3D inspection and inspection equipment after printing

This article briefly discusses the detection methods used after solder paste deposition, and compares the two-dimensional detection technology with the three-dimensional detection technology. Through comparison, the advantages of the three-dimensional detection technology and the technical performance of various detection equipment are mainly introduced. Keywords: three-dimensional inspection; solder paste deposition; printing inspection equipment. With the deeper development of higher density, smaller size, and more complex PCB hybrid technology for electronic assembly, it has become history to visually inspect the quality of printing. Although the process equipment is more advanced, it is still important to strictly control the quality of the PCB, because 70% of the defects generated during the electronic assembly process originate from the solder paste printing process, especially when depositing fine-pitch components. In addition, defects such as missing marks, too much or too little solder during the deposition of solder paste will cause bridging, short circuit and tombstone phenomena in the subsequent process (component placement), making the quality and reliability of the final product produced Sex is not guaranteed. For this reason, people pay more and more attention to solder paste inspection after printing. At present, surface assembly inspection equipment manufacturers provide several different post-printing inspection methods and various solder paste deposition inspection equipment, ranging from relatively inexpensive manual and offline inspection equipment to high-end, high-speed online inspection equipment of $ 100,000 . After fully understanding and weighing and comparing the pros and cons between the two-dimensional inspection equipment and the three-dimensional inspection equipment, the offline inspection equipment and the online inspection equipment, the sample inspection and the whole board inspection, choose your own assembly line Applicable solder paste inspection equipment.

1 Testing increases costs

Experts in the electronics industry agreed that soldering defects were caused by poor solder paste printing. Therefore, improving the printing quality or reducing the number of defective circuit boards entering the next step will help to improve the final quality, and the cost reduction can be achieved by reducing the amount of repairs and reducing the scrap rate.

1.1 Identify defects as early as possible After a summary of the costs of the repair or scrap of the defective circuit board detected during the online testing stage, it is found that the control printing process will bring many significant advantages to solder paste printing . Any kind of defects will consume money, and post-printing inspection can only help reduce the number of defects, and cannot completely eliminate the defects. However, in fact, before the cost of the circuit board increases, it is inspected during the processing process to identify defects as early as possible. Great effect. Cleaning the circuit board for reuse is much cheaper than repairing or retesting. The cost of repairing defects after printing is estimated to be $ 0.45, and the cost of repairing the same defects after online testing is approximately $ 30. Regardless of the dollar exchange rate, this relationship remains unchanged. Therefore, it is not a trouble to find defects as early as possible in the processing process, but a good opportunity to save costs.

1.2 Improve reliability The additional inspection process in the printing process can improve the reliability of the assembled circuit board for two reasons: first, inspection reduces the amount of rework, in addition; solder joints after rework are easy to damage, and are better than qualified Solder joints are more likely to break. Secondly, insufficient solder paste may also result in easily broken solder joints. Although it can pass the online test at that time, it will also break in the future. Although the circuit boards with these two problems can pass the final test, they are prone to failure during operation. These problems in the finished product will make the user dissatisfied or make the warranty cost very high.

1.3 Necessary inspection Due to the requirements of denser lead spacing, smaller ball grid array solder balls and more accurate printing gaps, more PCB assembly manufacturers have added solder paste inspection procedures in the assembly process. In some contract assembly plants, additional testing steps are added based on user requirements. According to the technical requirements, when solder paste inspection must be carried out, the next step is to determine which inspection equipment is most suitable for a specific application.

2 Select testing equipment

Solder paste inspection equipment can be purchased from several manufacturers. Each manufacturer provides testing equipment with different speeds, performances, and prices, but all report the measurement results of solder paste height, volume, and area (Table 1). There are two main types of testing equipment after printing: manual offline testing equipment , Including visual inspection and desktop measurement tools; automatic online inspection equipment, including the sample inspection system built in the printing machine and the entire printed board scanning inspection equipment.

2.1 Visual inspection The simple method of visual inspection has been used for a long time to determine the "pass or fail" of the sample. Until today, the advent of smaller devices, higher lead counts and finer pitch components made this method Not applicable anymore. Using a luminous magnifying glass or a calibrated microscope, a trained operator inspects the sample printed board and determines when calibration operations are required. Visual inspection is the lowest cost method in process monitoring, and the cost of correcting the operation mule in the printing process is the most reasonable. However, the visual inspection method carries people's subjective consciousness: the inspection results are different between operators and operators. The visual inspection tools are not calibrated and cannot provide the data required for process control. From the actual situation, with the continuous popularization of ultra-fine pitch and BGA devices, the visual inspection method is no longer used because it is no longer an effective method for monitoring the printing process.

2.2 Manual Laser Inspection To reduce defects, the next step is to use manual desktop cabin inspection equipment. These measuring tools use non-contact laser technology to measure solder paste height and record. By training the operators a little, these devices can usually produce consistent results, and the test results will not be different due to the different operators.

