3D detection and detection equipment after the hot

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This paper briefly discusses the detection methods used after solder paste deposition, compares the two-dimensional detection technology with the three-dimensional detection technology, and focuses on the advantages of the three-dimensional detection technology and the technical performance of various detection equipment through the comparison. Keywords: 3D detection; Solder paste deposition; With the in-depth development of PCB hybrid technology with higher density, smaller size and more complex electronic assembly, it has become a history to detect the quality after printing with the naked eye. Although the process equipment is more advanced, it is still emphasized to strictly control the quality of PCB, because 70% of the defects generated in the electronic assembly process originate from the solder paste printing process, especially when depositing components with fine spacing. In addition, defects such as missing marks, too much or too little solder produced in the process of depositing solder paste will bring bridging, short circuit and tombstone phenomena to the subsequent process (component mounting), so that the quality and reliability of the final product can not be guaranteed. Therefore, people pay more and more attention to solder paste detection after printing. At present, the manufacturer of surface assembly testing equipment provides several different post printing testing methods and various solder paste deposition testing equipment, ranging from the manual and offline testing equipment with a relatively low price and a low stress relationship of tr=-f/A to the high-end and high-speed testing equipment of 100000 US dollars. After fully understanding and comparing the advantages and disadvantages between two-dimensional testing equipment and three-dimensional testing equipment, offline testing equipment and testing equipment, sample testing and whole board testing, you should choose the applicable solder paste testing equipment for your own assembly line

1 testing increases costs

experts in the electronics industry agree that welding defects are caused by poor solder paste printing. Therefore, improving the printing quality or reducing the number of defective circuit boards entering the next process will help to improve the final quality, and the purpose of reducing costs can be achieved by reducing the amount of rework and reducing the scrap rate

1.1 find defects as soon as possible. After roughly counting the cost of repairing or scrapping the defective circuit boards detected in the test stage, it is found that controlling the printing process will bring many significant advantages to solder paste printing. Any kind of defect will consume money, and post printing inspection can only help reduce the number of defects, but can not completely eliminate defects. However, in fact, before the cost of circuit boards has not increased, testing in the processing process to identify defects as soon as possible can indeed reduce the additional cost caused by defects and improve the one-time inspection pass rate, which plays a great role in the basic production line. 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 testing is approximately $30. Regardless of the dollar parity, this relationship remains unchanged. Therefore, the early detection of defects in the processing process is not a trouble for the back door and tail brace pressure testing machine, which is mainly used in the automotive industry, but a good opportunity to save costs

1.2 improve reliability. The reliability of the assembled circuit board can be improved by adding a detection process in the printing process. There are two reasons: first, detection reduces the amount of repair, in addition; The repaired solder joint is easy to be damaged, and it is easier to crack than the qualified solder joint. Secondly, insufficient solder paste may also form solder joints that are easy to crack. Although they can pass the test at that time, they will also break in the future. Although the circuit board with these two problems can pass the final test, it is easy to fail during operation. These problems in the finished products will make users dissatisfied or make the warranty fee very high

1.3 necessary detection due to the requirements of denser lead spacing, smaller ball grid array solder balls and more accurate printing clearance, more PCB assembly manufacturers have added solder paste detection procedures in the assembly process. In some contract assembly plants, testing steps are added according to the requirements of users. According to the technical requirements, when the solder paste must be locked with a torsion spring for detection, the next step is to determine which detection equipment is most suitable for a specific application

2 select testing equipment

solder paste testing equipment can be selected from several manufacturers. Each manufacturer provides testing equipment with different speed, performance and price, but they 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 testing and desktop measurement tools; Automatic testing equipment, including the sample testing system built into the printing machine and the whole printed board scanning and testing equipment

2.1 visual inspection has been used for a long time. This simple method is enough to determine whether the sample is qualified or unqualified. Until the advent of smaller devices, higher lead numbers and finer spacing components, this method is not applicable. Use a luminous magnifying glass or a calibrated microscope, and the trained operator should check the sample printed board and determine when the calibration operation is required. Visual inspection is the lowest cost method in process monitoring, and the cost of its correction operation is the most reasonable in the printing process. However, the visual detection method has people's subjective consciousness: the detection results between operators are different. The visual inspection tools have not been calibrated and cannot give the data required for process control. From the actual situation, with the continuous popularization of ultra-fine spacing and BGA devices, the visual inspection method is no longer used, because it is no longer an effective method to monitor the printing process

