PCB Blog - Research on LED light source system for PCB board thick and dense board circuit inspection

The improvement of the process detection ability of PCB board is directly related to the actual ability and long-term development and progress of electronic products in the domestic and foreign electronics industries and other industries. With the development of PCB board technology, PCB boards show three major development trends: thinner lines, smaller and higher line spacing, and more obvious height differences, and this trend has been more clearly promoted and reflected in the past 5 years;, The development of instruments for the PCB board industry is lagging at home and abroad. The main reason is that it is limited by the development of light source technology. If the light source cannot effectively achieve lighting and information extraction for the target of interest, the back-end measurement Technology, including the application of image processing algorithm technology, precision electromechanical technology positioning, etc. are greatly restricted. In response to this, many domestic and foreign scholars have carried out extensive and in-depth experimental research and theoretical analysis. Due to the improvement of the light source lighting system itself, in addition to improving the clarity and contrast of the image, it is more important to ensure the authenticity of the extracted information from the image. The research of many scholars is based on the direct improvement of image quality. The direct consequence of adopting similar research methods will increase the difficulty of precision control, which is very unfavorable for industrialization and scientific detection. In 2006, Li Jun conducted research on the key technologies of machine vision light sources, and developed a color annular layered LED light source applied in electronic component placement systems, which directly improved the light source system to obtain better images and then improved accuracy. Taking the real object of the PCB board as the direct research object, and adopting a comparative research method combining the real object, light source illumination and image, in addition to effectively obtaining high image quality, the test characteristics of the actual object can also be fed back to the process of optical design of the light source. Therefore, in the process of light source design, effective control of measurement accuracy can be maintained at all times. The lighting effect of the 50° LED ring light source developed in this paper is different from the current ordinary dark field low-angle light source, which can clearly and truly reflect the circuit characteristics of the PCB board. However, it is worth noting that since the LED light source is developed for the inspection of PCB boards with thick and dense circuits, it has a good applicability for PCB board-related testing instruments. Blindly applying it to other test instruments without testing may cause poor lighting effects or bring system errors to the system.

1. Basic theory
1.1 Difficulties in the design of thick and dense plate light sources
In the development of the PCB board industry, there are currently three major trends: thinner lines, smaller line spacing, higher density, and more obvious height differences. The thick dense board used in this paper is a concentrated expression of this trend. As shown in Figure 1(a), it is the light path of a common light source applied to a PCB board with a large line spacing and a small thickness. Figure 1(b) is the light path of a common light source applied to a PCB with a small line spacing and a large thickness. When the ordinary light source is applied to the PCB with large line spacing and small thickness, the substrate, the bottom of the line and the top of the line can be illuminated, and the contrast is good. It has a good reflection on the actual size of the PCB board. When applied to thick and dense boards, ordinary light sources are easily blocked by thick and dense lines due to the small light angle. As a result, the lines and substrates do not have good contrast and brightness that can be distinguished by the naked eye. This makes further image processing-based measurements very difficult. It can be seen that the problems caused by the development of thick dense plates to the illumination of the light source cannot be solved by ordinary light sources.

1.2 Principle of adaptive ring light source based on fixed-angle design
From the previous analysis, it can be seen that in order to solve the problem of light source illumination of thick dense plates, the stability and improvement of accuracy can be maintained at the same time. It is necessary to effectively solve the selection of the incident angle and the parameter configuration of the ring light source suitable for the application object based on the geometric and optical characteristics of the thick dense plate. Based on this, this paper proposes an adaptive ring light source based on a fixed-angle design. It is composed of ring shell/circuit board/LED/diffuser plate. The inner and outer ring housings are used to hold the entire light source and internal components. There are series and parallel lines on the flexible circuit board to supply power to the LED. As a light-emitting device, LED is an approximate point light source. The emitted light has good directionality and is easy to illuminate into highlights. The area where highlights are formed will affect the output of adjacent pixels due to the electrical characteristics of the CCD, thereby affecting the measurement accuracy. Therefore, a diffuser plate is installed in front of the light source used in this paper to uniformize the brightness of the light.

2. Experimental results and discussion
2.1 Pseudo image error caused by ordinary light source illumination
The image captured by the current common light source has obvious edge deviation from the real cross-sectional view. Under the magnification of 2X, the size of the object surface corresponding to 1 pixel is 1.61 μm. The line width difference caused by false image edges is more than 3 pixels. This greatly reduces the system accuracy. The reason for the edge of the false image is that the illumination angle of the ordinary light source is too small. After most of the light is blocked by the thick and dense lines, the brightness and color of the substrate and the lower line width are very close. In general line width tests, this deviation is not included in the measurement results. Resulting in a systematic error in the inherent direction of the measured value, that is, if the error cannot be eliminated, the measured data is not a reflection of the circuit characteristics of the real PCB board. However, this error cannot be eliminated by changing the clarity and contrast of the image. It can only be eliminated or reduced by changing the design of the light source according to the characteristics of the thick and dense lines of the PCB.

2.2 Influence of geometric features of thick and dense lines on light source design
The line height/line spacing ratio is close to 1/2. That is, the angle is 27°. When using a thick dense board as the object of the line inspection, the required light source light incident angle is much larger than the light angle provided by the ordinary light source. To obtain a clear and contrasting image, the angle of the light source must be raised to an appropriate value, taking into account the optical properties of the substrate and the circuit.

2.3 The fixed-angle adaptive ring light source designed in this paper
Based on the above conditions, this paper designs a ring light source specially designed for the thick and dense board of the PCB. In order to evaluate the actual effect of the light source, the light source and the lens are aimed at the same area of the PCB board, and the lens magnification is also adjusted to the same. Using a common light source can only illuminate lines whose normal direction near the line width is close to horizontal 45°, such as the two horizontal bright lines in Figure 5(a). Due to the strong blocking effect of the thick and dense lines on the light, the brightness of the substrate part is very low and close to the brightness of the lower line width, and the difference between the meat and the meat is almost indistinguishable. Since the reflection on the surface of the line is close to specular reflection, the light reaching the surface of the line cannot enter the light-receiving cone of the lens, and the brightness within the line width is also very low. The image does not effectively record the circuit information of the PCB board, the accuracy of the line width under analysis is very low, and there is a fixed systematic error in the line width. The image using the adapted light source designed in this paper has good uniformity and sharpness. Due to the careful consideration of the light source angle, the brightness of the substrate/circuit transition/circuit surface presents a stepped rise and the transition area between different features is small, which is conducive to improving the test accuracy and realizing the lighting and information extraction of the line of interest. Ideal source images for image processing.

3. Conclusion
In order to solve the problem of the existing common light source for the inspection of thick and dense lines on PCB boards, this paper analyzes the illumination pseudo-image error of common low-angle light sources, and discusses the influence of the geometric and optical characteristics of thick and dense lines on the light source design., proposed and designed a special light source for PCB circuit inspection based on fixed-angle design. Experiments show that the lighting effect of this light source is different from that of ordinary dark-field low-angle ring light sources, and it can better solve the current problems in the detection of thick and dense PCB boards from the front end of the system. Illumination and accurate information extraction of the target of interest is achieved. High image uniformity and good contrast for substrate/transition area/line width. It is of great significance to back-end image processing and information extraction. It can be widely used in PCB board inspection equipment represented by line width inspection machine.