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Publication Date: 02/1/2008
Archive >  February 2008 Issue >  Special Feature: Components and Distribution > 

AOI: Leading the Way to Zero-Defect Production
Automated production line incorporating post-paste and post-assembly AOI.

The profitability squeeze on domestic PC board assemblers has tightened steadily over the last 10 years through a combination of component and board miniaturization, smaller and more varied lot sizes, conversion to lead-free solder, increased offshore outsourcing, and zero-defect customer directives. The survivors of these dramatic changes are striving to increase profit margins through improved inspection and process improvement to reduce waste and re-work.

To achieve zero-defect production, most small- and medium-size PC board assemblers in the U.S. have implemented a post-solder inspection to prevent bad product from getting out to customers. The systems of choice have been Automated Optical Inspection (AOI) and x-ray inspection, depending on the components and solder used. While this basic prevention step solves the supply chain quality problem, it does not address the process faults. "The only way to increase the bottom line is to do root cause analysis, improve the process, and ultimately lower costs," says Tim Anderson, Inspection Systems Sales Manager for Omron Electronics LLC.

"Many assemblers are adding a `post-paste' inspection to determine the quality and geometry of the solder paste brick," observes Joe McGonagle, Inspection Systems Sales Engineer at Omron. The shape can directly affect the solder results during reflow. "Two-dimensional inspections of the solder are now being built into the newer screen printers. However, three-dimensional inspections are the industry standard and deliver more accurate results."

"Ten years ago, AOI was not very capable, there were a high number of false calls and the system was slow to operate," recalls Anderson. In that time, chip components have miniaturized from 0402 (think small relays) down to 01005 (the size of rice grains) for an overall size reduction of one-quarter. Manual visual inspection was no longer viable. In-circuit testing after soldering where a "bed of nails" test fixture came down to test the different points also became relics when board density increased and component size reduced.

"The change to lead-free solder created new challenges for AOI technology due to the wider variety of colors and textures that result from solder composition and reflow temperature," continues Anderson. "The AOI technology has improved and now delivers zero defects with minimal levels of false calls at increased production line speeds." Both Anderson and McGonagle concur that typical inspection speeds are two to four inspections per minute for larger motherboards or multi-board panels, or 15 to 30 seconds for a complete PC board inspection.

Data Mining Results
By placing a post-paste inspection at the front end and post-solder inspection at the end, assemblers have the raw data to determine where faults occur in the process. "Right now, we are seeing assemblers address only about 50 to 60 percent of the problems that occur after soldering because it is so complicated to determine the root cause," says McGonagle. "By mining the data they collect, specific faults can be traced and corrected. Omron has developed an analysis software tool that performs the analysis and points out problem areas. Called Q-up Navi, the software has been used to reduce waste by correcting problems before more value is added to the board."

"As a manufacturer, Omron builds both consumer products and industrial automation equipment that contain circuit boards, and we needed to keep our manufacturing costs down," states Anderson. "In our sensor manufacturing plant, we perform AOI at all three locations in the assembly process: post-paste, post-placement and post-solder. On each line, the inspection systems feed data to Q-up Navi, and we can quickly identify the fault before a board moves to the next stage. The reduction in faults, counted as solder points in parts-per-million (ppm), has been dramatic.

AOI vs. System Analysis
When AOI alone was applied, the defect rate remained high but reduced by 9 ppm over the course of a month. The next month, data collected from the various inspection systems was integrated and mined by Q-up Navi software. Faults identified by the software in various aspects of the solder application and placement processes were corrected. By the end of that month, quality improved greatly, from 79 ppm down to 15 ppm. Over the following months, the plant was able to maintain the improved level of quality. Each of the AOI systems contributed valuable data that led to the finding the root causes of faults.

The parameters monitored at the solder paste inspection stage include detection of paste volume, height and incidence of bridging. The system identifies clogged apertures in the printing screen. When fully blocked, no solder is applied. When partially blocked, insufficient solder is deposited or the shape of the solder brick is deformed in a way that will cause an unreliable joint. Excess solder paste can spill over and create bridging between that will cause short circuits or functional failures. The three-dimensional solder inspection system used is Omron's model VP3000. It utilizes a color phase shift method to capture solder brick images for accurate measurements. From the image, the system performs height and volume calculations. This particular model divides the image into low, middle and high slices to determine the final profile of the solder brick. This enables highly accurate measurements that confirms correct solder geometry before reflow.

Post Placement Inspection
"Many PC board assemblers in the U.S. feel this inspection stage is unnecessary," says McGonagle. "While it is probably the least expensive of the inspection systems to implement, this provides one more opportunity to make sure everything on the board is correct before soldering." In the plant example above, an in-line Omron VT-RNS inspection system configured to do placement inspection is used. "We use it to optimize the process management by assuring all components are uniformly placed before reflow. In the U.S., assemblers have observed that the solder 'grabs' the component into the correct position upon reflow without post-placement inspection. It really boils down to the level of commitment to quality."

Post Solder Inspection
According to Anderson, "Omron pioneered a true color, color-highlight system that set the standard for reliable solder joint inspection. The patented technology proved to be so robust that when it came time to convert from eutectic tin/lead solder to a wide range of lead-free formulations, the inspection algorithms were only slightly modified. In the example above, an in-line model VT-RNS system configured for solder inspection was used. The combination of dome lighting that eliminates shadows and red-green-blue (RGB) processing provides color-highlighted images that provide three-dimensional detail of faults. Lead-free solder can solidify into unreliable joints due to variations in reflow temperature and cool-down time, and this inspection system has been able to help correct this type of problem."

The result of integrating the data collected by inspection systems at post-paste, post-placement and post-solder cause a dramatic reduction in faults, ultimately leading to less waste and re-work, and higher overall productivity. "Omron customers experience exceptional reductions in faults," say Anderson. "We have other successes that can be shared on an individual basis. Unlike the fast weight loss advertisements, the results they experienced are typical of the improvements of using a diagnostic software such as Q-up Navi."

For more information, contact: Omron Electronics LLC, One Commerce Dr., Schaumburg, IL 60173 800-556-6766 fax: 847-843-8081 Web:  

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