Laser 3D inspection equipment uses a laser beam to establish a reference point for measurement. This device can report the height of a single solder paste measured at a certain point on the pad irradiated by the laser beam, usually the center of the pad. This type of tester can also use the length of the pad multiplied by the width of the pad to obtain the area measurement. Then, the area measurement value is multiplied by the height measurement value to calculate the volume measurement value. The basic process control used by the offline testing equipment is to remove the sample circuit board from the production line, perform standard measurement, and record the test result data. The new PC-type testing equipment can store the data and provide it to SPC (Statistical Process Control) for analysis. However, before printing other defective circuit boards, the offline inspection equipment cannot immediately detect the defects.

2.3 Automatic detection system built into the printing press Several printing press manufacturers have introduced built-in 2D and 3D solder paste detection systems or fault finding systems. However, the detection system built into the printing machine shares hardware with the screen printing machine. Since the screen printing machine must be paused to perform the detection, the printing speed is reduced. Most built-in inspection systems use camera vision technology to evaluate solder paste area, coverage, and calibration. In addition to detecting the mark, you can also use this camera to check the screen to see if the template opening is blocked and there is too much solder paste.

Some printer manufacturers have added volume measurement to the printer. The method is to combine the laser beam height measurement with the vision system. In this way, the volume can be calculated by multiplying the area by a medium pad height measurement. This method has poor repeatability, and sometimes can detect pad defects that may occur at the end of the pad, but it cannot identify irregularities in the brick-shaped solder.

2.4 Automatic three-dimensional online sample detection equipment Compared with the detection system built into the printing machine, the automatic online sample detection equipment has two main advantages. First of all, because this equipment is an independent system, it can be tested without using the hardware of the printing machine without stopping the machine. Secondly, the measurement performance of the sample testing equipment enables you to obtain accurate and repeatable measurement results.

The online testing equipment after printing cannot measure every pad on each printed board. In order to collect SPC data, this equipment uses effective statistical techniques to detect many key problems in the field operations on the board. A study conducted by a process development engineer at IBM proved that the use of sample inspection equipment for BGA pads was more than enough. But there is still the possibility of accidental defects. In fact, the defect rate is much lower than the last detected defect rate. The commonly used sample testing equipment after printing is designed online, installed on the conveyor belt, and immediately behind the screen printing machine. It continuously detects the samples (usually fine gap or BGA) that the user requires in the printing process. The inspection equipment can compare the actual pad measurement value with the preset parameters and notify the operator when the solder paste mark deviates from the predetermined range.

The sample detection equipment is different from the laser tabletop and whole printed board scanning equipment. It uses a photosensitive device equipped with a detector, which can continuously take a quick print picture of the target solder paste mark. This image can create a high-resolution appearance of the detection area. First find the sum of all height data and multiply it by the known image area to calculate the volume of solder paste.

Compared with the whole printed board scanning equipment, the sample testing equipment has several advantages. First, it can be preset and programmed within a few minutes, and the entire printed board scanning device may take several hours to preset. Once set up, trained personnel can monitor sample testing equipment without the need for engineering skills. The collected data is automatically displayed on the monitor for the operator to view, and stored in a standard format for further analysis. Second, the number of test samples can be adjusted according to the matching of the test speed and the production line speed, so that it does not take too much time in the test. Finally, the lamp technology is configured to accurately measure the location of defects.

The disadvantage is that because the sample testing equipment cannot detect every position on each printed board, the chance of defect detection is high. The defined accidental defects will not appear in the regular graphics; when using the sample detection technology, there may still be a phenomenon of missed detection.

2.5 Automatic online whole printed board inspection equipment High-speed whole printed board inspection equipment is the most high-end equipment in this field, which can evaluate every inspection point on every printed board. This kind of equipment is expensive, but it is very fast and can inspect the entire printed board running on the production line.

The entire printed board inspection equipment uses a laser beam to scan the entire printed board on the production line, collect all the measurement data of each pad, and compare the actual measurement value with the qualified limit value that must be set. This equipment can inspect various types of imprints, including incidental defects, such as; missing pads caused by clogged template openings. The full scan also shows the imprint of the solder paste deposition pattern, including collapse, depressions, and solder bumps.

The main advantages of the entire printed board inspection equipment are: it can actually indicate the location of each printing defect, and can collect the actual height, area and volume data of each pad on the board. For defects with high potential cost or high unit cost, the entire printed board inspection is more applicable. Printed circuit boards used in the automotive, military or aerospace fields must meet high-reliability technical requirements, often requiring 100% inspection.

2.6 Automatic Optical Inspection (AOI) Equipment Automatic optical inspection equipment is currently the only equipment that can operate in parallel with the assembly line to detect the quality of solder paste deposition in parallel. It can detect more than 100,000 components per hour, which means that the online AOI device can detect 100% of the deposition points on the board. This device uses image analysis software, measurement components, edge value technology to confirm its value and polarity, and to ensure placement accuracy. Standard CAD and Gerber file programming is applied. Statistical process control (SPC) software tools were also applied, and a database was established, and a network connection was established with the rework station. It has many similar functions with automatic online PCB inspection equipment. Its most notable feature is its wide range of uses. It can not only check the printing quality, but also the quality of other processes, such as the accuracy of the placement machine.

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