Inspection moves into the mainstream

As inspection has evolved from an end-of-line procedure into an integral part of many production lines, inspection technology itself has also evolved. Stacy Kalisz Johnson, market development manager at Agilent Technologies in Phoenix, AZ, suggests that the logistics of electronics manufacturing have moved the goalposts, so to speak, as users demand more from the companies supplying their equipment.

Johnson said that the most obvious evolutionary issues relate to equipment cost. “A few years ago, the post-reflow niche for AOI [automated optical inspection] required moderate system reliability and fault coverage, with price less of a factor. Some systems sold for $250,000 or more. Using AOI systems for solder-paste and pre-reflow inspection was confined primarily to telecom, military, and automotive industries.

“Today, competition both among equipment vendors and among the manufacturers themselves have pushed prices down. Most significant are assembly's continued migration into such cost-conscious locations as Asia and Eastern Europe and the need to sell companies' products in those areas.”

Johnson explained that electronics manufacturers have increasingly accepted inspection as a standard part of the process, so equipment suppliers expend much less time and effort “justifying” the approach. “There is much less need to convince customers of the advantages of 3-D solder-paste inspection, for example,” she said. “They already appreciate the benefits of 3-D imaging to their products' long-term reliability.”

Johnson continued, “There has also been a major effort in the past couple of years to concentrate on defect prevention instead of merely detection, pushing more inspection up the line, which explains the increasing popularity of using AOI for solder-paste inspection.”

This classic Venn diagram shows the types of faults expected at each assembly step. Courtesy of Agilent Technologies.

In today's more globalized structure, factory managers often make the decisions for themselves, based on their own requirements. Which-ever process they use, they expect inspection-system suppliers to keep their offerings flexible enough and configurable enough to accommodate them.

“Today, we have to coordinate all parts of the sales and support process, maintaining communication across the regions,” said Johnson. “I not only know who the salespeople are in Asia, I talk to them almost every day. We travel to Asia to visit manufacturers there because that is where the action is. There is no way to 'un-globalize' the electronics industry. We travel around to assess our customers' needs.”

Where in the process manufacturers put their inspection equipment also depends heavily on circumstances. X-ray systems, once primarily relegated to product sampling or other forms of very targeted inspection, have become much more common in the mainstream manufacturing process, and they can now examine many more types of solder joints.

The decision about whether to use AOI after paste, before reflow, after reflow, or in some combination also depends on the type of product and on such issues as the consequences of failure. Even contract manufacturers, who often have inspection equipment available at every stage of the assembly process, may allocate their resources based on an economic cost-benefit analysis for each of their customers and for each particular project.

“In high-reliability cases, companies may perform AOI at all three process locations,” commented Johnson, “although makers of low- or medium-complexity products may not use them all on every production run.

“Instead, solder-paste inspection may be used only during ramp-up to full production. Pre-reflow could supply information for defect prevention. Once the process achieves stability, a manufacturer may reserve even post-reflow inspection for production samples only, to reduce costs and maximize throughput.”

Despite the industry's evolution, some aspects of the inspection task remain constant. Fault types, for example, have remained relatively constant since surface-mount components largely replaced through-hole and since reflow solder supplanted the old wave technique. Company strategies still detect certain fault types with corresponding classes of equipment. Only the proportions of those fault types at the board level have changed.

Johnson contributed the familiar Venn diagram in the figure, explaining, “The fundamental relationship still applies. AXI [automated x-ray inspection] can still provide the greatest test coverage overall, but the most economical balance includes the perfect combination of AOI, AXI, and in-circuit test [ICT].

“Such a mixed approach avoids committing solder-printing faults and component-placement issues to reflow, saving rework costs. Using inspection and in-circuit test to find the remaining faults after the reflow step prevents faulty boards from getting out the door to customers.

“Data collection and process-control tools that tie all of the pieces together are far more comprehensive than they were several years ago. They help to improve the overall process, producing fewer defects.

“They also permit analyzing the process to show how to take advantage of inspection's capabilities most efficiently and cost effectively. In fact, the application of data analysis often provides the differentiator when an OEM selects a contract manufacturer.”

Inspection, once an isolated part of the manufacturing process, is now integrated with test and process control techniques to provide the best product results. The interlinking and overlapping will continue until the line between inspection and other process steps fades into insignificance.