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AOI and AXI: One Powerful Combination
December 31, 1969 |Estimated reading time: 5 minutes
In the past, AOI has been pitted against AXI when a company implements automated inspection throughout its SMT line. AOI and AXI should not be viewed as competitive technologies. AOI and AXI should be combined and used in the manufacture of electronics products dealing with life-critical applications.
By Joseph Vilella
I have observed instances in which AOI is pitted against AXI when a company is deciding to implement automated inspection throughout its SMT lines. In reality, AOI and AXI should never be viewed as competitive technologies. Indeed they are complementary and, in many ways, mutually inclusive technologies. Whereas the use of AOI is more accepted as a critical and fundamental first line of defense against mediocre product quality in electronics products, the combined power of AOI and AXI should be used in the manufacture of every electronics product that deals with life-critical electronics applications such as aerospace, defense, medical, law enforcement, safety and security-related electronics systems.
Extensive coverage in those areas is essential because as we know, AOI is extremely capable of detecting and inspecting geometrically well-defined components and features that are on top, while AXI is extremely capable of detecting and inspecting hidden or hard-to-detect features such as traces underneath components, proper BGA alignment patterns and many hard-to-detect solder-quality defects. For 11 years, I have been immersed in the AOI field; and for the past two years, I have been the chairman of the Automated Inspections subcommittee of the IPC, which deals with the development of implementation standards for AOI, AXI and API (Automated Paste Inspection). Through these years, regardless of my level of involvement in AOI, I have considered AXI to be a genuine teammate of AOI, and the ultimate way to ensure rock-solid product quality in application-critical electronics manufacturing.
This article takes a closer look at the fundamental AOI/AXI relationship using a mission-critical case to illustrate the combined power of these technologies when properly applied to the SMT manufacturing process.
The Problem
A defense subcontracting company must manufacture a mission-critical board for a defense contractor. The board is a high runner used for a military communication device. Its demand presses the company to go into medium- to high-volume manufacturing. It has 900 components, of which 20% are leaded devices. The board/component layout requirements forced the designers to use J-leads in many of the leaded devices. Several critical gull-winged, custom-design IC packages also must be properly placed with a high degree of solder joint quality. The bottom line is that the manufacturer must ensure that all the devices are present, properly placed and that the solder quality is such that the device can withstand its required shock and vibration regime without falling apart. Adding to the manufacturing demand is the fact that the units have to be produced at SMT manufacturing speeds that can and do lead to typical random manufacturing defects, and those defects can consume profitability targets of the project. All of those concerns must be addressed with a solid automated inspection strategy.
The Solution
A cursory cost of quality analysis shows that considerable cost savings with reasonable product quality can be attained without impacting cycle time if the board goes though pre- and post-reflow AOI inspection. Such a strategy is not enough to ensure required product quality in this case. There are components on the board in the area of solder quality that are not visible to the AOI system, such as J-leads, or cannot be clearly inspected by the AOI systems due to obstruction to the field-of-view of the system caused by tight real estate within the board. The intrinsic difficulties that AOI encounters dealing with complex solder-quality issues, especially in the area of solder joint analysis also have to be considered. As we pointed out in the problem definition, there are critical joints whose profiles must be characterized with a high degree of certainty for the board to be cleared for operational use. Due to these solder-related issues, the need for the use of an AXI system with a strong track record in solder quality/joints inspection characterization is essential to provide a thorough volumetric-inspection analysis of critical joints, as well as good luck finding hard-to-detect solder-quality concerns. In this, as well as similar cases where safety of life could be tied to operational performance, the use of the combined power of AOI and AXI is essential to ensure the best possible quality of electronic devices. To use one system in place of the other simply will not do.
To solve this problem, AOI and AXI should be used to play to each other’s strengths. For example, due to its intrinsic nature, modern-day AOI has a better placement-error detection than AXI; and it can detect simple solder defects comparably. AOI also has the advantage of having the speed to deal with those errors, which are typically the majority of errors encountered in the average SMT process. Therefore, AOI is used pre-reflow to weed out errors in the placement process and early in post-reflow to deal with solder shorts, tombstones and other simple solder defects. The speed of these AOI systems should meet a reasonable cycle time. If they do not, they must be doubled up to eliminate a bottleneck.
On the other hand, AXI, which is superior to AOI in complex solder-defect detection, especially in the area of joint characterization, is used to perform those functions specifically. AXI may be slower, but it has to inspect considerably less than 100% of the board to perform its duty. As in the case of AOI, if cycle time does become an issue, a second AXI system can be brought into the process the same way that pick-and-place systems are doubled up to meet cycle times. One note of caution, do not use AXI to do functions that could be done by an AOI system, and vice versa. Every time you force these systems out of their performance “sweet spot,” they will slow down and, in many cases, may produce inspection errors. It is only natural that any piece of equipment that is overtaxed will degrade relative to its performance envelope. What is essential is for AOI and AXI systems to be allowed to operate at their peak efficiencies during the SMT process. If that is accomplished, an amazing thing will happen: The product will come out with an outstanding and repeatable quality at the end of the line. The first pass yield (FPY) will reach a high value; and the subcontractor will be rewarded with a good profit margin.
Conclusion
In this day and age, we cannot depend on saving money by cutting corners or underutilizing automated inspection technologies. Miniaturization has destroyed and, slowly but surely, will continue to destroy operations where automated inspection technologies are being under-utilized or not used at all. When we look at how to save money in mission-critical SMT manufacturing processes, the best way is to harness the combined power of AOI and AXI.
Joseph Vilella, developer of the Cost of Quality model and consultant in the field of automated inspection technologies, may be contacted at (619) 884-8360; e-mail: joevilella@mac.com.