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Non-contact Inspection Systems for Automated PCB Assembly
December 31, 1969 |Estimated reading time: 5 minutes
Ken Parlee
Vision inspection of printed circuit boards (PCB) is a straightforward process. A PCB is mechanically placed in the pick-and-place machine using the board`s edge or tooling holes as guides. There may be significant tolerance variations in the circuit`s pattern relative to the board edge and lesser variations with respect to the tooling holes. Also, there may be minute differences in the outside dimensions because device enclosures are generally not carefully controlled during manufacture. It is more difficult to determine pattern centroid positions in relation to board fiducials. The stakes rise directly with assembly volume and degree of added value throughout the assembly process.
PCB Vision Inspection
Vision systems are programmed to locate tough-to-measure features (or geometries) on the PCB. The machines guide and adjust the robot arms so that they place devices in the correct position and orientation. To do so demands a particularly powerful gray-scale edge-detection mechanism, a versatile video tool set, and strong illumination and software capabilities to adapt to such conditions (Figure 1). Dedicated 3-D range sensors on some units provide images with the data encoded in the gray-scale value rather than the amount of reflected light. The systems self-adjust for variations in product color and ambient light.
Focusing on Needs
To find the most appropriate inspection system for an operation, the following factors should be reviewed:
Part size. The bigger and heavier the part, the more a vision system with a larger field of vision is needed. Systems are often designed to handle heavier parts than those for inspection under microscopes.
Required inspection throughput. For low-volume work, a microscope may suffice. But as throughput requirements increase, so will the need for an advanced vision system. Throughput, in fact, is the main force driving the trend toward 3-D computerized numerically controlled (CNC) vision inspection.
Number of axes. For two axes, consider use of a microscope. However, a video machine offers the advantage of autofocus. For 3-D work, vision systems, which can measure all axes with a single setup, are the better choice.
Tolerances and resolution. Higher resolution requirements on multi-axes parts usually favor vision systems. This is because the image and part edge are developed by video and measured on a gray scale, which is more precise than that gauged by the human eye and results in greater reliability and faster throughput.
A System `Wish List`
Video imaging permits measurement of a greater number of points, in three axes, and to see patterns at once in a single setup. In operation, the optoelectronic system focuses on the component, measures specified dimensions in 3-D and retains the results, which can be printed out automatically as needed.
Higher end machine vision systems generally come with a top-of-the-line personal computer and powerful optoelectronics video technology, including high-resolution (380,000 pixel) charge-coupled-device (CCD) cameras with motorized zoom (Figure 2). This arrangement enables the projection of triangular patterns on the inspection surface and the ability to focus in on them - a feature ideal for applications with highly reflective, low-contrast or translucent surfaces.
An autofocus/autocalibration function operates in two modes - surface focusing and edge focusing. It can be pre-programmed for increased measuring speed and efficiency. A DC servomotor provides high resolution on a linear scale. Some manufacturers include a programmable power turret-tube lens changer with the auxiliary equipment. The lens changer comes complete with bright coaxial, stage and ring lights with a fiber-optic system for wide view fields and clear image detail.
For minimal thermal distortion, which can seriously compromise measurement accuracy of the image, a system with a separate QV controller is recommended. Cold-light illumination, transmitted through fiber optics, usually solves the problem.
Software Requirements
An open-architecture program and standard user displays should be included with the computer system. A Windows NT operating platform, for example, makes the system easy to run, connect to a CAD program or insert additional logarithms. A system that includes versatile processing tools for flexible measurement and intelligent, powerful algorithms as standard features is a must (see Sidebar). Also, it must permit multi-tool locations so that several measurement tools may be placed at one time and in desired positions. This greatly reduces total measurement time, especially when the part to be measured is complex.
Lastly, if failure rates are benchmarked, a machine that provides seamless performance, high-level statistical programs is recommended. Generally, this comes with the more advanced CNC vision systems.
Conclusion
Although video cameras and computers have been available for decades, only recently have they become viable tools for manufacturing. For example, in PCB surface mount inspection, 3-D CNC vision systems can help benchmark sigma failure rates and diminish defects early in assembly. They reduce the cost of rework and catch defects before the product is shipped. Speed need not be sacrificed for repeatability. The bottom line is, the faster and more accurate the inspection method, the better the process control, throughput, cost savings and quality.
KEN PARLEE may be contacted at Mitutoyo America Corp., Aurora, IL 60504; (626) 961-9661, ext. 4203; Fax: (626) 333-8019; E-mail: ken.parlee@mitutoyo.com.
Figure 1. An "economy-size" 3-D vision system is functionally comparable to standard units. With video and motorized zoom optics, it provides high resolution and gray-scale edge-detection accuracy.
Figure 2. A 3-D CNC vision system comes with a high-resolution CCD camera and motorized zoom. The focus feature projects triangular patterns on the inspection surface for greater detail while cold-light illumination, via fiber optics, usually solves any heat-distortion problems.
A Machine Vision Selection Questionnaire
Once the generic type of non-contact system requirements has been decided, the next step is comparing packages. In this process, a true "apples-to-apples" comparison is imperative.
Some basic questions include:
- What is included as standard? What is optional? Does the standard package include software, optics, videocam or image processing? Wide differences among vendors generally exist.
- Does the standard package provide four-quadrant programmable lighting? If high-accuracy edge detection is required, a greater light intensity will provide ideal conditions for enhanced vision measurement. Again, there exists a wide variation among vendors on what constitutes standard lighting.
- Does the standard package offer CAD connectivity? How easy is it to import a CAD file?
- Is the software based on an open-architecture platform? How easy is it to program custom algorithms, condition statements or report formats?
- Does the system offer statistical data collection? Does it have powerful algorithms? Does it meet in-house and customer needs? Now? In the future?
- What standards are used in claims of accuracy?
- Warranties and after-support of vendors must be compared. Are installation and training included in the quoted price?
- Are hardware and software upgrades offered? At what cost?
- Are the equipment and documentation user-friendly? Ease of use can affect skill requirements and accuracy.
- How are the suppliers` credentials rated? Do they manufacture a comprehensive line of inspection instruments to meet all present and future needs? Are they willing and able to handle unique technical problems?
- Does the supplier have a reputation for making significant investments in innovation? Is it a technological leader?
- Are prices, terms and conditions market-competitive? A "Buick" rather than a "Rolls-Royce" might serve present and future needs.