Optimization and Control of Reflow Soldering Profile in SMT Line
|School||Shanghai Jiaotong University|
|Course||Mechanical and Electronic Engineering|
|Keywords||Surface mount technology heating factor control method soldering reflow profile thermal time constant linear regression|
The rapid development of the electronics and information manufacturing industry in China drives the improvements of quality and efficiency of Surface Mount Technology (SMT) production line. As the crucial process in SMT line, the perfection of the quality and efficiency of a reflow soldering process relies on the control of the reflow profile. With the widely use of lead-free solder paste, the quality problem caused by inappropriate reflow profiles attracts more and more attention.To solve the above-mentioned problem, this dissertation focuses on the optimization and control of the reflow soldering process, the reflow profile in particular, in SMT line. Herein, experimental verification and comprehensive analysis from the view of material science and heat transfer are conducted as research tools. Thereby, the heating factor, as an optimization parameter of a reflow profile are introduced, the method for controlling the optimized profile is developed, and the strategy for controlling solder joints reliability is proposed. The whole process of optimizing and controlling the reflow profile is defined as Heating Factor Control Method. With this method, the high reliability and efficiency of the reflow process can be realized and ensured.The Heating Factor Control Method is developed in the following way. Firstly, by metallurgical analysis, it is revealed that the formation of Intermetallic Compound (IMC) between solder alloy particles and substrate pad is the essentials of a reflow process. And the IMC thickness is a key factor to the reliability of the solder joint. In view of the fact that the IMC thickness increases linearly with the heating factor, the reliability of reflow soldering can be basically controlled by confining the heating factor of the solder joints in a PCBA within a certain range. Since the formation of IMC is determined by fluxing performance, while the latter depends on the thermal input in preheating phase, which is closely related to the shape and length of the rising segment of the reflow profile. Accordingly, a satisfactory reflow profile, whose shape primarily conforms to what the solder paste manufacturer requires, is a must to ensure the high reliability. The solder joint reliability can be further enhanced by its fine grain structure with rapid cooling rate. Hence, the reflow profile is optimized from three respects: IMC formation, fluxing performance and solder joint microstructure.Secondly, to clarify the concept of the heating factor, this dissertation points out that the heating factor involves both heating and cooling phases. It consists of not only the heat input from thermal source within the heating part but also the holding energy within the cooling part at the soldering interface during liquid region. It demonstrates that the heating factor represents the total amount of heat absorbed by the liquid alloy component at the soldering interface between solder alloy and pad, but not the total heat input from thermal source at soldering interface.Thirdly, the acquisition of the heating factor value is explored from three aspects, including numerical heat transfer computation, temperature data collection and image processing, as the prerequisite of controlling heating factor. The attempt to acquire precise heating factor by numerical heat transfer computation is abandoned because of its complexity. So, a temperature data collection system, including the design of hardware and the program of software, is primarily developed. Together with EXCEL, the value of heating factor can be precisely acquired with the temperature data. Meanwhile, a MATLAB program is developed to compute the heating factor by image processing method with the image of a reflow profile attained in SMT line.Fourthly, with deep analysis of the heat transfer process and the temperature response of the solder joints in the reflow cycle, a method to get the inverse control solution of a reflow profile at any convectional oven is proposed and verified by experiments. Thus it is not a problem to make the shape of a reflow profile generally conform to what the solder paste manufacturer requires. Only simple computation and proper estimation is needed to acquire the solution of the reflow profile with this method. And the attained temperature profiles of the solder joints in a PCBA with its oven settings is taken as the reference for further control of the optimal range of the heating factor. Patent has been applied for this method.Fifthly, control of the heating factor is explored by experiments with above mentioned reference profile and oven settings. A strategy, making the heating factor of the cold point in the PCBA approach the lower limit of the optimal range, is proposed to control the optimal range of the heating factor. And the heating factor of the cold point is found to be linear with the temperature settings of all the top heat source of a reflow oven, which make the heating factor of the cold point be linearly controlled.Last but not least, optimization design and control plan of a reflow profile is synthesized and defined as Heating Factor Control Method. A comparison between Heating Factor Control Method and those based on the defect mechanism analysis is conducted, and the simplicity and universal application of the former method is fully demonstrated. In particular, Heating Factor Control Method can achieve high reliability of reflow soldering with rapid speed.