ver 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.”