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PCB Tech
What are the detection technologies for pcb boards?
PCB Tech
What are the detection technologies for pcb boards?

What are the detection technologies for pcb boards?


With the introduction of surface mount technology, the packaging density of PCB multilayer boards has increased rapidly. Therefore, the automatic detection of PCB multilayer films is not only basic, but also economical, even for PCB multilayer films with low density and average quantities. In complex PCB multi-layer board testing, the two commonly used methods are the bed of needle test method and the double probe or flying probe test method.

1. Needle bed test method

In this method, a probe with a spring is connected to each inspection point on the PCB board. The spring makes each probe have a pressure of 100-200g to ensure good contact at each detection point. This kind of probes are arranged together, called "needle bed". Under the control of the detection software, the detection points and detection signals can be programmed. Figure 14-3 is the typical structure of the bed of needle tester, which can get the information of all test points. In fact, only install probes at the test points that need to be tested. Although the needle bed test method can test both sides of the PCB multilayer board at the same time, when designing the PCB multilayer board, all test points should be on the soldering surface of the PCB multilayer board. The needle bed testing machine is expensive and difficult to maintain. The layout of the probe varies according to its specific application.

pcb board

A basic general-purpose grid processor consists of a drill plate and pins with a pitch of 100, 75, or 50 mils. The pins act as probes and are directly connected mechanically through electrical connectors or nodes on the PCB multilayer board. If the pads on the PCB multilayer board match the test grid, the polyacetate film drilled according to the specification will be placed between the grid and the PCB multilayer board to facilitate the design of specific inspections. Continuity detection is achieved by accessing the endpoints of the grid (defined as the X-Y coordinates of the pad). Because every network on the PCB multilayer board has been 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.

2. Double probe or flying probe test method

The flying pin tester does not depend on the pin pattern installed on the fixture or bracket. On this basis, two or more probes are installed on a micro-head that can move freely on the X-Y plane, and the test points are determined by computer-aided design (CADI)

Gerber data is directly controlled. The two probes can be moved within a distance of 4 miles. The detectors can move independently without restricting the distance between each other. The tester that moves the arms back and forth is based on capacitance measurement. The PCB multilayer board is used as another metal plate of the capacitor to be pressed tightly on the insulating layer of the metal plate. If there is a short circuit between the lines, the capacitance will be greater than a certain point. If there is an open circuit, the capacitance will decrease.

Testing speed is an important criterion for selecting a tester. The bed of needle tester can accurately test thousands of test points at a time, while the flying probe tester can only test two to four test points at a time. In addition, depending on the complexity of the circuit board, the bed of needle tester may only take 20-305 single-sided tests, while the flying probe tester requires IH or longer to complete the same evaluation. Shipley (1991) explained that even if manufacturers of high-volume PCB multilayer films think that the moving flying probe test technique is slow, this method is also a good choice for manufacturers of complex PCB multilayer films with lower yields. .

For bare board testing, there are special test instruments (Lea, 1990). A more cost-effective method is to use general-purpose instruments. Although this type of instrument is more expensive than a dedicated instrument at first, its initial high cost will be offset by a reduction in individual configuration costs. For ordinary 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 should be larger than 0.7mm. If the grid is small, the test pin is small, fragile, and easily damaged. Therefore, it is best to choose a grid larger than 2.5mm. Crum (1994b) clarified that the combination of a universal tester (standard grid tester) and a flying probe tester can make the detection of high-density PCB multilayer films more accurate and economical. Another method he suggested is to use a conductive rubber tester, which can be used to detect points that deviate from the grid. However, the different heights of the cushion blocks after the hot air leveling will hinder the connection of the test points.

Generally, the following three levels of testing are performed:

1) Bare board inspection;

2) Online testing;

3) Function test;