AOI equipment error
It is well known that AOI equipment can complete the qualitative detection of PCBA. As a qualitative detection, it mainly detects missing parts, wrong parts, dislocation, reverse sticking, erecting stele, side standing, empty welding, short connection, etc. As a high-performance AOI can also be used for quantitative detection of PCBA, such as detecting the displacement of components, wetting angle of solder joints, etc. If a brand AOI only provides good or not good reports, then AOI only conducts qualitative testing. Some people may think that if AOI gives some measurement data, it will be even if quantitative detection is carried out. In fact, the measurement data is only a necessary condition of quantitative detection equipment, as a sufficient condition is to show the effectiveness of the test data. Testing equipment manufacturers must let users know the systematic error of their equipment.
The hardware of AOI includes image acquisition system and motion control system
Computer and computer. It consists of three parts: light source, camera and image acquisition system. The main characteristics of the camera are: CCD pixel, CCD size, scanning mode, color, transmission mode and so on.
At present, the CCD pixels of cameras used in AOI equipment range from hundreds of thousands to millions. Under the same resolution conditions, the higher the pixel, the larger the field of view.
Pai standard is 752x582 pixels, about 430000 pixels, which is the lowest standard currently used in AOI. Generally, manufacturers of AOI equipment using lower CCD pixels do not provide this important parameter index. The size of CCD is generally 1 / 3 ", 1 / 2", 2 / 3 ", etc. the larger the CCD size, the clearer the image is. The size of 1 / 2 "CCD is 6.4mm x 4.8mm, and that of 2 / 3" CCD is 8.8mm x 6.6mm.
The scanning mode can be divided into one-dimensional scanning (linear CCD) and two-dimensional scanning (area array CCD). The one-dimensional scanning speed is relatively fast, but the scanning control requirements are relatively high. The mechanical requirements of two-dimensional scanning are relatively low, but the overall scanning speed is slow. At present, most AOI use area array CCD. In the aspect of color, there are black and white and color. The color image is intuitive. In fact, most of AOI image processing uses gray image processing. CCD color has little effect on AOI performance. At present, the AOI using single camera generally uses color camera, while AOI using multi camera uses monochrome camera. In terms of transmission, there are analog and digital, analog transmission camera noise is large, high requirements for the working environment. Digital transmission camera will not introduce noise in the transmission process, and the image is clear, especially in the harsh working environment.
AOI detection equipment error consists of three parts: motion error, image acquisition error and image processing error.
Motion error or mechanical error includes:
Motor error, transmission error, control error, positioning error, etc. Motor error is easy to understand.
Taking the stepping motor as an example, the stepping motor consists of a stack of punched toothed silicon steel sheets, and there are errors in the stamping and stacking process. Generally, the stepping motor needs further processing after stacking to eliminate (reduce) the error. No matter how to deal with the size and equiangular distribution of the teeth, there are always errors. The transmission error of synchronous belt is mainly caused by the flexibility of synchronous belt; the transmission error of screw rod includes: elastic error of coupling, linear error of screw rod and axial error of support bearing. Although the screw drive error includes many factors, generally speaking, the screw drive error is smaller than the synchronous belt transmission error. The control error of stepping motor includes linear error and out of step of output current of stepper motor driver. The control error of servo motor is mainly the error of encoder. The positioning error mainly refers to the origin positioning error. Generally, the photoelectric position sensor is used to indicate the reference position of the origin. The origin positioning error is related to the hardware facilities and positioning process of the positioning signal.
Image acquisition error or optical error includes:
Optical lens error, camera error, analog-to-digital conversion quantization error, optical system installation error. There are fixed and adjustable installation methods for optical system installation. No matter which installation, there are certain errors, and the design value of 20 μ M / pixel or 25 μ M / pixel cannot be guaranteed accurately. The amplification factor of fixed installation is basically fixed, the equipment consistency is good, the installation is flexible, but the adjustment at the same time is more time-consuming.
Image processing error or software error includes programming error and detection error.
Due to the noise in the image, if the convergence and stability of the algorithm are not good, there may be large error. The software error is complex and can only be confirmed by software evaluation.
1. PCB positioning
In AOI device software, mark is used to correct PCB placement error. Due to the difference between each PCB cutting or PCB placement, AOI software corrects the above errors through mark function. Mark is used in pairs. Two mark points are set during programming. The two mark points are separated as far as possible. Generally, two marks are selected on the diagonal line of PCB, and the pattern of copper clad plate is selected as mark as far as possible. AOI software can measure mark reference position of PCB for programming, detect mark position during detection, and correct the position of each detection point according to X and Y error of mark pair. When mark point cannot be found or PCB rotation is too serious, AOI software can report an error.
Generally, AOI software has considered the placement of the board, so it has two mark functions
One is mark function of PCB, the other is mark function of module. When using the mark function of PCB, there are only two marks. When using mark function of module, mark points are twice of the number of modules. It is well known that extra detection time is needed to detect mark points, so mark function should be selected reasonably according to the needs. If the connecting plate is fixed, that is, after welding is completed, the mark function of PCB is generally selected. If the connecting board is a piece of splicing before mounting, the mark function of module should be used.
The AOI device with mark function can automatically identify whether the module mark is reasonable or not when it is programmed, and can move the module position intelligently when Mark enters the detection range.
