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1 Mechanism of high-temperature aging When electronic products are manufactured, there are two types of quality problems caused by irrational design, raw materials, or technical measures. The first type is that the performance parameters of the products do not meet the standards, and the products produced do not meet the requirements. Requirements; the second category is potential defects, such defects can not be found using the general test methods, but need to be gradually exposed during use, such as silicon surface contamination, tissue instability, weld voids, chip and shell heat Resistant matching and so on. Generally, this kind of defect needs to operate for a thousand hours or so when the component is working at the rated power and normal operating temperature to be fully activated (exposed). Obviously, it is unrealistic to test each component for one thousand hours, so it is necessary to apply thermal stress and bias, such as high temperature power stress test, to accelerate the early exposure of such defects. That is, applying thermal, electrical, mechanical, or various external stresses to electronic products, simulating a harsh working environment, eliminating processing stress and residual solvents and other substances, allowing latent failures to occur in advance, and allowing products to pass through the early stage of bath failure as soon as possible. Stage, enter the high stable period. The failure curve of electronic products is shown in Figure 1.
After the aging, the electrical parameters are measured, and the components that have failed or changed are screened to eliminate the early failure of the product before normal use. In order to improve the reliability of electronic products and prolong the service life of products, the company conducts necessary assessment on the stability so as to eliminate potential "individuals" (components) that have "early death" defects and ensure excellent quality and life expectancy of the whole machine. The process is the principle of high temperature aging.
2 Space structure and heat insulation measures of high temperature aging room 2.1 Space layout of aging room According to the requirements of high temperature aging of electronic products and the actual conditions of our company, a factory building was renovated and renovated, with emphasis on space layout and thermal insulation design. The layout is shown in Figure 2. The room is divided into two parts. The outside is used as a control room and the control box is suspended on the wall of the control room. The inner room is a high-temperature aging room and is a closed space formed of a heat-insulating material. A steel keel ceiling is used at the top, and a movable panel is left at the corner of the ceiling so that maintenance personnel can enter the top for maintenance. The control line of the control room passes through the upper part of the ceiling and is then distributed to all parts of the aging chamber. The thermal insulation wall adopts a steel keel frame to ensure sufficient strength and rigidity. The thermal insulation wall is covered on both sides with a fireproof board, and the middle is filled with thermal insulation materials, such as rock wool, etc. (The thermal conductivity is about 0.04w·m-1·k-1 at 25°C. ). The door of the aging chamber is coated with double-sided aluminum-zinc steel plate, filled with thermal insulation material, and sealed with silicone rubber between the door frame and the door. The back wall sliding window and the front wall observation window adopt a double glass structure, which has good sealing and heat insulation effect, and is convenient for lighting and monitoring. Four fans are placed in the four corners of the wall of the aging chamber to allow indoor air circulation and uniform indoor air temperature. 2.2 Heat balance of aging chamber Calculate the heat required to increase the temperature in the aging chamber. * The heater is provided. The heater is made of stainless steel armoured structure. The heaters are connected by copper bars, fixed and secure. The outside is protected by galvanized iron mesh.
Irrespective of the ideal condition of heat loss, the heat required to set the aging temperature in the aging chamber is: q=(c1m1+c2m2)×(t1-t0)
C1 is the specific heat capacity of the aging indoor air (about 1.005 kj·kg-1·k-1, slightly different at different temperatures);
C2 is the average specific heat capacity of the product being aged (kj·kg-1·k-1);
M1 is the indoor air quality (kg);
M2 is the quality (kg) of the product being aged;
T1 is the set aging temperature (°C);
T0 is the initial ambient temperature of the aging chamber (°C);
In reality, sealing and thermal insulation cannot be ideal, so heat loss is unavoidable. Calculate the adiabatic coefficient of the whole system according to the structure of the aging chamber and the area of ​​the six surfaces of the room according to the different thermal conductivity μ(w·m-1·k-1) of the air and rock wool at the initial temperature and the highest setting temperature. ξ (m2·k·w-1), then calculate the actual amount of heat required for the entire system to reach the maximum set temperature within a certain period of time, so that the total theoretical power p of the heater can be calculated. Finally, the total actual power pt of the heater is calculated based on the system redundancy factor η. When customizing the heater, the voltage rating and connection of each heater must be considered, either delta connection, star connection, or star-triangle hybrid connection. Heater stainless steel heat sink to facilitate heat dissipation, prevent the heater from burning red.
