3D inspection and testing equipment after printing
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This paper briefly discusses the detection method used after solder paste deposition, and compares the two-dimensional detection technology with the three-dimensional detection technology. The advantages of the three-dimensional detection technology and the technical performance of various testing equipments are mainly introduced through comparison. Keywords: three-dimensional inspection; solder paste deposition; printing inspection equipment With the deeper development of electronic assembly of higher density, smaller size, more complex PCB hybrid technology, the use of the naked eye to detect the quality after printing has become a history. Despite the more advanced process equipment, it is still important to strictly control the quality of the PCB, because 70% of the defects in the electronic assembly process are derived from the solder paste printing process, especially when depositing fine pitch components. In addition, defects such as missing marks, excessive or too little solder during deposition of solder paste can cause bridging, short-circuiting and tombstoning in subsequent processes (component mounting), resulting in quality and reliability of the final product. Sex is not guaranteed. To this end, people are paying more and more attention to solder paste inspection after printing. At present, surface mount inspection equipment manufacturers offer several different post-printing inspection methods and various solder paste deposition inspection equipment, from relatively inexpensive manual and off-line inspection equipment to high-end, high-speed online inspection equipment of $100,000. . Choose your own assembly line after fully understanding and weighing the advantages and disadvantages between comparing 2D inspection equipment and 3D inspection equipment, off-line inspection equipment and online inspection equipment, sample inspection and overall board inspection. Suitable solder paste inspection equipment.
1 detection increases costs
Electronics industry experts agree that soldering defects are caused by poor solder paste printing. Therefore, improving the print quality, or reducing the number of defective boards entering the next step, will help to improve the final quality, and reduce costs by reducing the amount of repair and reducing the scrap rate.
1.1 Early detection of defects After extensive statistics on the cost of repairing or scrapping defective boards detected during the online test phase, it was found that controlling the printing process brings many significant advantages to solder paste printing. . Any kind of defect will consume money, and post-printing testing can only help reduce the number of defects and not completely eliminate the defects. However, in fact, before the cost of the board is increased, testing in the process to obtain defects as early as possible can indeed reduce the additional cost caused by the defects, and improve the one-time inspection rate to the basic production line. Great effect. Cleaning the board for reuse is much less expensive than rework or retesting. The cost of repairing defects after printing is estimated at $0.45, and the cost of repairing the same defects after online testing is approximately $30. Regardless of the dollar price, this relationship remains unchanged. Therefore, it is not a trouble to find defects early in the 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, the inspection reduces the amount of repair, and in addition, the solder joint after repair is easy to damage and is more qualified. Solder joints are more susceptible to breakage. Secondly, the insufficient amount of solder paste may also form a solder joint that is easily broken. Although it can be tested online, it will break later. Boards with both of these problems can pass the final test, but they can easily fail during operation. These problems in the finished product can make the user unsatisfied or make the warranty high.
1.3 Necessary Inspections Due to the tighter lead spacing, smaller ball grid array solder balls, and more precise print gap requirements, more PCB assembly manufacturers have added solder paste inspection processes to the assembly process. In some contract assembly plants, the detection steps are added according to the requirements of the user. When solder paste inspection must be performed according to technical requirements, the next step is to determine which test equipment is best suited for the particular application.
2 Select the test equipment
Solder paste inspection equipment can be purchased from several manufacturers. Each manufacturer offers testing equipment with different speeds, performances and prices, but the results of solder paste height, volume and area are reported (Table 1). There are two main types of testing equipment after printing: manual offline testing equipment. , including visual inspection and benchtop measurement tools; automatic in-line inspection equipment, including sample inspection systems built into the press and monolithic printed board scanning inspection equipment.
2.1 Visual inspection The simple method of using visual inspection for a long time is sufficient to determine whether a sample is “qualified or unqualified”. Until today, the introduction of smaller devices, higher lead counts and finer pitch components has made this approach possible. Not applicable. Use a illuminating magnifying glass or a calibrated microscope to have the trained operator inspect the sample printed board and determine when corrective action is required. Visual inspection is one of the lowest cost methods in process monitoring, and the cost of correcting the operating steps in the printing process is the most reasonable. However, the visual inspection method has people's subjective consciousness: the test results between the operator and the operator are different. The visual inspection tool is not calibrated and does not provide the data needed for process control. From a practical point of view, with the increasing popularity of ultra-fine pitch and BGA devices, visual inspection methods are no longer used because it is no longer an effective way to monitor printing processes.
2.2 Artificial Laser Detection In order to reduce defects, the next step is to use an artificial desktop cabin to inspect the equipment. These measurement tools use non-contact laser technology to measure solder paste height and recording. By training the operator a little, these devices usually produce consistent results without different test results depending on the operator.
