Minute components challenge AOI.

One of the major changes to affect automatic optical inspection (AOI) equipment used during PCB assembly and inspection is the greatly reduced size of the latest components. As stated in an October 2007 press release from Hitachi High-Technologies, "In recent years, the size of resistors, capacitors, and other electronic parts has generally fallen from 0402 (1.0 mm x 0.5 mm) to 0201 (0.6 mm x 0.3 mm) with the need to cope with size 01005 parts (0.4 mm x 0.2 mm) on the horizon." After describing how component sizes have decreased, the Hitachi press release announced the development of the GXH-3 Component Mounter capable of processing 95,000 chips/h.

To put the size of a 01005 part in perspective, it is smaller than the dot on this lower case i. Because these components are so small, several AOI machines feature a choice of magnification. It's a trade-off: Greater magnification reduces pixel size to allow accurate inspection of 01005 parts, but it also can decrease the machine's throughput.

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High magnification often means that a smaller area is inspected per unit time. To achieve high throughput on a machine with programmable pixel size, the magnification needs to be kept low for as much of the inspection process as possible.

This consideration applies to all types of AOI applications although it usually is associated with the multi-megapixel cameras used to inspect component placement before and after solder reflow. Nevertheless, it also relates to bare-board and paste-print inspection for which line scan cameras often are used. For example, the Viscom S3054QS Paste Print Inspection Machine uses an 8-kpixel line scan camera but offers either 22-[micro]m or 10-[micro]m pixel size. More scans are required to cover the same area with the higher-resolution option.

The very small parts also affect 3-D solder paste inspection machines. Many industry sources agree that solder volume is a critical factor in determining a good solder joint. Accurately measuring solder volume is especially important for 01005 components because so little solder is involved. Laser triangulation is a common technique used to acquire 3-D information in these machines.

As the use of 01005 components increases and the cost comes down, AOI rates will need to increase if today's throughput is to be maintained. This is not improbable based on recent performance improvements being delivered in real production environments by newer AOI machines.

Commenting on changes to components and AOI within the last 10 years, Jeff Bishop, product marketing engineer at Agilent Technologies, said, "During that time, optics resolution has gone from around 30 [micro]m/pixel to about 18 [micro]m. System inspection speeds have increased from 1 in.[.sup.2]/s to about 5 in.[.sup.2]/s today. And cameras have gone from 1-Mpixel analog to 4-Mpixel digital.

"In addition, machines are much more flexible, allowing their use for paste-print inspection, pre-reflow inspection, a mix of pre-reflow and paste, post-reflow, post-wave, and in some cases final assembly." Mr. Bishop continued, "This flexibility, along with very attractive return on investment, positions AOI as a popular production tool. Today, equipment is being used to measure placement accuracy and for volumetric paste analysis, component part identification, and tombstone and general solder-joint defect detection."

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Aside from purely technical considerations, the companies are changing their organizations, as well as their AOI machines to address a global redistribution of contract manufacturing services. This was the view of Lyle Sherwood, vice president and technology director at Landrex Technologies: "Local and regional low-cost markets are developing in Eastern Europe, and other both historical and new nontraditional markets are on the rise. Success in these areas will require more local-language GUIs as well as renewed focus on speed and ease of programming, inspection of smaller parts, and flexibility of the inspection process.

"In addition," he continued, "customer and product support strategies will need to become more creative to effectively support a wide-spread installed base. Time to travel, language, and new technological cultures will play a larger role in sales and support than in the past."

Brian D'Amico, president of MIRTEC, expanded upon the effects of these market changes. "Over the past few years, the electronics manufacturing industry has become increasingly polarized. There are the North American and Western European markets in which an emphasis on low- to medium-volume production will provide a competitive edge. In contrast, there are the Asian, Mexican, and Eastern European markets with a predominant focus on higher-volume production.

"An AOI machine that is quick and easy to program is ideal for low- to medium-volume manufacturing. On the other hand, high-volume production requires high-speed inspection," he explained. "Fast programming also is a benefit but is far outweighed by high-speed inspection capability."

