Flying Probe

Electrical testers can take on many forms, but are generally divided into two major categories, the bed-of-nails and the flying probe. Tooling for a bed-of-nails tester consists of outputting data to manufacture a fixture to accommodate the pins, which make contact with the circuit nets. When a flying probe tester is employed, the net data are fed into the machine, which uses its own software to configure the routine to probe continuity and perform isolation. 

Test fixture methods single fixtme with simultaneous access of all test points versus split-net fixtures versus flying probe... 

Test order is usually such that the continuity test is performed first. This verifies that each network is intact within itself, and that contact is established between any test fixture and the product. The isolation test can then be performed using only a single test point per network. Some test methods attempt indirect inference of continuity and isolation without making direct current (DC) measurements. These methods are commonly employed in flying probe systems. 

Test Methods Unique to Flying Probe Systems... 

Because flying probe systems contact pairs (or other limited numbers) of points at any one time, they cannot directly perform precisely the same isolation measurements as can universal grid (and other fixtured) systems. DC isolation is performed between pairs of networks, with the number of measurements reduced by determining which networks are adjacent to the network being tested and therefore likely to cause isolation problems. Otherwise, flying probers are capable of ordinary DC continuity and isolation measurement, as discussed earlier. In addition, most flying probe vendors have developed alternative measurement methods that reduce the number of measurements and therefore reduce time lost to mechanical motion. These methods are discussed in the following text. 

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. 

FLYING-PROBE/MOVING-PROBE TEST SYSTEMS ... 

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... 

FIGURE 38.18 Flying-probe test system. (Courtesy of atg Test Systems GmbH.)... 

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 dne to test-point density, these systems do slow down as additional test points are added. 

The primary limitation of flying-probe test systems is throughput. Although systems with up to 16 test heads are commonly available, these systems still lose a substantial fraction of the operating time due to mechanical positioning of the heads. Yet the cost savings derived from elimination of direct fixture costs, fixture support infrastructure, fixture debugging, and so on,... 

It is quite possible that, depending upon product mix, a testing department would be made more efficient with a larger investment in capital equipment (in the form of flying probe systems) and a smaller investment in fixtures. With smaller runs and/or smaller lots necessitating frequent fixture teardown/bring-up, this becomes more likely. 

FIGURE 38.20 Flying-probe throughput enhancement system. (Courtesy of Everett... 

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... 

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. 

In both methods the sample is contacted manually or automatically by fine metallized probes. The probes can be firmly set at the appropriate positions in an adapter, or else guided and positioned individually in a flying probe test. An important point to be kept in mind when testing metallizations on MID is that care is necessary to prevent sharp tips of probes from damaging the surface of the thin conductors on the soft plastic substrates. Using spring-mounted probes is one way of avoiding this risk. 

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