ig. 1 These post-placement scatter diagrams show component placement accuracy on two different production lines. Although the number of components that exceed the 8-mil gross defect limit is comparable, the number of “marginal passes” in b suggests that the machine is on its way to producing a greater number of gross defects and therefore needs adjustment. Courtesy of Landrex Technologies.
Post-placement seems best
Through their research, however, Lipson and Sherwood concluded that performing AOI at post-placement provides the best opportunity to catch the greatest number of defects and repair any process problems. “Post-placement inspection can uncover some design issues as well,” Lipson said. “In one case, a complex component had larger end pins than center pins, yet the board pads were all the same size. That mismatch produced a lot of post-reflow defects that simple post-paste inspection can never find. Analysis of only the joints without looking at the part relative to the pads will obscure this condition, and the root cause will go undetected.”
Lipson and Sherwood also found that waiting until post-reflow to perform inspection can lead to some misdiagnoses and, consequently, incorrect adjustments to the production process. “Our research showed that many placement-related defects get incorrectly diagnosed at post-reflow AOI,” Lipson said. “A grossly misplaced component can manifest as tombstoned or billboarded after reflow. Accurately determining the source of a defect increases the success of process adjustments made in response.”
She also explained that there are other placement defects that can be correctly diagnosed only through post-placement inspection: “If the placement machine pushes too hard when inserting a component, a post-placement inspection can detect deformation of the solder on the pad. Post-paste inspection will not detect a problem, because at that point, no problem exists. After reflow, the defect would show up as a solder bridge. If paste is applied correctly and the component is deposited correctly, the oven itself can't cause such a defect.”
Lipson added, “Post-placement inspection can also indicate when placement machines need adjustment before the process produces any defective boards.” Figure 1 shows the placement position for all components on a single board type on two production lines. Lipson explained, “Parts within the 4-mil circle generally indicate an accurate placement, while components placed more than 8 mils from the ideal position constitute 'gross defects'. Although both lines show a few such gross defects, the placement machine in Figure 1a demonstrates a much tighter profile than Figure 1b, suggesting that without adjustment, any further drifting of component positioning from the machine in Figure 1b will generate numerous defects. Again, the reflow oven may obscure the situation, and post-reflow—which focuses on solder joints and gross placement defects—may not detect such placement trends. Thus, without inspection post-placement, the need for adjustment will not be noticed until after a significant rise in the number of defective boards.”
Lipson concluded, “At one point in the study, we repaired all defects that the pre-reflow inspection step found to determine how many more defects would show up at post-reflow. We found—somewhat to our surprise—that virtually all the defects on those boards could have been detected or repaired before the board entered the reflow oven.”
For More Information
Lipson, Pamela, and Lyle Sherwood, "Process control keeps faults in check," Test & Measurement World, December 2005/January 2006. p. 43.
Scheiber, Steve, "Software is the future of AOI," an interview with Pamela Lipson, www.tmworld.com, May 27, 2008.