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

The improvement of PCB process detection capabilities is directly related to the actual capabilities and long-term development and progress of electronic products in the domestic and foreign electronic industries and other industries. With the development of PCB technology, PCB boards have shown three major development trends: thinner lines, smaller and higher line spacing, and more obvious height differences. This trend has been more obviously promoted and reflected in the past five years. In addition, the development of instruments for the PCB industry is lagging behind both 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 realize the illumination and information extraction of the target of concern, the back-end measurement technology, including the application of image processing algorithm technology and precision electromechanical technology positioning, will be greatly restricted.

In view of this, many scholars at home and abroad have carried out extensive and in-depth experimental research and theoretical analysis. Because the improvement of the light source lighting system itself, in addition to improving the clarity and contrast of the image, more importantly, it is necessary to ensure the authenticity of the information extracted from the image. The research of many scholars is based on the purpose of directly improving the image quality. The direct consequence of using similar research methods will make it more difficult to control the accuracy, which is very unfavorable for industrialization and scientific testing. In 2006, Li Jun studied the key technology of machine vision light source and developed a colorful ring-shaped layered LED light source used in the electronic component placement system, which directly improved the light source system to obtain better images and then improve the accuracy.

Taking the PCB board as the direct research object and adopting a comparative research method that combines the real object, light source illumination, and image, in addition to being able to effectively obtain high image quality, the actual object test characteristics can also be fed back into the light source optical design process. Therefore, during the light source design process, effective control of measurement accuracy can always be maintained.

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, and can clearly and truly reflect the characteristics of PCB circuits. However, it is worth noting that since this LED light source is developed for the inspection of thick and dense PCB circuits, it has a relatively good applicability to PCB-related testing instruments. Blindly applying it to other test instruments without testing may cause problems such as poor lighting effects or system errors in the system.

1 Basic theory

1.1 Difficulties in designing thick and dense panel light sources

In the development of the PCB industry, there are currently three major trends, namely, thinner lines, smaller line spacing, higher density, and more obvious height differences. The thick and dense board used in this article is a concentrated reflection of this trend. Figure 1(a) shows the light path of an ordinary light source applied to a PCB with a large line spacing and a small thickness. Figure 1(b) shows the light path of an ordinary light source applied to a PCB with a small line spacing and a large thickness.

As can be seen from the figure, when ordinary light sources are applied to PCBs with large line spacing and small thickness, they can illuminate the substrate, the bottom of the circuit, and the top of the circuit with good contrast. This has a good reflection of the actual size of the PCB. However, when ordinary light sources are applied to thick and dense boards, they are easily blocked by thick and dense circuits due to the small light angle. As a result, the circuits and substrates do not have good contrast and brightness that can be distinguished by the naked eye. This makes further measurement based on image processing very difficult. It can be seen that the problems caused by the development of thick and dense boards on light source illumination 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 and dense plates and maintain the stability and improvement of accuracy at the same time, it is necessary to effectively solve the selection of incident angle and the configuration of ring light source parameters suitable for the application object based on the geometric and optical characteristics of thick and dense plates. Based on this, this paper proposes an adaptive ring light source based on fixed angle design, and its basic principle is shown in Figure 2.

In the figure, the light source is composed of inner shell/outer shell/circuit board/LED/scattering plate. The inner and outer shells are used to fix the entire light source and internal components. There are series and parallel lines on the flexible circuit board to power the LED. As a light-emitting device, LED is a point light source. The light it emits 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 scattering plate is installed in front of the light source used in this article to even out the brightness of the light. [page]

2 Experimental Results and Discussion

2.1 Pseudo-image errors caused by ordinary light sources

FIG3( a ) is a thick and dense line width image obtained under ordinary light source illumination, and FIG3( b ) is a cross-sectional imaging after light source illumination.

It can be clearly seen from the figure that there is an obvious edge deviation between the image captured by the current ordinary light source and the real cross-sectional image. At a magnification of 2X, the object plane size corresponding to 1 pixel is 1.61μm. The line width difference caused by the edge of the pseudo image exceeds 3 pixels. This greatly reduces the accuracy of the system. The reason for the pseudo image edge is that the angle of illumination of the ordinary light source is too small. After being blocked by most of the light by the thick and dense circuit, the brightness and color of the substrate and the lower line width are very close. However, in general line width tests, this deviation is not included in the measurement results. This causes the measurement value to produce a systematic error in an inherent direction. That is to say, if this error cannot be eliminated, the measured data is not a reflection of the line characteristics of the real PCB. 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 circuit of the PCB.

2.2 The influence of geometric characteristics of thick and dense lines on light source design

FIG4 is an image obtained by slice acquisition of adjacent thick and dense lines.

As can be seen from Figure 4, the line height/line spacing ratio is close to 1/2. That is, the angle is 27°. When thick and dense boards are used as the object of line inspection, the required light source angle is much larger than the light angle that can be provided by ordinary light sources. In order to obtain a clear image with good contrast, the angle of the light source must be increased to a suitable value, while comprehensively considering the optical properties of the substrate and the line.

2.3 Angle-adaptive ring light source designed in this paper

Based on the above conditions, this paper designs an adaptive ring light source specifically for thick and dense PCBs. To evaluate the actual effect of the light source, the light source and the lens are aimed at the same area on the PCB. The lens magnification is also adjusted to be equal. The comparison image shown in Figure 5 is collected.

As can be seen from Figure 5, the use of ordinary light sources can only illuminate the lines near the upper line width whose normal direction is close to 45° horizontally, such as the two horizontal bright lines in Figure 5(a). Due to the strong shielding effect of thick and dense lines on light, the brightness of the substrate part is very low and close to the brightness of the lower line width, and the difference is almost indistinguishable. However, since the reflection of the line surface is close to mirror reflection, the light reaching the line surface cannot enter the light cone of the lens, and the brightness within the upper line width is also very low. The image does not effectively record the PCB line information, the accuracy of the analysis of the lower line width is very low, and the upper line width has a fixed system error. The image using the adapted light source designed in this paper has good uniformity and clarity. Because the light source angle has been carefully considered, the brightness of the substrate/line transition/line surface presents a step-by-step increase and the transition area between different features is small, which is conducive to improving the test accuracy and realizing the illumination and information extraction of the concerned line. It is a very ideal source image for image processing.

3 Conclusion

In order to solve the problem of existing ordinary light sources for PCB thick and dense line inspection, this paper analyzes the illumination pseudo-image error of ordinary low-angle light sources and discusses the influence of the geometric and optical characteristics of thick and dense lines on the light source design. Finally, a special light source for PCB thick and dense board line inspection based on fixed angle design is proposed and designed. Experiments show that the lighting effect of this light source is different from that of ordinary dark field low-angle ring light sources, which can better solve the current difficulties in thick and dense board PCB line inspection from the front end of the system. The illumination of the target of concern and accurate information extraction are achieved. The image uniformity of the substrate/transition zone/upper line width is high and the contrast is good. It is of great significance to the back-end image processing and information extraction. It can be widely used in PCB inspection equipment represented by line width inspection machines.