General Defect Detection Methods of PCB and Their Advantages & Disadvantages 

PCB is the carrier of many electronic components in electronic products. It provides the possibility for the orderly connection of various electronic components. PCB has become the core part of modern electronic products. With the rapid development of the modern electronic industry, the continuous innovation of electronic technology, the increasing PCB density, and the increasing number of levels, various reasons such as soldering defects in the production, resulting in a decrease in the qualification rate of the circuit board and an accident that affects the quality of the whole machine It's not uncommon. With the continuous improvement of the accuracy, integration, complexity, and quantity of printed circuit boards, the defect detection of PCB boards has become an important inspection content in the entire electronics industry. Among them, the traditional PCB defect detection technology such as manual visual inspection can no longer meet the requirements of modern industrial production level due to many disadvantages, so the development and application of new detection methods have become particularly important.

According to the reasons of PCB defects and the current common defect detection methods and their shortcomings, a general PCB defect detection method that meets the requirements of modern industry has been developed. 

They are including: automatic optical inspection technology (AOI), machine vision inspection technology (MVI), computer vision inspection technology (AVI).

1. Human visual subjective judgment method

With the aid of a calibrated microscope or magnifying glass, the pass rate of the circuit board is determined completely based on the operator's intuitive visual measurement, and the time for the correction operation is obtained. Although his budget is low and does not require test fixtures, this method is eliminated due to many shortcomings such as strong subjective judgment factors of personnel, low accuracy, high personnel cost, inconsistent defect detection, and difficulty in data collection.

2. Instrument on-line detection method

The on-line test method is to check the open circuit and short circuit of the circuit board welding and the function test of the faulty components and components through the simulation test experiment and the electrical performance test. If the layout density of the components on the board is too large, the setting of the test points may be difficult. At this time, boundary scan technology can be used. All points can be detected by the online tester. The on-line test technology based on the electric signal as the medium can detect the actual shape of the circuit board welding very close to the practical situation. Instrument online testing technology has the advantages of high conversion rate, low cost, large defect detection coverage, and easy manipulation. However, it requires a test fixture and the fixture manufacturing cost is high, the use is difficult, programming and debugging time is long and other shortcomings.

3. Function test method

The system function test method is to use special test equipment to test the function modules of the circuit board in the middle and end of the production line in order to confirm the quality of the circuit board in time. However, deep data such as component-level and foot-level diagnosis for process improvement cannot be provided by the functional test method, and a specially designed test process and special test equipment are required. Not only is the preparation of the test program complicated, but also the promotion and use are limited.

4. Visual inspection technology

Visual inspection technology covers knowledge of electronics, machinery, optics, computer software and hardware, etc. It is an important branch of computer science, involving image processing, PC applications, pattern recognition, signal processing, artificial intelligence, mechatronics and other fields It is an important means in the field of industrial inspection today. Based on the limitations of the above defect detection methods, the use of visual inspection technology to realize the detection of PCB lack of solder has become the main direction of current PCB defect detection research. The specific method is as follows:

(1) Automatic optical inspection technology (AOI): It comprehensively adopts multiple technologies such as automatic control, image analysis and processing, and electronic computer applications. Based on optical principles, it detects and processes welding defects encountered in production. It is a fast , The method of accurately detecting manufacturing defects. It mainly uses the camera to scan the PCB board to obtain the image of the solder joint area of the PCB board, and uses the vision processing technology to automatically detect PCB soldering defects at high speed and precision, and extract the characteristics of the corresponding solder joints. According to the comparison between the extracted solder joint features and the standard features in the database, the types of soldering defects are determined and marked, and the quality problems are analyzed to provide the detection result data. Wait for the relevant personnel to deal with it.

(2) Machine vision inspection technology (MVI): The application scope and practical functions of visual inspection technology are accompanied by the continuous development and progress of modern industrial automation, and it is becoming more and more extensive and perfect, especially CMOS and CCD cameras, image processing and modes The rapid development of recognition technology, digital image sensor, embedded technology DSP, FPGA, ARM, has rapidly promoted the progress of machine vision inspection. In short, machine vision inspection technology is to use machines instead of human eyes to make various measurements and judgments. As the most promising new technology, MV I takes the lead in the field of precision testing technology. Its powerful performance is that it is a collection of image processing technology (SRC), photoelectric detection technology (MSM), and computer application technology (Enterterms). The reason why machine vision inspection technology is introduced into industrial PCB inspection is that it has the outstanding advantages of fast speed, non-contact, and good flexibility, and it plays a very important role in the inspection of the electronic manufacturing industry. At the same time, machine vision can also realize automatic defect detection of PCB light boards, avoiding huge losses in the production process, and has high realistic economic value for the development of the electronic field.

(3) Computer vision inspection technology (AVI): Computer vision inspection technology based on computer vision research is an emerging industrial inspection technology. It uses an image sensor to realize the three-dimensional measurement of the size and spatial position of the measured object. The data obtained is extracted by a computer after comparing the standard and fault images or directly from the image, and guides the device to act according to the detection parameters. This kind of intelligent detection system based on vision sensor has the advantages of high time efficiency, simple structure and strong anti-disturbance, which is very suitable for modern industrial production requirements.

Cause analysis of welding defects on PCB board

1. Design defects affect welding quality

The PCB design size is too large, the printed lines are long, the impedance is increased, the anti-noise ability is weak, the heat dissipation is not good, and the electronic circuits that are arranged close together often affect each other. The more common situation is that the electromagnetic interference seriously interferes with the circuit board. Based on this, there is an urgent need to improve and optimize the design of the PCB board.

2. The solderability of the circuit board hole is related to the welding quality

If the solderability of the circuit board holes is low, it will cause false soldering problems in PCB soldering, which will directly cause errors in the technical parameters of the components in the circuit, which will cause the conduction of the inner line and the multi-layer board components to fluctuate, so that This leads to the loss of function of the entire integrated circuit area, and ultimately affects the production quality of the entire electrical product. The main influencing factors of PCB solderability are:

(1) The composition of the solder and the nature of the solder. In order to prevent the oxides generated by impurities from being dissolved by the flux, it is necessary to strictly control the proportion of impurities.

(2) The cleanliness of PCB surface and the influence of soldering temperature. Because the diffusion rate of solder is proportional to the temperature of soldering. When the activity reaches the highest level, the circuit board and the molten surface of the solder are rapidly oxidized, causing unavoidable soldering defects. At the same time, the surface contamination and low cleanliness of the circuit board will also reduce the solderability, resulting in a series of soldering defects such as: open circuit, open circuit, tin bead, tin ball, low gloss, etc.

3. Welding defects are closely related to warpage problems

(1) The electronic components and PCB boards warp due to the temperature imbalance of the upper and lower parts of the PCB during the welding process, and defects such as virtual welding and short circuit due to stress and deformation occur.

(2) When the components and the PCB board warp, defects such as empty soldering caused by warping will also occur. The occurrence of this defect is based on the solder joints in the center of the components being lifted off the PCB board, which brings great hidden dangers to the production quality of the entire electrical product. And when there is no solder paste to fill the gap and only flux is used, this situation will be more common. However, when only solder paste is used, the deformation will cause the solder ball to bond with the solder paste and cause a short circuit.