Seven common pcb inspection methods are as follows:
1. Manual visual inspection
Testing pcb manually is the most traditional testing method, which has the advantages of low initial cost and no need for testing fixture. Use a magnifying glass or a calibrated microscope to visually judge whether the pcb board is qualified or not, and determine when it needs to be corrected. The shortcomings of manual visual inspection are subjective and human error, high long-term cost, discontinuous defect detection and difficult data collection. With the increase of pcb output and the decrease of conductor spacing and component volume on PCB, the method of manual visual inspection is becoming more and more infeasible.
2. Dimensional inspection
Measure the position, length, width and position of the hole with a two-dimensional image measuring instrument. Because PCB is a thin and soft product, contact measurement is easy to deform, which leads to inaccurate measurement. Two-dimensional image measuring instrument has become the best high-precision size measuring instrument. After programming, the image measuring instrument can realize automatic measurement, which not only has high measurement accuracy, but also greatly shortens the measurement time and improves the measurement efficiency.
3. Online testing
There are several testing methods, such as needle bed tester and flying needle tester. Identify manufacturing defects through electrical performance testing, and test analog-digital and mixed-signal components to ensure that they meet specifications. The main advantages are low test cost of each board, powerful digital and functional test function, quick and thorough short circuit and open circuit test, programmable firmware, high defect coverage and easy programming. The main disadvantages are the need to test fixture, long programming and debugging time, high manufacturing cost of fixture and difficult use.
4. Functional system testing
Functional testing is the earliest principle of automatic testing. It is in the middle stage and end of the production line, using special test equipment to conduct a comprehensive test on the functional modules of the circuit board to confirm the quality of the circuit board. Functional system testing is based on a specific circuit board or a specific unit, and can be completed by using various devices. There are final product tests, the latest physical models and stack tests. Functional testing usually does not provide in-depth data to improve the process, but requires special equipment and specially designed test procedures. It is not suitable for most PCB production lines because it is very complicated to write functional test programs.
5. Laser detection system
Laser detection is to scan the printed circuit board with a laser beam, collect all the measured data, and compare the actual measured value with the preset qualified limit value. This is the latest development of PCB testing technology, which has been verified on bare board and is being considered for assembly board testing. The main advantages are high output speed, no fixed equipment and barrier-free visual access; Disadvantages are high initial cost and maintenance and use problems.
6. Automatic X-ray inspection
Automatic X-ray inspection is mainly used to detect defects in ultra-fine pitch and ultra-high density circuit boards, as well as defects such as bridging, chip loss and poor alignment during assembly. The principle of inspection is to use the difference of X-ray absorptivity of different substances to inspect the tested parts and find defects. Tomography can also be used to detect internal defects of IC chips, which is the only way to detect the welding quality of ball grid array and solder balls. The main advantage is that the welding quality of BGA and the ability to embed components can be detected without spending fixing devices.
7. Automatic optical inspection
Automatic optical inspection, also known as automatic visual inspection, is a relatively new method to identify manufacturing defects. Based on the optical principle, it uses image analysis, computer and automatic control technology to detect and deal with the defects encountered in production. AOI is usually used before and after reflow soldering and before electrical testing to improve the qualification rate of electrical treatment or functional testing. At this time, the cost of correcting defects is much lower than that after the final test, which is generally more than ten times.