2.2 artificial laser detection in order to reduce defects, the next step is to use artificial desktop cabin machine detection equipment. These measuring tools use non-contact laser technology to measure the height of solder paste and record. Through a little training for operators, these devices can usually produce consistent results, and the test results will not be different due to different operators

laser three-dimensional detection 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 point on the pad irradiated by the laser beam, usually the center of the pad. This type of tester can also multiply the length of the pad by the width of the pad to obtain the area measurement. Then, multiply the area measurement by the height measurement to calculate the volume measurement. The basic process control of offline testing equipment is to unload the sample circuit board from the production line, carry out 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 detection equipment cannot immediately detect the defects

2.3 automatic detection system built into the printing machine several printing machine manufacturers have launched built-in two-dimensional and three-dimensional solder paste detection systems or fault finding systems. However, the detection system built in the printing machine shares hardware with the silk printing machine. Because the silk printing machine must be in a suspended state to detect, the printing speed is reduced. Most built-in inspection systems use photographic vision technology to evaluate solder paste area, coverage and calibration. In addition to detecting the mark, this camera can also be used to check the wire to see if the opening of the template is blocked and there is too much solder paste

some printing machine manufacturers have added volume measurement function to the printing machine by combining 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. Sometimes pad defects that may occur at the end of the pad can be detected, but the irregularity of brick solder cannot be recognized

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

the post printing inspection equipment cannot measure each pad on each printed board. In order to collect SPC data, this equipment can detect the key problems of field operation on many boards by using effective statistical technology. A study conducted by a process development engineer of IBM company has proved that it is more than enough to use sample detection equipment for BGA pads. However, there is still the possibility of accidental defects. In fact, the defect rate is much lower than the undetected defect rate. The commonly used sample detection equipment after printing is designed to be installed on the conveyor belt and immediately behind the silk printing machine to continuously detect the samples (usually fine gap or BGA) required by users in the printing process. The testing equipment can compare the actual pad measurement value with the preset parameters, and inform the operator when the solder paste mark deviates from the predetermined range

the sample detection equipment is different from the laser desktop and the whole printed board scanning equipment. It uses a photosensitive device equipped with a detector, which can continuously take fast printing pictures of the target solder paste imprint. This image can establish a high-resolution appearance map of the detection area. Calculate the sum of all height data first, and multiply it by the known image area to calculate the solder paste volume

compared with the whole PCB scanning equipment, the sample detection equipment has several advantages. First, it can be preset and programmed in a few minutes, while the whole PCB scanning device may take several hours to preset. After setting, the trained personnel can monitor the sample detection equipment without engineering skills. The collected data is automatically displayed on the monitor for operators to watch and stored in a standard format for further analysis. Secondly, the number of test samples can be adjusted according to the matching between the test speed and the production line speed, so that too much time will not be spent in the test. Finally, the configured lamp technology can accurately measure the position with defect tendency

its disadvantage is that because the sample detection equipment cannot detect every position on each printed board, there is a high chance of defects missing detection. There will be no defined accidental defects in regular graphics; When sample detection technology is used, there may still be missed detection

2.5 automatic whole printed board testing equipment high speed whole printed board testing equipment is the most advanced equipment in this field, which can evaluate each test point on each printed board. This kind of equipment is expensive, but it is very fast and can detect the whole printed board running on the production line

the whole printed board inspection equipment uses laser beam to scan the whole printed board on the production line one by one, collect all the measurement data of each pad, and compare the actual measurement value with the qualified limit value to be set. This kind of equipment can test various types of marks, including accidental defects, such as; Pad leakage caused by blockage of template opening. Full scan can also show the imprint of solder paste deposition pattern, including collapse, depression and solder bulge

inspection of the whole printed board Napoleon Bonaparte, the most important founder of France 1

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