Investment in AOI Pays Off


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An AOI machine with color highlight and zoom capability helped one CM improve its yield for critical-use end customers.

Steven J. Beckman

Benchmark Electronics is a contract manufacturer (CM) that provides printed circuit board (PCB) assemblies and complete systems, from initial concept through the early engineering stages to product launch, development and distribution. At each location, comprehensive service is combined with automated manufacturing processes and technologically advanced manufacturing systems.

Within the past several years, the company`s Hudson, N.H., facility made an investment in automated optical inspection (AOI). Several of its customers require IPC Class 3 workmanship standards, which are usually reserved for mission/life-critical applications. These customers` applications include medical and data storage systems, for which board-related failures that could bring the system down would have far-reaching consequences. As a result, every measure is taken to ensure that the PCBs are assembled - and verified through test - to be free of defects.

The new AOI system supplements visual in-line manual inspection, X-ray inspection and electrical test platforms. With these test systems, yields in the low- to mid-90 percent range for large, complex assemblies were attained, but the company had hit a barrier toward achieving the customer`s requirements of 100 percent yield. Internal automated test equipment (ATE) test yield is defined as the percentage of boards that have zero defects on first power-up of the board. AOI systems would solve the human fatigue factor inherent in manual inspection, as well as find latent defects such as nominal wetting, solder balls, blow holes and insufficient solder that might only become evident as defects once they are out in the field.

The company recognized the value of using AOI based on its experience with a low-level AOI system it had previously purchased. While this system was capable only of discerning presence/absence and polarity, it enabled users to find many problems that would have manifested themselves as defects once the PCB was in the field. The company subsequently performed a "best-of-class" evaluation of several manufacturers` AOI systems and found that joint-level inspection was both practical from a return on investment (ROI) standpoint, and achievable from a technical standpoint. While several of the systems performed well, the company selected a system* not only for its reliability, consistency of measurement and tolerable false-call rate, but for two of its operational features: its color highlight system and zoom camera.

The patented color highlight technology incorporated into the system presents color images of a PCB to the system`s processor, enabling the machine to identify and the operators to verify PCB defects quickly and easily - reportedly more effectively than with traditional gray-scale images. The system works by using a fluorescent light source to project red, green and blue light onto the PCB at different angles. The camera captures these colors as they reflect off the board`s surface, producing a 2-D image that conveys 3-D information. Because only one continuous light source is used, the system`s single camera captures only one image for a specific fillet or component inspection task. Because only one image must be viewed and processed, repeatability is improved and programming is simplified.

The color highlighting allows users to build a topography of the fillet, providing a data-rich information set around the solder joints. In addition to highlighting obvious flaws, these color images also enable the system to inspect for latent defects that often go undetected by visual inspections or by customers` functional tests. In addition to saving customers the time of returning defective boards (and saving the cost to rework the boards), they have the knowledge that few, if any, boards will fail in the field because of potential manufacturing defects that would otherwise pass undetected.

The zoom capability gives users the option of selecting the ratio of detail vs. throughput for each job. With the level of customization that is provided, this type of flexibility is crucial. A system that has one or two fixed magnification levels is at a significant capability and flexibility disadvantage.

Since implementing the system to inspect for surface mount and bottomside defects in the post-wave area, the company claims to have achieved close to a 10 percent improvement in first-pass yield of boards with no defects. In addition, the company has been able to discover defects earlier in the process and make data available for process control.

As the company becomes more familiar with the advanced capabilities of AOI systems, it plans to deploy the technology at other facilities, as well. At the Hudson facility, the system is being brought in-line with the other elements of the assembly process. They will provide immediate feedback to other parts of the process and will enable the company to take preventive or corrective action with minimal delay.

The system has enabled this CM to remain a "step ahead" in a highly competitive industry. The company claims that the investment has paid off in terms of drastically reducing the quantity of defective PCBs that leave the facility. Advanced technologies like this must be weighed in terms of the value added. There is a fine line between investing in advanced technologies that will truly benefit customers, and pouring money into a cutting-edge technology just to have the newest thing available.

* VT-WIN PCB Solder Inspection System, Omron Electronics.

STEVEN J. BECKMAN is director of operations engineering for Benchmark Electronics Inc., 65 River Rd, Hudson, NH 03051-5228; (603) 885-8855.

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