I. Introduction

In the surface mount technology (SMT) assembly process, automated optical inspection (AOI) equipment plays a crucial role. With the continuous development of the electronics manufacturing industry, the demands for quality and efficiency in SMT production are increasing.  The rational selection and evaluation of AOI equipment has become a key factor for companies to ensure production quality, improve production efficiency, and control costs. Accurate selection and evaluation can ensure that AOI equipment performs optimally on the production line, thereby improving overall production efficiency.

II. Key Points for SMT AOI Equipment Selection and Evaluation

(I) Detection Functions of AOI Equipment

In the SMT assembly process, AOI equipment plays a critical role in quality control, and its detection function is a key element in selection and evaluation. AOI equipment needs to perform well in both solder paste inspection and component inspection. In solder paste inspection, due to its high detection difficulty, there are two technical types: color 3-color light source and multi-angle. AOI equipment should not only provide information on the area of ​​printed solder paste, but also provide information on the volume and height of the solder paste. If only the area is considered and the height is ignored, even if there is insufficient solder paste on the pad, an incorrect detection pass result may occur. According to relevant research, after the solder paste is cured, multi-stage camera technology, multiple lighting angles, and a 2-D system can quickly and sensitively capture color changes in the PCB, solder mask, and solder paste appearance. The 3-D system provides more comprehensive solder paste information, including area, height, and volume, and can also determine the solder paste thickness based on metal traces, PCB, or solder mask surface. For component inspection, AOI inspection systems are usually installed before and after reflow soldering in the SMT production line. The methods used include learning based on qualified samples and detection based on SMT CAD data. The former is mainly based on statistical pattern recognition, establishing a qualified image pattern through learning, and then obtaining the detection and recognition results through comparison and calculation of the detected object image and sample image. Pre-reflow inspection can detect most defects in solder paste printing and machine placement, and can generate quantitative process control information at this stage, providing a basis for calibrating high-speed chip mounters and fine-pitch component placement equipment; post-reflow inspection, because it can detect all assembly errors, has become a more popular choice for current AOI inspection.

(II) Detection Accuracy of AOI Equipment

Detection accuracy significantly impacts product quality control. Based on resolution, AOI equipment is classified into 0402 component AOI equipment and 0201 component AOI equipment, with different equipment offering varying levels of component detection accuracy. High-precision AOI equipment can detect minute component defects, such as those related to 0201 chip components and 0.3-pitch integrated circuits. In terms of solder joint inspection, precise detection is achieved through high-resolution cameras and advanced image processing algorithms. For soldering quality issues, such as solder joint short circuits, cold solder joints, varying degrees of insufficient or excessive solder, and visual defects such as component misalignment, tombstoning, polarity reversal, multiple components, and missing components, all should be detected. This requires the equipment to have highly accurate comparison capabilities between the standard or predetermined specifications and the actual detected objects. Any subtle deviations should not be missed or misjudged to ensure that the detection results reflect the true product quality, thereby reducing the risk of defective products reaching the market.

(III) Working Efficiency of AOI Equipment

The role of AOI equipment in the production line determines the importance of its working efficiency. The working efficiency of AOI equipment can be considered from various aspects, such as detection speed and image processing speed. Based on structure, there are simple manual offline AOI equipment, offline AOI equipment, and online AOI equipment, and the efficiency of different types of equipment may vary. Online AOI equipment, because production and inspection are completed simultaneously, can reduce product inspection turnaround time. Statistics show that in an SMT production line, every minute increase in inspection turnaround time may lead to a certain degree of decline in overall production line efficiency. Online AOI equipment can avoid this problem, achieving fully mechanized operation, from the PCB entering the SMT production line to the PCB exiting, enabling continuous inspection of a large number of PCBs, meeting the needs of modern high-efficiency production. Offline AOI equipment also demonstrates improved efficiency. For small SMT manufacturing companies, if the order volume is not extremely large, offline AOI equipment can improve efficiency and save costs while meeting basic production and inspection needs by running batch inspections, increasing the number of product batches produced, and achieving on-time delivery.

