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FIGURE 39.2 Pneumatically actuated shorting plate.
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critical, as the rubber or plate must be replaced when excessive dirt is embedded. As with the coupled plate, certain signal topologies are untestable or difficult to test. Signals that loop from one DCA or CSP pin to another and go nowhere else are untestable, as there is no external probe site with which to verify either continuity (to the DCA or CSP shorting pad) or isolation. Any signal that connects two DCA or CSP pads on the same package may be partially untestable, even if the signal continues to an external probe site. In this case, it is possible to verify that the external signal arrives at the device, and it is possible to verify isolation of this segment. But there is no simple way to discern whether the two device pads are joined to each other. (If they are already shorted together on the board and engaging, the conductive rubber has no effect.) This latter case may sometimes be mitigated by segmenting the shorting pad, such that each of the target device pads is in a separate segment. Then, if the connection on the board is good, the halves will be joined through the board.
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Several designs for fixture systems employing conductive rubber as the basic probe element have been offered commercially. In some cases, the rubber is a specialized material in sheet form and is conductive only in the z-axis (through the thin sheet vertically, but not sideways across its surface).The fixture is itself made from a circuit board, with slightly raised pads to compress the rubber tightly against desired product sites. The fixture board connects to the grid electronics at its reverse side. Other designs have included various types of locally deposited rubber dots, usually of conductive rubber that is not sensitive to orientation. Again, a rubber probe is formed. Problems with cost, complex manufacturing, complex repair, dirt sensitivity, and suitability to very small pad areas (which limit contact quality to the rubber) seem to have prevented widespread adoption.
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Optical inspection has been discussed elsewhere. It is generally applied early in the fabrication process as a yield improvement and data collection tool, not as a means of final product qualification. However, with improvements in resolution, the type of defect that may escape undetected becomes somewhat more limited, and optical inspection is argued as a possible means of final testing. With complex multilayer product, optical inspection will not be able to identify assembly/contamination-induced defects internal to the board in any case, and may still be limited in cases of contaminants or very fine-geometry shorts and opens on external layers. The equipment is somewhat slower than universal grid test systems, particularly when run at very high resolution. For such reasons, it is still not common practice to employ optical inspection (by itself) as a substitute for electrical testing. This may be a method that develops further in the future or that finds acceptance in special circumstances.
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Quite possibly the greatest daily irritant (and cost) in the operation of a typical test area today revolves around the cost of building test fixtures. Customers still hate to pay for them, and the creation of fixtures burdens the board shop with an entire manufacturing process that seems to add no value and that distracts from the main productive purpose of the factory. The only fully effective commercially available systems that require no test fixtures are flying-probe
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test systems and the related throughput enhancement systems. In addition, extensive work has been done by several organizations to attempt to develop alternative noncontact means of fixtureless test.To date, no fully successful noncontact method has yet emerged with a capability for parametric measurement of the normal range of both continuity and isolation, yet some brief description of various techniques seems warranted.
Electron Beam Methods When a test system contacts a product with an electromechanical contact, it uses this contact to inject or remove electrons. That is, current flows. The sort of electron beam common to the picture tube in a television might be used to do the same thing without contacting the product physically. Experimental systems have been built by several firms to date, but have shared some common problems. First and foremost, the amount of current delivered by the electron beam is so small that only a very poor continuity test is possible. Increased interest in fractional ohm and/or high-current testing to detect latent continuity defects runs counter to this limitation. Lab systems have been limited to continuity thresholds of 100,000 ohms or more, and this only very slowly. As most users wish to test at 10 to 100 ohms, this is a major compromise. (A smear of contaminant across an open circuit would appear to be a perfectly good conductor.) Test speeds are affected by product capacitance, as a longer time is required for the limited current to achieve significant voltage effect in a highly capacitive environment. Also, such systems must operate in an extremely high vacuum of laboratory grade. Expensive pumping systems are required. Costly air-lock systems with multiple stages and robotic product handling are probably required to keep any reasonable flow of material through the system. (Staging product in and out through a series of chambers avoids the time delay of constantly pumping down the main chamber when loading new product.) At the present time, flying-probe systems seem to offer superior test and measurement coverage at more practical operating costs.
Photoelectric Methods Subjecting a metal surface to an intense beam of light, such as that from a laser, can cause electrons to be ejected from the metal. As in the case of the electron beam, a very small current flow can be induced. Again a vacuum is required (so that the ejected electrons can be measured before they collide with air molecules), though the vacuum requirement is less stringent than that for the electron beam technique. The resistance at which continuity measurements can be made is quite limited, as in the case of the electron beam. (See discussion of electron beam method in Sec. 39.9.1.) Again, test speed suffers due to product capacitance. No practical system has resulted from investigative work.
Gas Plasma Methods Fluorescent light bulbs emit light because a gas is subjected to an electric field, which adds energy to the electrons orbiting the gas molecules until some break free. As they attempt to reattach themselves to the gas molecules, they eject their excess energy as photons (light). The plasma consists of a mixture of gas molecules, ionized gas molecules (missing electrons), and free electrons. In this state, the gas can conduct an electric current. If a jet of plasma is directed from a small nozzle to the surface of a circuit board, the effect is that of building a gas probe. In broad terms, the gas probe can be used just as any other probe or test is completely possible. Generally a noble gas such as argon is used. The gas residue is nontoxic, only a tiny amount of gas is consumed, and substantial current can flow to the product. Several companies have developed experimental systems in the form of flying-probe systems.
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