A few years ago, Eliyahu Goldratt wrote a business novel—The Goal
—that captured the attention of the manufacturing world. Its main message: the importance of continuous improvement.
Agilent engineers took that message to heart as they faced the challenge of designing the SJ50 Series II, the company's flagship AOI machine. Their marching orders from circuit board manufacturers: higher performance, higher throughput, increased reliability, and lower costs.
What's more, the design needed to be flexible. The Series II would operate at multiple locations on a surface mount line, including post-reflow solder joint inspection, pre-reflow component location measurement, and post-paste 2D solder paste inspection. In these processes, the machine would need to identify missing, offset or skewed components, lifted and bent leads, excess or insufficient solder, bridging and polarity. Finally, it would perform optical character recognition and validation on components.
A Transatlantic Effort
As the engineers began work on the project, they drew on a solid history of machine vision expertise, which had its roots in an Irish company called Machine Vision Technology (MVT). Purchased by Agilent in 2001, MVT had started building machines in the mid-'90s when AOI was still in its infancy at the Machine Vision Center at Trinity College in Dublin.
"In 1996, one of our first big challenges was working with Motorola, which wanted to come up with a non-contact measurement technology as part of implementing a Six Sigma process in SMT," recalls Malachy Rice, a Ph.D. product engineer. "At that time, AOI as an industry didn't really exist. In effect, we got in through the backdoor because of our early work with machine vision."
What followed was development of a series of MVT-designed machines for component placement inspection, gauging, and solder joint inspection—featuring progressively more sophisticated camera and lighting technology. By the year 2000, for example, MVT machines featured colored LED lighting heads, automated calibration of lighting, and the first use of digital cameras.
The SJ50 Series II, featuring leading-edge motion
control and machine vision technology, meets the tough call accuracy
standards required by circuit board manufacturers. Engineers in Ireland
and the U.S. joined forced on the
On the motion control side, the company moved from early screwdrive designs, in which the inspection system was mounted above the PCB conveyor line, to a solid-frame gantry system with linear motors and a 1-micron encoder. "This gave us better repeatability and better accuracy," notes design engineer Jim Tracey, also based in Ireland.
But after the Agilent purchase and the launch of the Series II project, the engineers wanted to make even more improvements in these crucial machine vision and motion control areas. As Tracey explains it, the Series II would feature big changes in the imaging chain, including:
A new, 25 percent faster 2-megapixel digital camera
with 23 µm per pixel resolution and Camera Link interface.
New lighting controller and lighting system designed to deliver quadrant lighting and a more even field of view.
These advances gave the system better results in pre-reflow and post-reflow inspection, allowing for better imaging of joints. "Previously we were lighting a 360-degree ring LED," Rice adds. "In quadrant lighting, we split the light into different segments, which allows us to get a richer image."
Motion Engineering's XMP controller, along with the
motion control drivers, sharply reduces cabling once required in earlier
inspection systems using analog
During inspection, the lighting head projects RGB (red, green, blue) light from different angles onto the board, while the high-speed, high-resolution camera captures multiple images of the same field of view with different angles and colors of light. This enables the system software to build a lifelike 3D visualization of the component or solder joint, that Agilent calls Solid Shape Modeling.
To upgrade the motion control system, the team brought in Barry Eppler, a veteran mechanical engineer based in Loveland, CO. He was to make several trips to Dublin to work with the Irish design team.
"When I became involved with the project, the history had been to buy a complete bundled motion control system from one vendor," Eppler recalls. "But there was a price premium involved in getting an integrated gantry and control system from one supplier."
Eppler not only wanted to find a lower-cost solution to motion control, but he also wanted to find a system that could be applied across other Agilent product platforms, such as x-ray inspection and fiber-optics inspection. A system with more commonality would help reduce development time as well as simplify training and support. Agilent might also be able to improve its negotiating position with suppliers if it could leverage the core technology across more machines and more platforms.
Close-up view of the machine's optics head. The
system employs a 25 percent faster 2-megapixel digital camera with 23 um
per pixel resolution and Camera Link
Performance and Cost Efficiency
Eppler looked at a variety of motion solutions: analog motion controllers, networked controllers, software-based solutions, and chip-level approaches. The team soon agreed on three core requirements:
The motion control architecture would be based on a
digital network. This would reduce cable requirements, which would cut costs.
Fewer connector points also meant increased reliability.
It would be an open system, which could accept motors
and drives from a variety of vendors.
The system would also need to meet Agilent's high performance standards for its AOI machines.
The system that the engineers fashioned features an XMP motion controller from Motion Engineering Inc., a Danaher company. The controller card, which resides in the Series II PC, operates together with the SynqNet®
digital motion network interface. Connected to the XMP are two CD SynqNet servo drives, supplied by Kollmorgen and housed in a separate control panel.
Eppler notes that in a digital servo network, the network takes the place of a traditional controller chassis and backplane, making the design scalable without the costs of a chassis, power supply, and connectors. The digital network also makes it easier to implement automated configuration and download functions, as well as automated diagnostics.
