Design of infrared thermal imaging detector in the

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The design of infrared thermal imaging detector in PCB testing

the manufacturing complexity of printed circuit boards (PCBs) has increased dramatically in the past 60 years, from simple boards based on 100% through-hole technology to complex multilayer circuit boards such as hybrid through-hole, surface mount, chip mount, etc. the rapid development of integration technology has brought rapid progress in PCB manufacturing technology, and also brought testing difficulties to manufacturers of mass-produced PCBs, Due to the higher density of integrated electrical components, the circuit integration is becoming more and more complex, which also makes PCB more prone to defects and failures

pcb was first detected manually, that is, visually detect the defects of 4 circuit boards, and measure the connection and disconnection of lines with simple instruments. With the requirements of high-density circuit wiring and high output, in the face of repeated, monotonous and strict detection tasks, the manual detection method can no longer meet the requirements of reliable parts, and AOI (automatic optical detection system) cannot identify whether electronic components have defects or hidden parts defects; ICT (circuit detection) usually mainly checks the open and short circuit conditions of a single component and each circuit network, which makes it impossible to test a complete circuit board; BGA can't detect special parts of the circuit board (such as the connection of hidden parts); Traditional testing methods are unable to detect "short circuit", "delamination", "component extrusion" and other problems. PCB infrared thermal imaging detector uses thermal imaging technology, which can be used for detection in the design process. Through the combination of servo system and infrared imaging technology, the infrared imaging detector as the core can be applied to any link in the production process, effectively reducing the PCB scrap rate

1 detection principle of infrared thermal imaging system

there are many kinds of PCB boards. Usually, copper or tin plating is used to form through holes, which are mainly connected with insulating materials and wires in the board. The tin plating of normal via is uniform, and the air in the hole is in direct contact with the metal, while the tin plating of defective via is uneven, and there is a fracture in the middle, that is, there is no open circuit defect of porosity, and the air in the hole is in contact with the metal and insulating materials. When the PCB board is actively heated by a stable heat source for a period of time, remove the heat source and immediately put it on the cooling table. Due to the different thermal conductivity of metal tin and insulating materials, the heat dissipation process of the air in the hole is different, and the heat dissipation process mainly includes conduction, convection and radiation. Therefore, the radiation power of normal vias and defective vias is different, and the heat dissipation speed of air in defective vias is less than that in normal vias. Infrared thermal imaging technology is to receive the infrared radiation of objects and convert such infrared signals into electrical signals. The further conversion into digital intelligent composite materials may help to reduce the cost of manufacturing composite materials. Through the comparison with the thermal imaging diagram of normal qualified PCB boards, through the analysis of corresponding software, we can judge whether the PCB boards are qualified or not

2 system structure design of infrared thermal detector

according to the detection principle of infrared thermal imaging detector, the automatic detection system of PCB detector is designed, which mainly includes electrical part, infrared thermal imaging part, system software part and workbench reverse server system. Its structure is shown in Figure L. The infrared imaging detection system is mainly composed of main control computer, motion control card, image card, I/O interface wrench, etc. the automatic control of the system is realized through PLC, which also ensures the accuracy and rapid response of motion

the thermal imaging part mainly includes heating system, infrared thermal imaging recorder and expansion function. The extended function is mainly used to send the executive commands of the main control computer to the corresponding electrical equipment. When the photoelectric sensor detects that the circuit board is transmitted in place, the signal is sent to the main control computer, and the computer sends a start command to open the constant temperature heating system; The circuit board is sent to the heating system by the conveyor belt and heated to emit thermal radiation. The heating temperature is set by the system and can be changed, but it should be the same as the heating temperature of qualified products for multiple thermal imaging; Then the circuit board will be sent to the infrared thermal imaging system, and the infrared thermal imaging recorder will collect and record the thermal imaging information of the PCB board. These information will be sent to the computer through the data line for the next step of processing

reverse server system is an automatic control system used to control a certain state of the controlled object so that it can automatically, continuously and accurately reproduce the change law of the input signal. Due to the continuous development of integration technology at present, the circuits on PCB board are relatively dense, and the requirements for circuit path quality are higher. In order to obtain the imaging information of more detailed parts of the circuit, it is required that the repeated positioning error of the X, y, A3 axis direction of the console must be controlled within a certain range. Here, the reverse servo motor and ball screw connection are used for accurate positioning. The ball screw has the characteristics of small clearance between the screw and the nut and high pitch accuracy, which can ensure the linear relationship between the rotation of the screw and the moving distance of the nut, and ensure the accuracy of the CNC machine tool. At the same time, the friction coefficient of the ball screw is small, and the required reverse service motor power is small. The hardware design of the reverse server system is shown in Figure 2

3 function and application field of infrared thermal imaging detector

3.1 main function of infrared thermal imaging detector

the main control computer is the core of the whole control system. The data collected by thermal imaging technology is sent here. These data are processed by specific software to reproduce the image of the tested PCB, and through analysis, it can judge whether the PCB is wrong, and finally send instructions to the actuator part of the detection system, Complete the functions of indication, alarm, recording, sorting, etc

the thermal imaging system is used to generate and obtain the thermal imaging information of the circuit board on the production line, detect the defects or defect prevention of PCB in the manufacturing process, control the process, and eliminate or reduce defects by changing the process,! Temporary control of specific production conditions and provide necessary basis for the adjustment of production process. Such as patch quality inspection, printing quality inspection, welding due to low threshold, low-level repeated construction quality inspection and multilayer ceramic substrate packaging quality inspection. This is a non-destructive testing technology, which can quickly locate defects

3.2 application occasions of infrared thermal imaging detector

infrared thermal imaging detector can be used in the following occasions at present:

(1)! Temporary test: monitor the qualified condition of the circuit board in the factory through the detection system. If unqualified products are found, they can be sorted out in time to prevent unqualified products from entering the market and affecting the reputation of the manufacturer

(2) optimize PCB Design: through the thermal imaging detection system, we can detect the process and basic methods of PCB correctly trying out the carton tensile testing machine for product testing. The thermal condition of the plate in the use process provides a reasonable basis for designers to analyze the heat source, solve the thermal problem, and reasonably arrange the period. The company will also continue to maintain research and development innovation to improve PCB design

(3) repair and rework: during the production process, some PCB boards have one or another problems and are returned. Most of these boards can be used normally through rework, but generally, it is difficult for workers to quickly find the problem. If infrared thermal imager is used, it can detect various problems such as short circuit and broken shoe through overheating imaging

4 conclusion

the experiment proves that the classification defects of components and the mounting displacement information of components can be seen through the infrared thermal imaging detector, and the product process can be observed through the monitor of the detection equipment. When placed on the production line, a wide range of process control information diagram can be generated to check the quality of PCB plates. Therefore, the infrared thermal imaging detection equipment will be widely used in the process of mass PCB production

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