Once you understand why AOI machine capabilities are valued as they are by different users, the various models and feature mixes make more sense. Nevertheless, one machine by itself may not solve your problems no matter which machine you choose.

Don Miller, president of YESTech, proposed using several machines collaboratively, especially when working with 01005 components: "For assemblies using 01005 chip components, the post-paste print AOI system serves mostly as a process monitoring tool because there really is no effective inline repair mechanism for random paste defects. Any manual manipulation tends to introduce more problems than it can correct.

"The best inspection strategy for 01005 assemblies uses multiple AOI machines at several points in the SMT process. By combining the defect reports from the upstream AOI machines, the post-reflow machine can ensure the defect will not escape detection.

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"In an ideal situation," he continued, "there are three AOI machines on the line. The post-paste system should be tuned to detect stencil misalignment and insufficient solder, the pre-reflow machine can detect component defects such as missing parts or incorrect polarity, and the post-reflow system should detect bridging and tombstone defects. Dividing the tasks and combining results provide optimal detection."

Magnification

You can change the size of a pixel by changing the magnification of the camera's lens. However, with components as small as 01005 parts, maintaining a single magnification may be impractical. On the one hand, you need to image more than some minimum number of pixels to define the smallest dimension of a part or marking. On the other hand, if an imaging system is designed to acquire sufficient pixels to inspect even 01005 components, its resolution will be an overkill for larger parts.

One way to cope with this situation is to change the magnification to suit the inspection job. For example, the Agilent Medalist sj5000 Automated Optical Inspection System provides 19-[micro]m/pixel resolution scalable from 21 to 12 [micro]m/pixel. The datasheet's 44.7-mm x 32.8-mm field of view (FOV) corresponds to a 4-Mpixel monochrome camera at the nominal 19-[micro]m/pixel resolution.

The 0.1-mm height of a 01005 component is equivalent to only five 19-[micro]m pixels. That quantity may not be sufficient for reliable inspection if you need to identify cases of slight tombstoning, for example. At the 12-[micro]m/pixel resolution, the same measurement would be based on 8 or 9 pixels.

However, the camera's FOV will scale in direct proportion to the pixel size. So, for a 12-[micro]m pixel, the FOV is 28.2 x 20.7 mm or an area reduction of 60%. The FOV for 12-[micro]m pixels is only 40% that for 19-[micro]m pixels. This means that the inspection speed changes significantly if the magnification is altered to cope with smaller parts.

Pixel size in the sj5000 can be changed to any value between 21 [micro]m and 12 [micro]m but must remain constant throughout the inspection process. The machine's software takes into account the actual pixel size when measuring components.

One approach that avoids changes to the FOV uses multiple cameras. In the latest Viscom AOI machines, four 4.5-Mpixel cameras are used to provide a 57.6-mm x 43.5-mm FOV and either 23.4-[micro]m/pixel or 11.7-[micro]m/pixel resolution. The 23.4-[micro]m size corresponds to about 4.5 Mpixels for the given FOV and results from running all four cameras in a low-resolution mode. When the cameras are switched to high-resolution mode maintaining the same FOV, four times the basic resolution results, equivalent to 2x magnification.

It may be that processing four times as many pixels takes longer, but no change to the optical system is involved. Viscom calls the feature OnDemandHR-Operation. It offers selective rather than fixed high-resolution imaging.

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YESTech's new M1 AOI uses a 3-Mpixel camera to provide a 40.96-mm x 30.72-mm FOV with a 20-[micro]m pixel size. The lens can be changed to give greater magnification equivalent to a 12.5-[micro]m pixel size with a corresponding reduction in FOV to 25.6 mm x 19.2 mm. The M Series Machines are intended for high-volume applications, and you choose the more appropriate pixel size when developing the inspection program. Either 20-[micro]m or 12.5-[micro]m pixels must be used throughout an inspection routine.