Another prime benefit of the MEI controller and the Kollmorgen drive package is the sharp reduction in cabling. "When we started five or six years ago, you had analog systems with multiple cables between your controller cards and drive amplifiers, and then to your motors," Tracey explains. "Now, with the digital system, it is much simpler, with one Ethernet cable going between the MEI card and the Kollmorgen drives."
Among other key contributors to the project, Anorad supplies the 600 × 600 mm gantry assembly, which sits on a steel frame about 1m above the floor. The package includes: the base, the risers, X and Y axes, two linear motors, and two 0.5-micron linear encoders from Renishaw. The LC-80-240-D-NC linear motors, which move the inspection head, provide a continuous force of 276N. Some applications, such as paste measuring, also require a ball screw Z axis for vertical positioning of a height-sensing laser.
On the Series II project, which took about a year to complete, the engineers in Ireland relied on AutoCAD for electrical design and SolidWorks for mechanical structures. For programming the system's motion control code, Eppler used development tools supplied by MEI, such as Motion Programming Interface, Motion Console and MotionScope. Says Eppler: "We like the fact that there are both C libraries for our customer-shippable application code, and ActiveX tools for engineering tests and prototyping."
Agilent AOI systems provide 3D images of devices and
board features, allowing repair technicians to verify defects without a
The Machine in Action
In a typical application, such as pre-reflow, the Series II sits on an SMT production line after the placement machines. The PCB goes through a paste printer where the paste is applied. The board then goes through a chip shooter for component placement. After traveling through a series of fine-pitch placement machines, the board enters the Series II on a belt-driven conveyor controlled by an Omron PLC (model CPM2A) in the AOI machine's control panel. A NEMA 23 stepper motor and Parker XL drive power the conveyor. Providing the communications link between the Series II and both upstream and downstream machines is the SMEMA interface (Surface Mount Equipment Manufacturers Association).
When a board is available for inspection from an upstream machine, the Series II computer receives a signal via the SMEMA interface and—if the AOI machine is free—the conveyor starts running. As the board enters the Series II, it activates an in-position sensor that slows the conveyer down until the board reaches the stop pistons. After the conveyor stops, pneumatic-powered clamps, supplied by SMC, keep the board in a fixed position, and the PLC via the RS 232 port signals the PC that the board is in position and ready for inspection.
Then the motion control system takes over, moving the inspection head in point-to-point fashion around the board, capturing images in the frame grabber and relating this data to algorithms for each component being inspected. Fiducial marks on the boards serve as reference points for the inspection head. This process can determine, for example, if a part is misplaced and by how much. Depending on the size of the board, the machine can take as few as two or three images, say, for a small cell phone board, or 10 images or more for a large motherboard. All this at a speed of about five moves per second.
"We are doing point-to-point moves with our inspection head, and we must get there as quickly, accurately, and as repeatable as possible," Tracey explains. "And that's where the linear gantry system comes into play."
When the inspection is complete, the clamps are released, and the conveyor starts moving again. The Series II outputs a file for each board that it inspects, including a list of components on the board and which upstream machine placed the components. Also shown are the results of the inspection, which might include information on missing components, misplaced or skewed parts and the like. Boards that pass inspection move downstream to a reflow oven. If defects are detected, the Series II PC sends a signal down line to have the board shunted to a repair station.
Meeting the Customer's Agenda
How well is the Series II measuring up to customer demands? Rice notes that the measure of a top-performing AOI is its ability to perform well in five principal areas: minimizing false fails, preventing defective parts to escape down the line, optimizing uptime, fast programming, and high-speed operation. "You've got to make continuous improvements on all of these fronts," Rice says, "or you won't succeed in this market."
Agilent is confident that the Series II offers the features that address those critical concerns:
The 3D Solid Shape Modeling helps achieve a very high
level of call accuracy (minimizing escapes and false fails). That means being
in the parts per million range in level of correctness—in effect, better than
99.9 percent correct calls.
A flexible platform allows customers to quickly
transform the machine from paste inspection to a pre- or post-reflow system by
simply swapping the interchangeable lighting head.
Most users can generate programs in an hour or less,
thanks to the Windows 2000 interface, click-and-drag graphical user interface,
and two complete algorithm engines for geometric pattern matching.
And when it comes to speed, the Series II can inspect boards at the rate of 21 cm˛/min (3.25 inch˛/sec).
Those benefits, say the Agilent engineers, have produced a wave of satisfied customers. "Nothing has sold as well as the Series II," Rice says, reflecting on the succession of machines developed from the early MV Technology days. "It's the most successful machine we've ever done."
Moreover, the Agilent engineers have succeeded in their internal goal of designing a platform that will share many of its features with future generations of machines, even across technology and product platforms.
Even so, the job is not finished. Says Rice: "We know our customers will continue to design boards with smaller components and more functionality while running their production lines ever faster. So we must work to continuously redesign our inspection systems. It's a constant challenge."