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SMT technology is an electronic assembly technology with a very high degree of automation. With the development of miniaturization of electronic products, a fundamental revolution has taken place in the SMT production process. At present, in the production and assembly of SMT processing plants, there are 4types of SMT production, single-sided SMT, single-sided SMT assembly hybrid with through-hole soldering, double-sided SMT, double-sided SMT assembly hybrid with through-hole soldering. This article will continue the introduction of "Quality and Reliability of SMT Chip Processing and Production".
Controlling of reflow soldering is generally divided into three stages: preheating stage, heat preservation stage and reflow stage. Before reflow soldering, it is necessary to set the temperature of these three stages, and determine the temperature curve of each production, so as to ensure the high reliability of each production. According to the requirements of the reflow soldering equipment, combined with the specifications and parameters of the solder paste, develop more ideal temperature profile.
The purpose of preheating is to heat the printed circuit board from room temperature to the desired temperature for activating the solder paste, but the heating rate must be controlled within an appropriate range. If it is too fast, thermal shock will occur to the circuit board and components. For example, some components are sensitive to internal stress, if the external temperature of the component rises too fast, it may be damaged.
If it is too slow, the solvent in the solder paste will not evaporate sufficiently and affect the quality of soldering. After analysis and repeated practice, it is believed that the heating rate of 3℃/s in the preheating zone is the most appropriate, and the preheating effect is best when the temperature rises to 180℃. In this way, a good preheating effect can be obtained, and it can also effectively limit boiling and splashing, and prevent the formation of small solder beads.
The heat preservation stage refers to the area where the temperature rises to the melting point of the solder paste, which generally occupies 33% to 50% of the heating channel. Provide enough time in this area for the temperature of larger components to catch up with smaller components. The main purpose of this stage is to stabilize the temperature of each component on the printed board, close to the melting point of the solder paste, and minimize the temperature difference. Make it easier to transition to the reflow stage; another purpose of this area is to activate the flux in the solder paste to make it fully volatilized, remove the oxide on the pad and the lead, and leave a clean surface where the solder can adhere. The applicable temperature range of the heat preservation stage is 120℃～180℃. If the temperature is set too high, the flux will not have enough time to activate, which will eventually affect the soldering effect.
The temperature of the reflow stage is set to the highest among the process, its function is to increase the temperature of the printed board from the active temperature to the peak temperature, so that the solder paste, the pad and the lead can form a eutectic alloy layer. For example, in tin-lead soldering, the alloy is Sn63/Pb37. The eutectic tin-lead alloy has a single melting point of 183°C, the typical temperature range of the reflow stage is 205°C to 220°C.
Setting the temperature too high in the reflow stage will cause warpage, delamination or burnout of the printed board, and will damage the integrity of the component. The ideal temperature setting is to minimize the area that exceeds the melting point of the solder. The printed boards sent to the conveyor belt of the reflow oven go through preheating, heat preservation, and reflow stages in sequence without interruption. It takes about 4 minutes. After natural cooling, the soldered printed boards can be obtained from the conveyor belt exit.
Increasing the convective thermal conductivity can increase the amount of heat transferred from the heat source to the PCB. As the heat transfer speed increases, the time for the PCB to reach the same final temperature can be shortened, thereby increasing the speed of the conveyor belt. If the speed of the conveyor belt is kept constant, the set temperature of the heating zone can be lowered, thereby reducing energy consumption.
Familiarity with the quality inspection process is the key to improving the quality and reliability of SMT products.
In order to ensure the normal operation of SMT processing and production, it is necessary to strengthen the quality inspection of the processed PCBA in each process to monitor its operating status. Therefore, it is necessary to set up quality control points after some key processes, so that quality problems in the previous process can be found and corrected in time. Prevent unqualified PCBA products from entering the next process and reduce the economic loss caused by quality defect to a minimum. The setting of the quality control point is related to the specific production.
"Reliability" is the ability of an electronic product to maintain its function within a specified time. One is the continuation of product quality with the time; the second is the anti-interference ability of the product; the third is the natural quality characteristics of the product. It is measured by high reliability, high stability, and high consistency. In surface assembly technology, the quality assurance of solder joints is the most important. This involves all aspects of the problem, we must work hard on quality management to minimize various factors that affect the quality of solder joints, so as to provide good quality assurance and improve the overall reliability of electronic products.
