1 Bed of Nails Test Method
This method consists of probes with springs attached to each test point on the board. The spring applies 100 - 200g of pressure to each probe to ensure good contact at each test point, and the probes are arranged in what is known as a "bed of needles". The test points and signals can be programmed under the control of the test software. Figure 14-3 shows a typical bed-of-needles tester configuration, where the tester is informed of all the test points. Only the probes for those test points to be tested are actually installed. Although it is possible to test both sides of the board using the bed-of-needles test method, when designing a board it is important to have all test points on the solder side of the board. Needle bed testers are expensive and difficult to repair. The needles are arranged in different probes depending on their specific application.
A basic general-purpose grid processor consists of a drilled board with pins spaced 100, 75, or 50 mils apart on center. the pins act as probes and make a direct mechanical connection using electrical connectors or nodes on the board. If the pads on the board are mated to a test grid, a polyvinyl chloride film punched to specification is placed between the grid and the board for design-specific probing. Continuity detection is achieved by accessing the end points of the grid (which have been defined as the x-y coordinates of the pads). Since continuity detection is performed for every network on the board. In this way, an independent inspection is accomplished. However, the proximity of the probes limits the efficacy of the bed-of-pins test method.
2 Dual Probe or Flying Pin Test Method
Flying pin testers do not rely on a pin pattern mounted on a fixture or bracket. With this system, two or more probes are mounted on tiny magnetic heads that move freely in the x-y plane, and the test point is directly controlled by CADI
Gerber data. The dual probes can be moved within 4 mils of each other. The probes can move independently and there is no real limit to how close they can be to each other. The tester with two arms that move back and forth is based on capacitance measurements. A circuit board is placed firmly pressed against an insulating layer on a metal plate that serves as the other metal plate for the capacitor. If there is a short circuit between the lines, the capacitance will be greater than at a definite point. If there is - a broken circuit, the capacitance will be smaller.
Testing speed is an important criterion in selecting a tester. A bed-of-needles tester can accurately test thousands of test points at a time, while a flying probe tester can test only two or four test points at a time. In addition, a bed-of-nails tester may take as little as 20 - 305 seconds to perform a single-sided test, depending on the complexity of the board, whereas a flying probe tester takes Ih or more to complete the same evaluation. Shipley (1991) explains that even though high-volume printed-circuit-board producers find the moving-probe test technique slow, it is still a good choice for lower-volume producers of complex circuit boards. manufacturers of lower-volume complex boards.
For bare board testing, there are specialized test instruments (Lea, 1990). A more cost-optimized approach would be to use a general-purpose instrument, and although this type of instrument is initially more expensive than a dedicated instrument, its initial high cost would be offset by the reduced cost of the individual configurations. For a generic grid, the standard grid for boards with pinned components and surface mount devices is 2.5mm. At this point the test pad should be greater than or equal to 1.3mm. For an Imm grid, the test pad should be designed to be greater than 0.7mm. If the grid is small, the test pins will be small, brittle and easily damaged. Therefore, it is better to use a grid larger than 2.5 mm. Crum (1994b) showed that the combination of a universal tester (a standard grid tester) and a flying probe tester can make the inspection of high-density boards both accurate and economical. Another method he suggests is the use of a conductive rubber tester, a technique that can be used to detect points that deviate from the grid. However, the different heights of pads treated with hot-air leveling will prevent the test points from connecting.
The following three levels of testing are typically performed:
1 ) Bare board testing;
2) In-circuit testing;
3 ) Functional testing.
Using a general-purpose type of tester, it is possible to inspect a class of styles and types of circuit boards, as well as for special applications.
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