In order to ensure the production quality of PCB board, the manufacturer has gone through a variety of detection methods in the production process, and each detection method will target at different PCB board defects. It can be divided into two main categories: electrical testing method and visual testing method.
Electrical testing usually uses a Wyeth bridge to measure the impedance characteristics between test points to detect all conductivity (i.e. open and short circuits). Visual testing detects defects by visually checking the characteristics of electronic components and the characteristics of printed circuits. Electrical testing is more accurate when looking for short circuits or open circuit defects, visual testing can more easily detect the problem of incorrect gaps between conductors, and visual testing is generally carried out in the early stages of the production process to find defects and repair, to ensure the highest product pass rate.
The common detection methods of PCB board are as follows:
1. PCB board manual visual inspection
Using a magnifying glass or calibrated microscope, the operator's visual inspection is the most traditional method of inspection to determine whether the circuit board fits and when correction operations are required. Its main advantages are low upfront cost and no test fixture, and its main disadvantages are human subjective error, high long-term cost, discontinuous defect detection, data collection difficulties, etc. At present, due to the increase of PCB production, the reduction of wire spacing and component volume on PCB, this method is becoming more and more impractical.
2. PCB board online test
Through the detection of electrical properties to find out the manufacturing defects and test analog, digital and mixed signal components to ensure that they meet the specifications, there are several test methods such as needle bed tester and flying needle tester. The main advantages are low testing cost per board, strong digital and functional testing capabilities, fast and thorough short and open circuit testing, programming firmware, high defect coverage and ease of programming. The main disadvantages are the need to test the clamp, programming and debugging time, the cost of making the fixture is high, and the difficulty of use is large.
3. PCB board function test
Functional system testing is to use special test equipment in the middle stage and end of the production line to carry out a comprehensive test of the functional modules of the circuit board to confirm the quality of the circuit board. Functional testing can be said to be the earliest principle of automatic testing, which is based on a specific board or a specific unit and can be completed by various devices. There are types of final product testing, the latest solid model, and stacked testing. Functional testing usually does not provide deep data such as pin and component level diagnostics for process modification, and requires specialized equipment and specially designed test procedures. Writing functional test procedures is complex and therefore not suitable for most board production lines.
4. automatic optical detection
Also known as automatic visual inspection, is based on the optical principle, the comprehensive use of image analysis, computer and automatic control and other technologies, defects encountered in production for detection and processing, is a relatively new method to confirm manufacturing defects. AOI is usually used before and after reflow, before electrical testing, to improve the acceptance rate during the electrical treatment or functional testing phase, when the cost of correcting defects is much lower than the cost after the final test, often up to ten times.
5. automatic X-ray examination
Using the different absorptivity of different substances to X-ray, we can see through the parts that need to be detected and find the defects. It is mainly used to detect ultra-fine pitch and ultra-high density circuit boards and defects such as bridge, lost chip and poor alignment generated in the assembly process, and can also detect internal defects of IC chips using its tomographic imaging technology. It is currently the only method to test the welding quality of the ball grid array and the shielded tin balls. The main advantages are the ability to detect BGA welding quality and embedded components, no fixture cost; The main disadvantages are slow speed, high failure rate, difficulty in detecting reworked solder joints, high cost, and long program development time, which is a relatively new detection method and needs to be further studied.
6. laser detection system
It is the latest development in PCB testing technology. It uses a laser beam to scan the printed board, collect all the measurement data, and compare the actual measurement value with the preset qualified limit value. This technology has been proven on light plates, is being considered for assembly plate testing, and is fast enough for mass production lines. Fast output, no fixture requirement and visual non-masking access are its main advantages; High initial cost, maintenance and use problems are its main shortcomings.
7. size detection
The dimensions of hole position, length and width, and position degree are measured by the quadratic image measuring instrument. Since the PCB is a small, thin and soft type of product, the contact measurement is easy to produce deformation, resulting in inaccurate measurement, and the two-dimensional image measuring instrument has become the best high-precision dimensional measurement instrument. After the image measuring instrument of Sirui measurement is programmed, it can realize automatic measurement, which not only has high measurement accuracy, but also greatly reduces the measurement time and improves the measurement efficiency.
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