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Closing the Large Format Digital Color AOI Loop
December 31, 1969 |Estimated reading time: 6 minutes
By Joseph L. Vilella
Introducing large format digital color into automated optical inspection (AOI) can provide the key to increased first pass yields and reduced costs.
As the cost of SMT quality continues escalating with the introduction of higher complexity boards and ever-smaller components, the industry must take positive steps to reduce the cost of quality while increasing production first pass yields. Balancing those two factors is critical to ensure adequate electronics product performance, reliability and customer satisfaction. Today, with the introduction of large format digital color AOI* in conjunction with a sophisticated parametric inspection strategy, it is possible to achieve the proper balance of those factors. This article discusses the rationale for the strategy as well as the critical steps that one company** has taken to provide component placement manufacturers with a reliable solution they can verify and integrate.
The term "closing the loop" typically has been associated with a real-time feedback process. This is a process by which a primary system produces an event while a secondary system (called the feedback system) provides data back to the primary on how well it is performing with a view toward its optimization. In essence, by permitting the feedback system (actually AOI) to provide critical performance information to the primary system e.g., a chipshooter or pick-and-place machine on how well it is performing, one can say that the "loop" between the primary and feedback systems is closed. This improves process quality and savings directly related to the costs of inspection, rework, repair and scrap.
How closing the loop between AOI and placement systems provides a qualitative and quantitative means toward high first pass yields and reduced costs.
Achieving Cost SavingsIt is well known that as much as 80 percent of the manufacturing defects created by the surface mount process typically happen before the board gets to the solder reflow oven. In fact, the potential defects contributed by the "pre-reflow" machines greatly outnumber those produced during reflow operations and are even greater in the case of the high-speed assembly lines. The latter generally is characterized by multiple chipshooters and pick-and-place machines pressed into service to increase throughput. If the number of defects produced during pre-reflow could be minimized, the cost of quality would be reduced significantly. There are three steps to achieving this:
Cost-saving Step 1. It is at the solder pre-reflow stage of the process where defects can be prevented quickly before considerable amounts of defective boards are produced. With a well-implemented closed-loop strategy, defects data can be fed back to the placement systems quickly, which then can "decide" to continue the process or to stop the line and "request" corrections.
Cost-saving Step 2. Correcting defects during pre-reflow is easier and faster simply because, by definition, the components are not yet soldered to the board. Thus, it can take only seconds for a rework-and-repair operator to correct a specific pre-reflow defect vs. the several vital minutes generally required to deal with the same problem after soldering. Hence, the closed-loop strategy will support rework and repair early in the process wherein one person can perform the work of many and significantly reduce costs.
Cost-savings Step 3. Today's components placed on boards present a massive challenge to inspectors. It can take several hours to perform 100 percent inspection of a single board. Further, the inspection accuracy of human operators typically degrades with time, leading to many defect "escapes," which may end up as an unreliable product in the field.
Generally, operator-performed inspections are implemented after solder reflow when repair costs are much higher. In such a scenario, a line may produce a full day of products before a systemic error is discovered, affecting all produced products. This could lead to hundreds of hours of rework and repair or, in some cases, to the scrap disposition of hundreds of boards. By contrast, a well-implemented closed-loop strategy can provide 100 percent AOI inspection of all printed circuit boards (PCB) and eliminate the bulk of non-value-added costs related to operator-only inspection.
Closed-loop Strategy RequirementsWhat is required of an AOI system to close the loop around component placement systems and why large format digital color AOI can achieve it may be described in terms of five essential "abilities" (Figure):
The Ability to Add Large Amounts of Pixels per Feature. Feature extraction is fundamental to a solid AOI inspection strategy. The more pixels a system can place on a feature such as a chip, lead, short, character, etc., the more detail the system will have to make a decision based on the feature's physical characteristics. The large format, high-resolution digital color AOI camera brings a 6,000,000-pixel sensor to the task, thereby providing a high capability for adding large quantities of pixels per feature. (Just ahead in state-of-the-art technology: an 11,000,000-pixel camera will be introduced to further boost the feature extraction.)
The Ability to Extract Large Amounts of Data per Pixel. This critical factor for accurate AOI inspection goes straight to the issue of feature contrast discrimination in the color space. The more data per pixel, the higher the accuracy level in the decision-making process. Each pixel present is capable of displaying one of 16,000,0000 tones of color, bringing the power of high-definition color to perform advanced feature discrimination.
The Ability to Measure. There is no sense having great quantities of data on hand if they cannot be used fully to minimize decision uncertainties. Decisions pertaining to a site under test must be made as accurately as possible. Thus, comparing one site to a limited number of "good" examples does not provide the necessary detection accuracy margins. Rather, measurement is the best solution.
The system must know the physical characteristics of the units under test (UUT) and measure them to confirm authenticity and correct placement. To this task, large format digital color AOI technology is fully parametric in measuring all elements essential in making a decision on a UUT. This includes essential spatial placement data on X/Y positions and component rotation.
The Ability to Minimize False Alarms. False calls are produced when the detection uncertainty relative to the target is such that the AOI system must "err" on the conservative side. A robust AOI technology having solid data per feature from which equally powerful artificially intelligent decisions can be made will minimize the incidence of false calls or eliminate them from the process. This is essential because component placement systems must not be overwhelmed by false calls; rather, they must be able to handle true problems.
Nevertheless, when the inevitable false calls get through, they must be reduced to a minimum. The amount and quality of data per feature available in large format digital color AOI technology in conjunction with its parametric strategy accomplishes this.
The Ability to Provide the Necessary Data to the Placement Systems. The final important factor of large format digital color AOI is that of providing the placement systems with the data necessary to perform their functions. This strategy must be implemented so that, for example, the high-speed chipshooters receive pertinent data on the specific sites in the placement operation. The multi-site management capabilities provided by the parametric strategy of the enhanced AOI technology makes selective data reduction and transfer possible in a simple, straightforward manner.
ConclusionThe time has come to close the loop between placement systems and AOI. Large format digital color AOI provides a qualitative and quantitative opportunity to do this. Once successfully implemented, this strategy will provide significant savings to SMT production lines.
*K2-AOI*Vectron Inc.
Joseph L. Vilella is president and CEO of Vectron Inc., 10109 Carroll Canyon Rd., San Diego, CA 92131; (858) 621-2400; E-mail: joev@vectroninc.com.