(IV) Key Factors in SMT AOI Equipment Selection

1. Cost Factors: Cost is a crucial factor that companies must consider when selecting AOI equipment. Initial equipment purchase costs directly impact capital investment, and there are significant price differences between different types of equipment. For example, simple manual offline AOI equipment is much cheaper than online AOI equipment. Although online AOI equipment is more expensive, its high degree of automation and integration offers a natural advantage in simultaneous production and inspection, making it suitable for companies with large-scale, high-efficiency production needs.  Simple manual offline AOI equipment, on the other hand, is more suitable for companies with limited budgets, smaller production scales, or those that don't require very high inspection speeds, as it helps save initial capital investment. In addition to purchase costs, operating costs include equipment energy consumption, parts replacement, and maintenance costs. Some AOI equipment consumes a lot of power during operation, resulting in high operating costs over the long term; if the equipment uses special optical or mechanical components, these parts can be expensive to replace, increasing the company's operating costs. Furthermore, maintenance costs can vary significantly between different brands and models. Some equipment has complex designs, requiring specially trained technicians for maintenance, and the lack of interchangeable parts increases maintenance costs. Therefore, from a comprehensive cost perspective, companies should choose suitable AOI equipment based on their budget, production scale, and profit margins.

2. Equipment Suitability: Equipment suitability encompasses several aspects. First is its adaptability to the production process. For SMT production lines, AOI equipment must be able to work in conjunction with the production characteristics and quality inspection requirements of each step, such as before and after solder paste application, before and after reflow soldering, or for bare board inspection in the PCB industry. For example, before component placement after solder paste application, the AOI equipment needs to accurately and efficiently detect errors such as bridging, misalignment, missing solder, and insufficient solder. After reflow soldering, the AOI equipment needs to accurately detect problems such as insufficient/excessive solder, missing solder, short circuits, solder ball leakage, polarity issues, misalignment, and bent pins. In addition, the suitability of AOI equipment in the production environment is crucial. If the production workshop environment experiences significant temperature and humidity fluctuations, slight dust, or electromagnetic interference, the AOI equipment must still maintain accuracy and stability in its inspection. Furthermore, PCB boards produced by different companies vary in type, size, and the types and density of mounted components. This requires AOI equipment to be adaptable to different PCB board specifications by adjusting parameters and replacing certain components. For example, high-quality AOI equipment can detect PCB boards ranging from those with few components and large spacing to those with dense components and extremely small spacing. It can also adapt to the detection requirements of PCB boards of different sizes, ensuring effective application in various production scenarios.

3. Reliability and Stability: The reliability of AOI equipment directly relates to the credibility of the inspection results. If the AOI equipment frequently produces false positives or malfunctions during use, product quality cannot be effectively guaranteed, which in turn affects the company's production efficiency and reputation. Reliable AOI equipment should first use high-quality components in its hardware. For example, the camera quality, whether it's a single-camera, dual-camera, or multi-camera structure, requires the imaging quality, resolution, and color reproduction of the camera to remain stable over the long term to ensure that the captured images accurately reflect the condition of the product's solder joints and components; at the same time, the stability of the light source is also extremely important. During the inspection process, if the brightness, color, or angle of the light source fluctuates unnecessarily, it may seriously affect the inspection results. In terms of software algorithms, AOI equipment needs to process images based on accurate algorithms. The algorithms for statistical judgment of defect rates and the definition of acceptance criteria must be accurate and reasonable. For example, using pattern recognition technology based on pre-processed images, the algorithm design should be able to handle the complex situations of different types of components and solder joints, accurately determine whether the product is qualified, and reduce misjudgments of classifying qualified products as defective or vice versa. In addition, the stability of the equipment during long-term operation is crucial. In the continuous production process of an enterprise, the AOI equipment is required to work continuously and stably, avoiding frequent shutdowns and debugging due to the instability of the equipment itself. This places high demands on the equipment's heat dissipation performance, the stability of the mechanical structure, and the anti-interference ability of the internal circuits. For example, if the equipment has poor heat dissipation, it may cause the internal motherboard or other electronic components to malfunction, thereby affecting the stability of the equipment's normal operation.

III. Methods for Selecting and Evaluating SMT AOI Equipment

(I) Clarifying Production Needs

Before conducting selection and evaluation, enterprises must first clarify their own production needs. This includes an analysis of product types. For example, if a company primarily produces consumer electronics such as mobile phones and tablets, these products typically require miniaturization and high integration.  The corresponding PCBs have densely packed components and are relatively small, thus requiring higher precision from AOI equipment.  The equipment needs to be able to detect tiny components and solder joints with very small pitches, such as 0201 chip components or solder joints with a 0.3 Pitch.  Furthermore, due to the large number of product batches and high production volume, the equipment's detection speed and efficiency are also crucial.

Secondly, determining the production scale is important. If the company is a large-scale manufacturer with production lines requiring continuous operation, then inline AOI equipment may be more suitable. It can be seamlessly integrated with other equipment on the production line, enabling fully automated operation and improving production efficiency. However, for small production companies with unstable order volumes and relatively light production tasks, offline AOI equipment or simple manual offline AOI equipment may suffice, providing basic quality inspection while saving costs. Furthermore, an analysis of the production process is necessary, as different SMT production processes tend to produce different types of quality problems. For example, if a company's solder paste printing process has some instability, then the AOI equipment needs stronger correction and detection capabilities in solder paste inspection. It should be able to promptly detect defects in the solder paste printing process, such as bridging and insufficient solder, to ensure the smooth operation of subsequent placement and reflow soldering processes.

