Where to put AOI?

Ever since automated optical inspection (AOI) became a mainstay of a manufacturer's test strategy, planners have debated where to place it in the production process. Do you inspect the solder paste before component placement, inspect the component presence and position after placement and before reflow, inspect the entire board after reflow, or perform some combination of these steps?

To help sort out the advantages and disadvantages of these alternatives, Pamela Lipson, CEO of Imagen, and Lyle Sherwood, VP and director of technology for Landrex Technologies, collected data about defects from several customers of Landrex's AOI systems. They then conducted a study with one customer, a manufacturer of high-volume, high-complexity boards, in which they inspected boards at post-placement, repaired the post-placement defects, and inspected the boards again at post-reflow.

The study let them compare the benefits of post-placement vs. post-reflow AOI. Lipson presented their analysis at April's APEX show in Las Vegas. She subsequently explained the results to me in an exclusive interview.

“We worked with both OEMs and contract manufacturers with a wide range of volume, mix, and complexity,” explained Lipson. “In the data we captured with both types of users, the greatest number of defects resulted from post-placement defects, including misinstalled parts and missing parts.”

Lipson said that they did not examine the value of using AOI at the post-solder-paste stage, “because most of the customers we surveyed do some inspection at that point anyway. They use it to detect gross problems, such as stencils that need cleaning.” She continued, “Generally, post-paste inspection results in a go/no-go decision. That is, the manufacturer doesn't repair portions of the board, but wipes it clean and returns it to the beginning of the process. Post-paste inspection can predict some future defects, but post-placement can often find those defects as well. Therefore, we regard it [post-paste inspection] as orthogonal to post-placement or post-reflow inspection. Thus, we focused on the latter two inspection steps.”

Lipson explained that the boards in the study used primarily lead-free solder. She noted that the reduced wetting characteristics of lead-free solder (compared to leaded solder) make it less likely that a placement problem will resolve itself in the reflow oven.

Finding the best place to position the AOI system is crucial to a manufacturer's ability to adjust the production process properly. Conventional wisdom, Lipson acknowledges, says that the earlier in the process you inspect parts and correct process flaws, the more money you can save.

“Defects uncovered before reflow cost less to repair than those found later,” she said. “In addition, post-placement AOI generally occurs in-line with production in real time, so any process-related defects can be identified immediately and the process corrected. Post-reflow, on the other hand, is often applied to board batches that come off the line during a certain period of time. If a defect goes undetected until that point, the process will likely create many more defective boards before correction.”

Yet, Lipson noted that the stage at which AOI is used is sometimes chosen for logistical reasons rather than technical or economic ones. “Process groups,” she explained, “are often responsible for post-placement AOI, for example, while the responsibility for end-of-line AOI rests with test groups. Members of each group naturally prefer the solution that belongs to them.”

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.” 

Fig. 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.