3 temperature control system This control system uses the pid control device for temperature control. When the temperature of the aged electronic product collected by the temperature sensor deviates from the desired given value, the pid control device performs the ratio (p) based on the feedback deviation. Integral (i), differential (d) operation, output an appropriate control signal to the actuator (heater), prompting the measurement value to return to a given value, to achieve the purpose of automatic temperature control.
3.1 Control mathematical model The control object is a first-order system with hysteresis links. The control system uses closed-loop delay output pid adjustment. The pid control technology is mature and flexible.
Continuously adjusted pid differential equation is u=kp(e+)+u0
For microcomputer control, to make the discrete control form close to the continuous control form, the sampling period must be sufficiently short. In this way, the differential equation describing the system regulation law can be changed to the difference equation, which facilitates the programming and realizes the digitalization of analog control. .
The pid difference equation is un=[en+·t+()]+u0
Un is the nth output volume u0 is the initial output volume en is the sensor's nth acquisition amount of the offset en-1 is the sensor's n-1 time's acquisition deviation is the proportional coefficient is the integration time is the differential Time 3.2 Controller Parameter Adjustment The proportional operation refers to the first-order differential quotient relationship between the output control quantity and the input quantity. The larger the scale factor setting value of the meter is (the smaller the proportional band δ is), the lower the control sensitivity is, and the smaller the setting value is, the higher the control sensitivity is. Increasing the proportional coefficient helps to reduce the static error and speed up the response of the system. However, if the proportional coefficient is too large, the system will produce a large overshoot, and even oscillate, deteriorating the stability. The purpose of the integral operation is to eliminate the static difference. As long as the deviation exists, the integral action moves the control amount to the direction in which the deviation is eliminated. The integration time is a unit that indicates the strength of the integral action. Increasing the integration time is beneficial to reducing the overshoot and reducing the oscillation, so that the system tends to be stable, but the elimination of the system's static difference is slowed down. The shorter the integration time set by the meter, the stronger the integral action. The proportional action and integral action are corrective actions for the control result, and the response is slower. The differential action is supplemented by eliminating its disadvantages. The differential action corrects the output according to the speed at which the deviation occurs, allowing the control process to return to its original state of control as quickly as possible. The differential time is the unit of the differential action strength, and the longer the differential time set by the meter, the differential correction is used. The stronger, is conducive to accelerating the response of the system, reducing the overshoot, increasing stability, but reducing the system's ability to suppress disturbances, making the system too sensitive to interference. In the actual debugging process, several aspects must be taken into consideration. After repeated debugging, the controller is in the best condition.
3.2 Structure of temperature control system The temperature sensor collects the temperature of the aging chamber, and then passes it to the controller. The controller compares it with the internal set value and adjusts the thyristor conduction angle according to the deviation value output control quantity. The change, which is to control the load current changes, so as to achieve the purpose of automatic temperature control in the form of closed-loop control. In addition, the controller has also set an upper limit temperature trip protection, so that when the PID controller fails, it can play a dual protection. The controller is connected to a remote computer through a standard serial communication interface. The background computer can call the field data of the controller to set the internal data of the controller and print the real-time temperature curve.
4 Concluding remarks After two years of actual operation, the stability and dynamic response of the system all meet the requirements for use. The temperature control accuracy is within plus or minus 1 degree.
The principle of high temperature aging of electronic products
The principle of high-temperature aging of electronic products With the development of electronic technology, the degree of integration of electronic products has become higher and higher, the structure has become more and more subtle, more and more processes, and manufacturing processes have become more and more complex, which will produce in the manufacturing process. Latent defects. For a good electronic product, it not only requires a higher performance index, but also has a higher stability. The stability of electronic products depends on the rationality of the design, the performance of the components, and the manufacturing process of the complete machine. At present, high-temperature aging technology is widely used at home and abroad to improve the stability and reliability of electronic products. Through high-temperature aging, the potential defects in the production process of defects, welding, and assembly of components can be exposed in advance to ensure that products leaving the factory can be Can afford the test of time.