stroke speed for a screen printer, squeegee pressure, under-stencil clean- ing, etc. The bottom line is that it is more dif- ficult to diagnose screen printing issues based on data collected by post-reflow AOI. Johnson: For somebody who's a more tradi- tional thinker in this industry, then the one test at the end of the line is how they think about test and inspection. The more factory automation you put into place, the more places you'll need measurement—mini feed- back loops. With the measurement data cap- tured, it becomes a case of how sophisticated the machinery is that you're feeding test data back to. Can the machinery self-adjust, for example, or does it need a human operator to make the fix? D'Amico: Adjustments are typi- cally made based on trends within the inspec- tion data. For example, let's say that the SPI machine detects an offset trend in solder depo- sition with regard to pad location. Under these circumstances, the SPI machine would report the trend to the screen printer. The screen printer, in turn, may then use this data to adjust the location of the stencil with respect to the pads to eliminate the offset. As you stated, however, this really does depend on the level of sophistication of the screen printer to accept this data and then make the necessary changes to the specified parameters to resolve the issue. Please keep in mind that the goal is not to control the screen printer but to provide actionable data which may be used to eliminate the offset trend and potential Brian D'Amico

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