In this method, a probe with a spring is connected to each detection point on the circuit board. The spring enables each probe to have a pressure of 100 – 200g to ensure good contact at each detection point. Such probes arranged together are called “needle bed”. Under the control of the detection software, the detection points and detection signals can be programmed. Figure 14-3 is a typical needle bed tester structure, and the tester can get the information of all test points. In fact, only the probes at the test points that need to be tested are installed. Although the needle bed test method may be used to test both sides of the circuit board at the same time, when designing the circuit board, all test points should be on the welding surface of the circuit board. The equipment of needle bed tester is expensive and difficult to maintain. The probes are arranged differently according to their specific application.
A basic general-purpose grid processor consists of a drilled board with pins spaced at 100, 75 or 50mil centers. The pin acts as a probe and uses the electrical connector or node on the circuit board for direct mechanical connection. If the pads on the circuit board match the test grid, the polyvinyl acetate film perforated according to the specification will be placed between the grid and the circuit board to facilitate the design of specific detection. Continuity detection is achieved by accessing the end point of the grid (which has been defined as the X-Y coordinates of the pad). Since each network on the circuit board is tested for continuity. In this way, an independent test is completed. However, the proximity of the probe limits the effectiveness of the needle bed test method.
The circuit board has small volume and complex structure, so professional observation instruments must also be used for the observation of the circuit board. Generally, we use a portable video microscope to observe the structure of the circuit board. Through the video microscope camera, we can clearly see the very intuitive microstructure of the circuit board from the microscope. In this way, it is easier for us to design and test the circuit board. The portable video microscopes msa200 and vt101 used in the factory are more convenient than traditional microscopes because they can realize “observation, detection and discussion at any time”!
Flying needle test
The flying pin tester does not depend on the pin pattern installed on the fixture or bracket. Based on this system, two or more probes are installed on a small magnetic head that can move freely on the X-Y plane, and the test point is directly controlled by CADI Gerber data. The double probes can move within a distance of 4mil from each other. The probes can move independently, and there is no real limit to how close they are to each other. The tester with two movable arms is based on the measurement of capacitance. Place the circuit board tightly on the insulating layer on one metal plate as another metal plate of the capacitor. If there is a short circuit between lines, the capacitance will be greater than at a certain point. If there is an open circuit, the capacitance will decrease.
Test speed is an important criterion for selecting tester. The needle bed tester can accurately test thousands of test points at a time, while the flying needle tester can only test two or four test points at a time. In addition, the needle bed tester may only spend 20 – 305 for one-sided test, depending on the complexity of the board, while the flying needle tester requires IH or more time to complete the same evaluation. Shipley (1991) explained that even if manufacturers of high-yield printed circuit boards think that mobile flying pin testing technology is slow, this method is a good choice for manufacturers of low-yield complex circuit boards.
For bare board testing, there are special testing instruments (Lea, 1990). A more cost optimized approach is to use a general-purpose instrument. Although this kind of instrument is initially more expensive than special instruments, its initial high cost will be offset by the reduction of individual configuration costs. For general purpose grids, the standard grid for boards with pin components and surface mount devices is 2.5mm. At this time, the test pad should be greater than or equal to 1.3mm. For the grid of IMM, the test pad is designed to be greater than 0.7mm. If the grid is small, the test needle is small, brittle and easy to damage. Therefore, it is best to select a grid larger than 2.5mm. Crum (1994b) stated that the combination of General tester (standard grid tester) and flying needle tester can make the detection of high-density circuit board accurate and economical. Another method he suggests is to use a conductive rubber tester, which can be used to detect points that deviate from the grid. However, the height of the pad treated by hot air leveling is different, which will hinder the connection of the test points.
Generally, the following three levels of detection are carried out:
1) Bare board inspection;
2) Online detection;
3) Function test.
The general type tester can be used to detect a kind of style and type of circuit board, and can also be used for the detection of special applications.