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38.1 INTRODUCTION
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The largest volumes of bare boards are tested on fixtured systems. However, for smaller volumes or special purposes, flying probe systems may be preferable. This chapter discusses them both to enable the user to make the most effective decision based on testing objectives and volumes.
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38.2 SYSTEM ALTERNATIVES
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38.2.1 Fixtured Systems Each dedicated, or hard-wired, test system presents some sort of standardized interface pattern of test points. This may consist of a universal grid s continuous array of points (bed of nails) or may be a connector pattern of some type. In the most primitive systems, a simple cable connector may be presented. Each of these types of equipment uses one or more types of customized test fixtures to connect this standard electrical interface pattern to the unique contact pattern of a particular product to be tested. Each fixtured system includes a measurement unit that can be connected to any of some thousands of test points by a solid-state switching matrix. A central computer controls the measurement unit and switching matrix. This computer also controls the press (or vacuum mechanism), which can compress the product against spring-loaded test points presented to the product by a customized test fixture. To imagine the system s operation, first consider the measurement unit to be an ohmmeter. The computer commands the switch matrix to connect the red lead of the meter to, say, test point 17. It then connects the black lead of the meter to test point 1027. The measurement system can then be commanded to measure the resistance of the product under test between test points 17 and 1027.The test program dictates a long series of such measurements organized to test the product completely. Because the switching and measuring occur at the high speeds possible with modern electronics, a complex board can be tested in seconds. 38.2.2 Dedicated (Hard-Wired) Fixture Systems Originally the most popular system type, dedicated systems are generally being displaced by systems employing less expensive and more accurate fixturing methods. The word dedicated
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refers to the fact that it is generally impossible to salvage very much material from these fixtures for reuse in other fixtures. The cost of these fixtures derives from this fact, and from the high labor and material costs involved in the original construction. 38.2.2.1 Advantages of Dedicated Fixture Systems. A primary advantage of these systems is that, because each test point is routed within the fixture by a flexible wire, it is possible to position any test point at just about any location. If your product requires 8,000 active test points, then you need no more points than this in the test system. One hundred percent utilization of available test points is possible. This contrasts with the universal grid approach discussed later, wherein points can be shifted only a small distance from the original location on the grid. In that case, you must have enough electronic test points to cover the entire surface of the product at some constant density, and any given fixture will generally use only a portion of these points. Thus, the capital equipment cost of a dedicated system may be lower, but the ongoing fixture costs may be much higher. Dedicated systems are generally built with no more than about 10,000 test points, with smaller numbers common. 38.2.2.2 Construction of Dedicated Fixtures. The test fixture for such systems generally consists of a rigid box structure with one side facing the product to be tested and some other side or surface presented to the interface pattern of the test system. The side facing the product includes the probe plate. This plate of insulating material is drilled to mount an array of spring-loaded pins able to make electrical contact with the product. The reverse side of these pins is connected within the box by physical wires to the system interface connector on the opposite side of the fixture. Fabrication of the fixture involves the usual data-processing steps, followed by spring probe type selection, drilling of the probe plate, installation of spring probes, drilling of the interface plate, and installation of system mating contacts or connectors. Many spring probe tip styles are available to suit different diameters of target pads and holes (see Fig. 38.1).
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FIGURE 38.1 A variety of spring probes and receptacles. (Courtesy of Everett Charles Technologies.)
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