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FIGURE 38.17
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Transfer point clamshell fixture for dual-side access.
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To use this method, it is necessary that the lower grid be larger than the product under test such that a number of transfer test points adequate to provide for the upper fixture can be placed outside the product area. Again, the cost of the extra contact points, spring probes, wiring, and other special material and labor processes associated with the topside fixture may well justify an equipment upgrade. 38.3.4.3 Dual-Side Access Universal Grid. The third and most widely accepted method of providing for volume testing of dual-side product is the double-sided universal grid. Such systems provide electronics and grid probe fields on both the top and bottom sides. Upper and lower translator fixtures are built for dual-side products using the standard techniques discussed earlier, providing economical fixtures, reusable pins and hardware, full fault coverage, excellent throughput, reliable contact, and accurate targeting.
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BARE BOARD TEST EQUIPMEMT
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Press Units The major mechanical component of the grid is a press unit. The three major compression methods used to compress the product onto the fixture are hydraulic, pneumatic, and electric. The total force requirement can be quite large. The spring probes employed in grids usually require a force in the range of 4 to 10 oz. each. A large, complex board may have 20,000 active points per side. This amounts to 5,000 to 12,500 lb. of force, depending upon the probes used. Hydraulic drive is probably the least used compression method. Although it provides tremendous power, it is difficult to maintain and fluid leakage is a problem. Pneumatic and electric compression are popular and successful. Electric drives are easy to control, but become expensive and/or slow when larger forces are required (as in the case of larger grids). Pneumatic systems are fast and powerful, but require a good supply of clean, dry, compressed air. With any method, accurate compression control is critical to reliable fixture performance. As noted, universal grids present the test points as an array of spring-loaded contacts. A variety of contact tip shapes have been used, but most contacts used today offer waffle tips. Waffle tips are nominally flat, but with an H-shaped pattern of grooves cut into the surface to provide a somewhat aggressive contact surface. Such a surface is better able to cut through thin contaminants on any surface it contacts.
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FLYING-PROBE/MOVING-PROBE TEST SYSTEMS
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Moving probe, flying probe, and x-y prober are all names for test systems that make use of two or more test points that can be accurately positioned anywhere on the board surface by means of a computer-controlled motion system (see Fig. 38.18). Probe tips can be retracted in a z-axis direction away from the board surface, then moved in the x and y directions to a new board
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FIGURE 38.18
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Flying-probe test system. (Courtesy of atg Test Systems GmbH.)
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FIGURE 38.19
Two probe tips contacting board. (Courtesy of Probe Inc.)
location and extended again to make contact. Figure 38.19 shows two probe tips contacting board. Dual-side systems generally provide a minimum of two independently moving probes per side for a minimum total of four heads.
Advantages of Flying-Probe Systems The major advantage of these systems is elimination test fixtures, making these systems ideal for small-to-moderate volume production. Advanced flying-probe systems provide highly accurate probe placement, and contact the board with minimal force, leaving no discernable witness marks on most surfaces. They are very well suited to testing the finest pad sizes. Although not subject to limitations due to test-point density, these systems do slow down as additional test points are added. Direct current (DC) continuity test measurements are accomplished by placing one probe tip at each end of the continuity to be verified and performing a measurement.The probes then retract, move to the next measurement site of interest, make contact, and make the next measurement. Similarly, it is possible to perform DC isolation testing of a network by placing one probe (probe A) on the network, while another (probe B) checks all other networks in sequence for the presence of a short or leak. Probe A can then be stepped to the next network to be tested and the process repeated. This continues until each network has been checked against all others.
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