(II) Researching Equipment Models and Brands on the Market

After clarifying production needs, it is necessary to conduct extensive research on AOI equipment models and brands available on the market. First, relevant equipment information can be collected through online searches and industry reports to understand common brands and the equipment models offered by each brand, including their general functions, performance indicators, and price ranges. For example, some well-known brands have a high reputation in the market, and their AOI equipment may perform excellently in terms of detection accuracy or work efficiency, but the price is also relatively high; while some emerging smaller brands, although less well-known, may have advantages in certain specific functions or cost-effectiveness. Regarding equipment models, various technical parameters can be considered, such as resolution. Different models may be suitable for inspection scenarios with different precision requirements. For example, 0201 component AOI equipment is often used for inspecting tiny components requiring high precision. In terms of camera structure, single-camera, dual-camera, or multi-camera systems will differ in imaging capabilities, field of view, and inspection speed. It is also helpful to refer to the experience of other companies in the industry, consulting with peers about the advantages and disadvantages of the AOI equipment brands and models they have used, whether they meet their production needs, and the after-sales service situation. This helps companies narrow down a list of suitable AOI equipment candidates and brands.

(III) Testing and Comparison

After selecting some potential AOI equipment, contact the equipment suppliers to provide equipment samples or conduct on-site testing and demonstrations. Only through actual testing can you truly confirm whether the equipment meets the company's production needs. A series of test cases can be set up, including tests under actual production environment conditions. For example, test the company's commonly used PCB boards on the candidate AOI equipment, observing what types of defects the equipment can detect.  Focus on checking the accuracy of detecting common errors before and after reflow soldering or wave soldering, such as the accuracy of detecting different degrees of insufficient solder, excessive solder, and component misalignment.

Compare the differences in detection results between different equipment models, and examine the false positive rate. If the equipment classifies many qualified products as defective, it will increase unnecessary production costs. Conversely, if defective products are mistakenly allowed to proceed to the next process, it may affect the quality of the final product. In addition to detection functions, compare the working efficiency of each device. For example, record the time required for each device to inspect the same number of PCB boards. In terms of ease of operation, evaluate whether the equipment programming is convenient and simple, and the difficulty of parameter adjustment. If an AOI equipment is complex and cumbersome to program, it will increase the learning cost for operators and the possibility of operational errors. Furthermore, it is necessary to pay attention to the stability of the equipment during long-term operation, running it continuously for several hours or even days, and checking whether the equipment malfunctions or the detection results show significant deviations. Analyze the data generated by the equipment, such as whether the statistical reports provided after detection are detailed and helpful in assisting the company in quality control of the production process.  Comprehensive analysis and comparison of these results will help determine the most suitable AOI equipment for the company's needs.

IV. Characteristics of High-Quality SMT AOI Equipment

(I) Precise Detection Capabilities

High-quality SMT AOI equipment must possess precise detection capabilities to ensure the accuracy of component and solder joint defect detection. The camera used in the equipment is crucial; high resolution is a prerequisite for ensuring image quality. For example, high-precision equipment can achieve a resolution of 15 micrometers/pixel, enabling it to clearly capture the minute features of components and the details of solder joints, leaving no tiny defects undetected.

When inspecting solder paste, in addition to providing information on the area, height, and volume of the solder paste, the equipment should also be sufficiently precise in detecting the uniformity of the solder paste on the PCB board, avoiding subsequent soldering problems caused by neglecting small irregularities. For component inspection, whether using a learning-based detection method based on qualified samples or a detection method based on SMT CAD data, the accuracy of the algorithm determines the detection results. The equipment should possess high-precision judgment capabilities in aspects such as component polarity determination, position offset measurement, and misplacement identification. Furthermore, when the equipment inspects different types of circuit boards, it should be able to automatically adapt to different component layouts and solder joint shapes, maintaining stable detection accuracy without a decrease in detection capability or misjudgment due to the complexity of the circuit board or the diversity of components. For example, when inspecting PCBs with multi-layer circuit structures or high integration, it should still be able to accurately identify whether each component and solder joint meets the requirements.  In complex circuit boards like mobile phone motherboards, which have a large number of components with different specifications and performance requirements, high-quality AOI equipment must be able to accurately detect them while maintaining a low false positive or false negative rate.

(II) High-Speed ​​and Efficient Detection Efficiency

Modern SMT production lines pursue speed...