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AOI test technique and check position in PCB board

In the process of PCB copying, especially to copy some high precision circuit boards, testing is an essential step, and only tests can evaluate whether the PCB boards are qualified. As you all know, the most commonly used test equipment in the circuit board is the flying needle test machine and the test rack test. In fact, there is also an electronic test instrument called AOI. AOI is a new type of testing technology which has been developed in recent years, but it is developing rapidly. Many manufacturers have introduced AOI testing equipment. When the automatic detection, automatic scanning machine through the camera PCB, image acquisition, compare the parameters of qualified test in the solder joint and the database, through image processing, check out the defects of the PCB copy board, and through the display or sign the automatic defect display / marked dressing for the technical personnel.
1, implementation objectives: the implementation of AOI has the following two main objectives:
(1) final quality (End, quality). Monitor the final status of the product when it goes down the production line. This goal is preferred when production issues are clear, product mix is high, quantity and speed are critical factors. AOI is usually placed at the end of the production line. In this position, the device can produce a wide range of process control information.
(2) process tracking (Process, tracking). Use inspection equipment to monitor the production process. Typically include detailed defect classification and element placement offset information. This is a priority for manufacturers when product reliability is important, high-volume manufacturing is low, and supply of components is stable. This often requires placing the inspection equipment several locations on the production line, monitoring the actual production status in real time, and providing the necessary basis for the adjustment of the production process.
Although the AOI can be used in multiple locations on the production line, each location can detect specific defects, the AOI inspection device should be placed in a position that can identify and correct the most defects as soon as possible.
2, there are three check position is the main:
(1) after solder paste is printed. If the solder paste printing process meets the requirements, the number of defects found by ICT can be substantially reduced. Typical print defects include the following: lack of solder on the A. pad. Excessive soldering on the B. pad. C. solder butt welding disc does not coincide properly. Solder bridge between D. pads.
On ICT, the probability of defects relative to these conditions is directly proportional to the severity of the situation. A slight lack of tin rarely causes defects, while severe conditions, such as the fundamental Wuxi, almost always cause defects in the ICT. Insufficient soldering may be a cause of component loss or open solder joints. Still, deciding where to place AOI requires recognizing that component loss may have occurred for other reasons, and that these reasons must be included in the review plan. This location check most directly supports process tracking and characterization. Quantitative process control data at this stage include printing offset and solder volume information, as well as qualitative information about printed solder.
(2) before reflow. Inspection is done after the component is attached to the solder paste inside the board and the PCB circuit board is sent into the reflow furnace. This is typically a placement check machine location, as most defects from solder paste printing and machine bonding can be found here. Quantitative process control information generated at this location provides information on calibration of high-speed chip machines and dense pitch element mount equipment. This information can be used to modify component placement or to indicate that the mounter needs calibration. This location check satisfies the goal of process tracking.
(3) after reflow soldering. The final step in the SMT process is examined, which is the most popular option for AOI at this time, because this position can detect full assembly errors. The reflow test provides a high level of safety because it identifies errors caused by solder paste printing, component mount and reflow processes.