Navigating AOI for low-volume, high-mix users: a series of companies describe how they use inspection.

Defect prevention and containment is a very important part of today's production. Cost pressures are greater than ever and quality is scrutinized in new ways. This, coupled with the speed at which new products are being generated, and the advancements in technology with finer pitches and more detailed components, put PCB assemblers in a challenging position. The choices and options for automated optical inspection are many. AOI can be implemented in multiple ways if you consider the location (i.e., pre- or post-reflow) along with inline vs. offline inspection. It is difficult to navigate the murky waters to determine the AOI test strategy implementation best suited for a given product, production environment, and to yield the best results. This is particularly visible in lower-volume environments with a high mix of products. In high-volume manufacturing, most lines are equipped with AOI. In many cases, they have 100% inline post-reflow, solder paste inspection and in some cases, pre-reflow inspection too. But for a situation of high-mix, low-volume, or just a few lines, it becomes complex to balance the investment and return for AOI. I recently sat down with several AOI users from different product areas and assembly segments to see what they are doing. A few of their summaries are described, along with some conclusions from the exercise. This may provide some considerations to help with the implementation considerations facing your organization.

Low-volume, high-mix telecom. One telecommunications company building products in a low-volume, high-mix environment uses AOI in a pre-reflow function. It has been doing so for about 18 months. Every board is inspected at 100%, through inline pre-reflow AOI. The company mainly catches missing parts, misalignments and polarity issues. The final yields did increase within the first few weeks/months after implementation of AOI in the pre-reflow position, and there has been a consistent stream of caught defects over time. Funding has prohibited the adoption of SPI and post-reflow inspection, but those are desired. On a daily and weekly basis, the AOI data are compared to the automated x-ray inspection data to address major issues and quickly localize the area of production impacting the issue.

Defense-based OEM. An OEM building medium-complexity applications for military use has two surface mount lines. The manufacturing environment is low-volume, high-mix. It uses one offline AOI system, colocated with production, to service both lines. Its AOI is used for post-reflow and has been in use for about seven years. The motivation for post-reflow in this instance was to combat the investment in in-circuit test fixtures in a high-mix environment; over seven years, the company has relied more each year on AOI. For newer products, the main failure observed at AOI is wrong parts or incorrect polarity. For more mature products, tombstoning or missing solder have been the most prevalent defects. Results are pulled daily, and alerts sent to the various product steps, AOI programmer, process engineer, etc.