With the surface mount technology cited, circuit board packaging density increased rapidly. Therefore, even for the density is not high, the general number of circuit boards, circuit board automatic detection is not only basic, but also economic. In the complex circuit board test, two kinds of common method is the testing method and double probe or flying probe testing method.
1: testing method
The method is connected to each detection point on the circuit board by a spring probe. Spring makes each probe has the pressure of 100-200g, to ensure that each detection point of contact is good, so the probe arranged together is called "bed". In the control software, can be programmed to detect and signal detection, figure 14-3 is a typical testing instrument structure, the detector can learn all the test points information. Actually only those probes that need to be tested are installed. Although the use of testing method can simultaneously detect on both sides of the circuit board, the circuit board design, should make all the detection point in the welding surface of the circuit board. The equipment testing instrument is expensive and difficult to repair. Needles are selected according to their specific application.
A general grid processor consists of a board composed of the drilling, the center pin spacing is 100, 75 or 50mil. Pin up the probe, and direct mechanical connection with the circuit board of the electric connector or node. If the pads on the circuit board match the test grid, the poly (vinyl acetate) film perforated in accordance with the specification will be placed between the grid and the circuit board in order to design a specific probe. Continuity detection is achieved by accessing the end point of the mesh (which has been defined as the X-Y coordinates of the pad). Since every circuit on the circuit board is continuously tested. In this way, an independent detection is completed. However, the proximity limit probe testing method of performance.
2: double probe or fly needle test method
Flying probe tester does not depend on the installation in the fixture or bracket pin pattern. Based on this system, two or more probes mounted on the X-Y plane are freely movable on the micro heads, the test points from CADI Gerber data direct control. Dual probes can move in the range 4mil apart from each other. The probes can move independently and do not really define the extent to which they are close to each other. A tester with two movable arms moving back and forth is based on capacitance measurements. The circuit board is pressed tightly on an insulating layer placed on a metal plate, as another metal plate of the capacitor. If there is a short circuit between the lines, the capacitance will be larger than at a certain point. If there is a strip circuit, the capacitance will become smaller.
Test speed is an important criterion for selecting testers. A testing instrument can accurately test thousands of test points, and a fixtureless tester can only test two or four test points. In addition, the single test bed testing instrument, may take only 20-305, depending on the complexity of the board, and the flying probe will need Ih or more time to complete the same assessment. Shipley (1991) explained that even if the high yield of printed circuit board manufacturers that mobile flying probe testing technology is slow, but this method for complex circuit board with low yield production business is still a good choice.
For bare board testing, there are dedicated test equipment (Lea, 1990). A more cost optimized approach is to use a general purpose instrument, although it is initially more expensive than a dedicated instrument, but its initial high cost will be offset by a reduction in the cost of individual configurations. For general grids, standard grids with pins and boards and surface mount devices are 2.5mm. The test pad should be greater than or equal to 1.3mm. For the Imm grid, the test pad is designed to be greater than 0.7mm. If the grid is small, the test needle is small and brittle and easily damaged. Therefore, it is best to choose more than 2.5mm grid.
Usually the following three levels of detection:
1) bare board test;
2) on line detection;
3) function test.
Using common type tester, can be a type and type of circuit board detection, can also be used for special application detection.