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PC-Based Vision Simplifies SMD Component Placement
The Genesis Series 2 GC-120Q surface-mount component placement system from Universal Instruments uses a quad-beam positioning system for high-volume electronics assembly.
By John Lewis, Market Development Manager, Cognex Corporation, Natick, MA
Populating today's PCBs requires high-speed and high-accuracy component placement. The Genesis and Advantis
surface-mount placement systems from Universal Instruments Corp. has helped PCB builders at both original equipment manufacturers' and contract electronics manufacturers' facilities with reliable performance. For many years, these systems relied on a VME-based vision system to inspect and center components and ensure accurate placement of components on a PCB. The firm recently migrated from a VME-based vision system to a personal-computer (PC) platform, selecting the SMD4 placement guidance software package from Cognex for its algorithm performance, support for industry-standard cameras, and experienced engineering support team.
According to Damon Ashman, Vision Manager for Universal Instruments Corp., "Since moving to a PC-based platform, we have monitored the performance of the new machines in the field and found it to be significantly improved." He added that "the new vision platform has allowed us to simplify our hardware requirements while adding capability. Moving forward, it will enable us to provide more value to our customers by delivering a wide range of new functionality."
Surface-mount component-placement platforms from Universal Instruments Corp.
offer placement capabilities ranging from the smallest passive components to the largest ball-grid-array (BGA) packages. These systems employ a single, dual, or quad-gantry positioning system driven by the company's patented linear motors. These motors use variable-reluctance-motor (VRM) technology for high positional repeatability and high-speed component placement. One of three types of placement heads — 30-spindle rotary Lightning
, 7-spindle InLine7, or 4-spindle InLine4 — rides on the gantry and picks components from various fixed feeder locations.
Accurate placement of surface-mount-device (SMD) components requires high levels of speed and accuracy as well as the capability to handle many different component types and sizes. Vision systems are involved from the point when a camera attached to the placement head determines the position of components in a waffle pack, tape, or other component source so they can be picked for placement on a PCB. Each time the placement head picks up a component, it rapidly passes it over another camera. The vision system acquires the image from the camera and inspects the part. If small component features such as leads or balls are missing, damaged, or out of tolerance, the part is rejected. If the leads and balls are good, the vision system determines their location. Another camera system attached to the carriage captures images of the fiducials and placement sites on each PCB to locate the board position. The machine software then uses the known position of the component on the pick nozzle and the board to position the component within a tolerance ranging from 10 to 50 microns.
Dual cameras help the Lightning
placement head from Universal Instruments handle a wide range of component sizes.
As Ashman noted: "Our previous generation of vision systems provided excellent performance but the integration costs and complexity of the previous VME-based vision hardware were excessive." He added that "Our goal was to maintain the performance of the vision system while consolidating its functions into the PC that already exists in our machine." Ashman's firm decided to continue using its existing product line of camera systems, including integrated optics and lighting, that it has developed specially for this application. But the company then set about selecting a supplier of software algorithms and vision tools to optimize the performance of its systems under PC control.
Selecting Vision Software
In the search for the best possible component-placement vision tools, engineers from Universal Instruments evaluated the leading vision tools designed for SMD placement applications. They measured the performance of each vendor's algorithms against a library of component images they had collected over the years. They compared the speed, positional accuracy, and inspection accuracy of each algorithm. They also considered each vendor's ability to support off-the-shelf cameras, its roadmap for developing and improving its technology, and its total cost of ownership.
Ashman explained: "We selected Cognex because its algorithm performance was outstanding and because the company presented a compelling vision for the future of its product." He added that "in addition, Cognex has an extensive engineering team that is fully committed to supporting their vision products." At the time the decision to select the Cognex software was made, Cognex had recently ported its vision process software to the PC platform. Since then, Cognex has also provided networking support for inexpensive industry-standard cameras using the IEEE 1394 FireWire
Geometric Image Analysis
At the core of the SMD4 toolset is Cognex's PatMax
geometric image analysis technique for accurately locating devices and device features even with wide variations in device appearance or image contrast. This is useful when reflections or variations appear in a device image. The PatMax technique helps to minimize the effects of the device image variations and locate the device reliably. PatMax also helps the SMD4 software to accurately locate fiducials that have been degraded by oxidation, hot-air solder leveling (HASL), and other PCB manufacturing processes. This helps increase PCB alignment yield while reducing production downtime and the need for operator intervention.
placement head from Universal Instruments has a 0.55 mil/pixel (high-magnification) camera for small component feature recognition, and a 2.8 mil/pixel (large field of view) camera for larger components.
The SMD4 software package is an open, PC-based development architecture for building SMD placement guidance applications. It includes calibration tools, nozzle location tools, graphical diagnostics, and device description data structures. It also offers direct access to three levels of software tools. The first level is a library of high-level software tools for locating and inspecting chips, leaded devices, BGA packages, chip-scale-package (CSP) devices, and flip-chip devices. These tools, based on Cognex's proven solutions, automatically locate the device, determine the precise X, Y, and theta location of each device, and examine the device for missing or damaged leads, solder balls, or bumps.
Mid-Level Software Tools
The second level contains a library of flexible, mid-level software tools for locating and inspecting device features such as leads, device bodies, and ball patterns. These tools can be easily grouped in user-defined sequences and are excellent for those who wish to create custom device inspection procedures to optimize the speed and placement accuracy of their particular mounter. The third level contains an extensive library of low-level, general-purpose image processing and analysis tools. With these tools, users can write their own device inspections from scratch or add other functions to their SMD application such as post-placement inspection or PCB identification.
During the implementation process, the SMD4 platform was integrated with Universal's vision code, library code, graphical user interface (GUI), and servo control technology. The basic interaction between the platform and machine begins with the control software directing the SMD4 platform to acquire an image. In the case of a BGA device, the SMD4 platform captures an image from the camera and processes it to determine whether all balls are in place and positioned within tolerances on the device and the distance by which the balls are off the center of the vacuum tool. The SMD4 platform then transfers the information back to the control software.
Ashman said that the new PC-based architecture will expedite the introduction of new devices: "For example, instead of having to spend time manually entering parameters into the system in order to create device descriptions, Cognex tools enables users to simply present a device to the vision camera and the system will automatically learn the device geometry." He noted that "this improves the accuracy of device descriptions, and speeds up the process of creating and testing descriptions - especially in situations where new devices are frequently introduced or where uniquely shaped devices need to be described."
The SMD4 placement guidance package has been integrated into the Genesis Series 2 and Advantis 3
surface-mount-component placement platforms from Universal Instruments Corp., with excellent results and system performance levels. The Cognex SMD4 Placement Guidance Package offers distinct advantages in terms of the speed, accuracy, and flexibility required for SMD placement compared to earlier VME-based vision tools.
Contact: Cognex Corporation, One Vision Drive, Natick, MA 01760-2059;
877-264-6391 fax: 508-650-3344 E-mail:firstname.lastname@example.org Web:
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