The laser three-dimensional inspection device uses a laser beam to establish a reference point for measurement. This device reports the height of a single solder paste measured at a point on the pad to which the laser beam is exposed, typically the center of the pad. This type of tester can also take the area measurement by multiplying the length of the pad by the width of the pad. The volume measurement can then be calculated by multiplying the area measurement by the height measurement. The basic process control used by off-line inspection equipment is to remove the sample board from the production line, perform standard measurements, and record the results of the inspection. New PC-based inspection equipment stores data and provides it to SPC (Statistical Process Control) for analysis. However, offline inspection equipment cannot detect defects immediately before printing other defective boards.
2.3 Automated inspection systems built into the press Several press manufacturers have introduced built-in 2D and 3D solder paste inspection systems or fault finding systems. However, the inspection system built into the printing press shares the hardware with the screen printing machine, and since the screen printing machine must be detected in a paused state, the printing speed is lowered. Most built-in inspection systems use photographic vision technology to evaluate solder paste area, coverage, and calibration. In addition to detecting the imprint, the camera can also be used to check the screen to see if the template opening is blocked and the solder paste is too much.
Some printer manufacturers have added volume measurement capabilities to the press by combining the laser beam height measurement with the vision system so that the area is multiplied by a medium pad height measurement to calculate the volume. This method is poorly repeatable, and sometimes it can detect pad defects that may occur at the end of the pad, but does not recognize the irregularity of the brick solder.
2.4 Automatic 3D Online Sample Testing Equipment Automatic online sample testing equipment has two main advantages over the built-in inspection system. First of all, since the device is a stand-alone system, it is possible to perform inspection without using the hardware of the printing press and without stopping the machine. Second, the measurement performance of the sample testing equipment allows you to obtain accurate, repeatable measurements.
The printed in-line inspection equipment cannot measure every pad on each printed board. To collect SPC data, this device uses effective statistical techniques to detect key problems in field operations on many boards. A study by a process development engineer at IBM proved to be more than adequate for using sample inspection equipment for BGA pads. However, there is still the possibility of accidental defects, in fact, the defect rate is much lower than the last detected defect rate. The sample testing equipment commonly used after printing is designed on-line, mounted on a conveyor belt, immediately behind the screen printing machine, and continuously detects the samples (usually a small gap or BGA) that the user requires to detect during the printing process. The inspection device compares the actual pad measurement to the preset parameters and notifies the operator when the solder paste is deviated from the pre-specified range.
The sample inspection equipment is different from the laser benchtop and monolithic printed board scanning equipment, which uses a photosensor equipped with a detector to continuously capture a fast print of the target solder paste imprint. This image creates a high-resolution appearance of the inspection area. The volume of solder paste can be calculated by first summing all the height data and multiplying it by the known image area.
Sample testing equipment has several advantages over a monolithic printed board scanning device. First, it can be preset and programmed in a matter of minutes, and the entire printed board scanning device can take several hours to preset. Once set up, trained personnel can monitor sample testing equipment without engineering skills. The collected data is automatically displayed on the monitor for viewing by the operator and stored in a standard format for further analysis. Secondly, the number of samples to be tested can be adjusted according to the matching between the detection speed and the line speed, so that it does not take too much time in the detection. Finally, the configured lamp technology allows accurate measurements of defective locations.
The disadvantage is that since the sample detecting device cannot detect each position on each printed board, the chance of defective defect detection is high. The defined accidental defects do not appear in the regular graphics; when using the sample detection technique, there is still the possibility of missed detection.
2.5 Automatic online monolithic printed board inspection equipment High-speed monolithic printed board inspection equipment is the most advanced equipment in this field, which can evaluate each inspection point on each printed board. This equipment is costly, but it is very fast and can detect the entire printed board that runs on the production line.
The monolithic printed board inspection equipment uses a laser beam to scan a single printed board on a production line, collects all measurement data for each pad, and compares the actual measured value with the required acceptable limit value. This device can inspect a variety of different types of imprints, including accidental defects such as pad smear caused by clogging of the stencil opening. The full scan also shows the imprint of the solder paste deposition pattern, including collapse, dents, and solder bumps.
The main advantages of the monolithic printed board inspection equipment are: the ability to actually indicate the location of each print defect and the ability to collect actual height, area and volume data for each pad on the board. For potentially costly defects or high unit costs, the entire printed board test is more suitable. Printed circuit boards used in automotive, military or aerospace applications must meet high reliability technical requirements and often require 100% inspection.
2.6 Automatic Optical Inspection (AOI) Equipment Automatic optical inspection equipment is currently the only equipment that can detect the quality of solder paste deposition in parallel with the assembly line. It can detect more than 100,000 components per hour, which means that online AOI equipment can detect 100% of the deposition points on the board. This device uses image analysis software, measurement components, edge vision technology to confirm its value and polarity, and to ensure placement accuracy. Standard CAD and Gerber file programming is applied. A statistical process control (SPC) software tool was also applied, and a database was established and networked with the rework station. It has many similar functions to the automatic online monolithic PCB inspection equipment. Its most remarkable feature is its versatility, which not only can detect the printing quality, but also the quality of other processes, such as the accuracy of the placement machine.