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This molded spacer provides a series of flat surfaces upon which individual plates sit. In a top view, the shape of each surface is modified such that plates slip easily over the top of the spacer, but are locked into position when the spacer is rotated 90 . As long as the spacer is accurately molded, no cumulative plate spacing error results. For midfixture plate supports, a similar part of smaller diameter is used. Short screws at the top and bottom plate secure the support in place. Alternative methods use machined bars around the periphery of the fixture. Either method results in accurate fixtures, reduced parts count, and ease of assembly. 38.3.3.9 Headed Music Wire Pins. For very large through-holes, the preferred probing method is to probe the annular ring of the plated through-hole. If, however, this ring is insufficient, a headed pin may be required. Such pins are manufactured with a machined tip attached to a standard music wire pin. Various head shapes are available. These headed music wire pins work in the fixture design much the same as the headless pin, that is, flush in the top plate (see Fig. 38.16), with the exception that the headed pin must be loaded from the top individually.
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FIGURE 38.16
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Headed pin in a tilt-pin fixture.
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38.3.4 Dual-Side Testing Considerations Modern SMT boards often have networks that terminate at component locations on both sides of the board. Continuity testing of such networks requires that the top and bottom terminations of such networks be probed simultaneously. There are three ways to test boards that require dual-side access:
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Flip test Clamshell upper fixture Dual-side universal grid fixture
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These methods vary in terms of test coverage, fixture cost, labor cost, risk of test escape, and required infrastructure. Each method is discussed in the following subsections. 38.3.4.1 Flip Testing. Flip testing is a last-resort method employed where up-to-date dual-side equipment is not available and compromising in test coverage is acceptable. The flip test method requires building either two fixtures, one with the top-side image and one with the bottom-side image, or one fixture with both images on it. The board is then tested one side at a time. Although this does provide partial fault coverage, it does not test those plated holes that link the top-side half of a network to the half on the bottom of the board. This poses a significant risk that bad boards will escape the test process. The problem only becomes worse if these interconnecting vias are tented (that is, if they have solder mask over them), preventing probing. In this case, the traces that run from the tented via to the first pad on the network that can be probed are also untested at both sides of the board.
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Flip testing is expensive. Not only are two fixtures built, but also many test points are duplicated between the fixtures. Much additional board handling and potential for confusion are involved. The cost and risk incurred would likely justify an equipment upgrade to dual-side test capability. 38.3.4.2 Clamshell Upper Fixture. Clamshell testing is another compromise method employed where dual-side equipment is not available. This method provides top-side access through the use of a wired (dedicated) top-side fixture. The method is superior to flip testing in that it can provide complete simultaneous access and full test coverage. However, fixture costs are quite high, especially for top-side test points. In this scenario, a more-or-less normal pin translator grid fixture is built for the bottom side of the board. The board should be oriented such that the busy side of the board faces down, as this minimizes the costly top-side fixture (see Fig. 38.17). Additional test points (called transfer points) are added outside the product area on both the top and bottom fixtures, conducting test points from the lower fixture and grid to the upper fixture assembly. Normal wired fixture construction methods are used within the upper fixture to relocate these points to desired top-side product target positions.
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