1. SMT assembly quality inspection
There are many inspection methods for the assembly quality of SMT products. The inspection methods currently used mainly include the following four types: manual visual inspection; electrical testing; automatic optical inspection and X-ray inspection.
(1) Manual visual inspection
Manual visual inspection in SMT assembly is to use human eyes or with the aid of a simple optical magnification system to perform manual visual inspection on solder paste printing quality and solder joint quality. At this stage, high-tech testing equipment is still being improved. During the period, it is still an effective method with little investment. Especially in the case of low technological level, imperfect technological equipment and testing equipment, it still plays an important role in improving the design, craftsmanship and improving the quality of circuit components. It is still widely used in the SMT assembly process.
The contents that can be manually inspected include: printed circuit board quality, glue dot quality, solder paste printing quality, patch quality, solder joint quality, and circuit board surface quality.
Manual visual inspection has major limitations, such as poor repeatability, unable to accurately and quantitatively reflect the problem, high labor intensity, not suitable for large-scale centralized inspection, unable to inspect invisible solder joints, and peeling off of the metal layer in the solder end of the lead. The formation of failed solder joints, etc. cannot be inspected, and the micro cracks on the surface of the components cannot be inspected. Nevertheless, it is still an effective inspection method commonly used by many electronic product manufacturers. It is of great significance for finding defects as soon as possible, eliminating them as soon as possible, preventing the recurrence of defects, optimizing the assembly process and improving the design of circuit components.
(2) Electrical test
Electrical testing is mainly to conduct contact testing on circuit components. In the assembly process of SMA, even if very strict process management is implemented, defects such as wrong polarity, solder bridging, virtual soldering, short circuit, etc. may occur. Therefore, contact inspection and testing of circuit components must be carried out after assembly and cleaning. The electrical characteristics and functions of the components. Among them, online testing (ICT) is the main contact testing technology.
On-line testing is to test the components on the SMA individually and one by one by using a fixture needle bed or flying probe on the SMA where the components are installed. The current online tester has a more comprehensive test function, which can detect almost all manufacturing faults (manufacturing faults) including assembly faults and device faults, and plays an extremely important role in the quality control of the SMT assembly process.
However, because the online testing technology is based on product testing and aims at the final inspection, it is still the main method of SMT assembly failure detection, and there are still large defects. The main problems are as follows:
① The cost of repairing is high. Production failures detected by online testing must be repaired by the repair workbench with repair equipment (as shown in Figure 1.5). Due to the high failure rate of SMT production, the difficult repair process, and the expensive equipment to repair, the repair cost is high. Statistics show that when the final inspection and repair methods are used in online testing, 15% to 25% of the manufacturing cost of SMT products is wasted in rework.
SMT online test repair diagram
② Low production efficiency. The speed of online testing is relatively slow, and it generally needs to be performed off the production line. Coupled with the low efficiency of the rework process, the overall production efficiency of the product is low.
③ The test cost is high. For an SMT assembly production system, multiple online testing equipment and needle beds are often required, and multiple sets of needle fixtures are required for different products, so the cost of testing is high.
④ The quality feedback information is lagging behind. The assembly quality information found through online testing, after statistical analysis, and then processed by manual feedback, has greatly lags behind the real-time requirements of assembly quality control. Especially for multi-variety, small-batch production or single-piece production of scientific research products, its quality information is difficult to play a real-time feedback control role.
Although the flying probe online tester can realize the non-offline and needleless fixture test, the above-mentioned basic problems still exist. To this end, in addition to online testing, the current advanced assembly equipment itself is equipped with some self-checking functions, such as the screen printing machine can be equipped with a solder paste thickness detector, the placement machine has an optical self-checking system for component positioning, and so on. At the same time, in the quality control of the production process, inspection points are often arranged in key process links such as solder paste printing and placement, and the process quality is randomly tested using optical inspection equipment or manual visual inspection. These equipment self-inspection functions and process sampling methods can form the self-inspection feedback correction function or the partial process feedback correction function of the partial process of the assembly equipment. With manual cooperation, the quality of each assembly process is strictly controlled, so as to eliminate the source of assembly failure. In each process, it has a very positive significance for assembly quality control.
(3) Automatic optical inspection
With the thinning of circuit patterns, the miniaturization of SMD and the high density of SMA, traditional manual visual inspection methods are difficult to meet the requirements of SMA. Therefore, in recent years, automatic optical inspection (AOI) technology has developed rapidly . This detection technology is characterized by the use of computer technology, high-speed image processing and recognition technology, automatic control technology, precision mechanical technology and optical technology. It is a product that integrates a variety of high-tech products, with automation, high-speed and high-resolution detection capabilities; it greatly reduces human labor intensity, improves the objectivity and accuracy of quality judgment, and reduces special tools and fixtures. Strong versatility; especially it reduces the time of testing and troubleshooting, and can provide real-time feedback information to the assembly system. Most AOI systems are also equipped with user-orderable software, which provides production process control (SPC) data. Therefore, AOI technology is now popularized as a tool for process control. There are several main types of AOI currently used in circuit assembly: bare board appearance inspection technology, circuit component appearance inspection technology, solder paste printing, etc. Assembly process quality inspection technology and laser/infrared solder joint inspection technology.
⑷X-ray inspection
With the emergence of BGA, CSP, flip chip and ultra-fine pitch devices and the continuous improvement of circuit assembly density, the above detection and testing techniques and methods are difficult to meet the requirements of assembly process quality control, such as solder short circuit, bridging, and insufficient solder , Detection of defects such as missing chips, poor component alignment, and quality detection when the solder joints are not visible on the bottom of the device. This problem can be solved by X-ray inspection technology. At present, the X-ray inspection systems used in circuit assembly are mainly online or offline, 2D or 3D, etc. In principle, X-ray tomography and tomographic synthesis technology are mainly used. The main feature of these inspection technologies is that they are intuitive, can accurately detect the type, size and location of defects, provide valuable reference data and true images for further analysis and repair, and improve the effect and speed of repair.
2. Raw material inspection
The measurement and control of the source of assembly failure is of greater significance than the post-mortem inspection and repair. For this reason, incoming inspection and quality control of raw materials are also very important tasks. There are many kinds of inspection items and inspection methods for raw materials such as components and PCBs. The most important one is the solderability inspection of components and PCBs. This is also the most commonly used inspection item. The inspection methods include edge dipping, solder ball, and lubrication. Wet weighing method, wet balance test, rotary dipping test, wave solder dipping test, etc. On many occasions, SMT assembly quality problems are often caused by defects in the physical and chemical properties of the materials used. Good inspection and quality control of raw materials are the basis for ensuring the quality of SMT assembly.
3. Component inspection
Component inspection is a non-contact inspection of the SMA. It does not touch, damage, or damage the inspection part, and can inspect the parts that cannot be detected by the contact test. The simplest method of component inspection is visual inspection, which can only make a rough observation of the appearance quality of SMA, and cannot perform comprehensive and accurate inspection of components. This detection method cannot check the internal structure of the component, so the scope of application is greatly restricted. With the thinning of PCB conductor patterns, the miniaturization of SMD and the high density of SMT components, AOI technology has developed rapidly. It is widely used in PCB appearance inspection, SMA appearance inspection and other component inspections. X-ray inspection, infrared inspection and ultrasonic inspection are also widely used in the quality inspection of solder joints.
