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Study on the Creep Behavior of SnAgCuRE Solder Joints

Author: ChenZhiGang
Tutor: ShiYaoWu
School: Beijing University of Technology
Course: Materials Processing Engineering
Keywords: SnAgCu Rare earths Lead-free solder Creep Constitutive equation
CLC: TG407
Type: PhD thesis
Year: 2003
Downloads: 564
Quote: 49
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Abstract

The EU WEEE directive have been issued, clearly 2006 will be a total ban on the use of leaded solder. Facing the pressures of commercial competition, the national electronics manufacturers, and research institutions have carried out extensive research of lead-free solder. The new lead-free solder must not only have a good process performance is more important to have higher mechanical properties, in particular solder joint creep resistance, in order to meet the growing electronics industry reliability requirements, to ensure that its reliable operation in the service process. SnAgCu alloy has excellent wetting properties and mechanical properties, and is considered to be the most promising alternatives SnPb solder. In many SnAgCu alloy, Sn-3.8Ag-0.7Cu solder because of its higher creep resistance, which has been widespread concern. However, the trend of ultra fine pitch design of electronic circuit boards as well as the increasingly harsh service environment requires a new type of lead-free solder has a higher temperature creep resistance. Having a surface active elements added to the alloy the solder creep resistance can be effectively improved. This article is more promising in the field of lead-free solder Sn-3.8Ag-0.7Cu solder, to add a small amount of Ce-based mixed rare earth, rare earth creep properties of Sn-3.8Ag-0.7Cu alloy : creep rupture life test to study the impact of different rare earth content on creep rupture life of Sn-3.8Ag-0.7Cu brazed joints; creep strain test to determine the Sn-3.8Ag-0.7Cu and SnAgCu- The stress index 0.1RE, activation energy and constant A was established to reflect the constitutive relation between the steady-state creep rate, temperature and stress. The addition of rare earth also have an impact on other performance and microstructure of Sn-3.8Ag-0.7Cu solder. Sn-3.8Ag-0.7Cu containing different content SnAgCuRE solder alloy physical properties, process performance and conventional mechanical performance testing and comparison. Finally, the analysis of the impact of the addition of rare earth Sn-3.8Ag-0.7Cu solder alloy and its microstructure of brazed joints. , Does not appear that the misch metal added does not make the low-melting eutectic peak, SnAgCu alloy to produce low-melting eutectic composition in the alloys studied. Adding trace of misch SnAgCu solder melting temperature of the alloy has little effect, the liquidus temperature substantially between 220 ° C to 225 ° C. Physical the SnAgCuRE solder process and conventional mechanical performance tests show that the SnAgCu alloys of rare earth from the overall performance point of view, the amount should not exceed 1.0wt%. The creep rupture life tests showed trace mischmetal may be added to significantly improve the creep rupture life of the room temperature SnAgCu solder joint. Under ambient conditions, the rare earth content was 0.1 wt%, the highest creep rupture life, can be achieved in more than 7 times that of the Sn-3.8Ag-0.7Cu. When the rare earth content exceeding 0.1wt.%, The creep rupture life of the connector downward trend. Particularly when the rare earth content of 1.0wt.%, The creep of the connector off the LT; WP = 4 GT; cracking life is even lower than the Sn-3.8Ag-0.7Cu solder welding head substantially with SnPb solder joint creep rupture life flat. Therefore, SnAgCu solder alloy, the amount of rare earths to be added should not exceed 1.0wt%. Improve not is up to more than twice the SnAgCu solder joint creep rupture life at 65 ℃. Therefore, the optimum range of the added amount of rare earth is between 0.05wt.%-0.25wt.%, Rare earth-doped difficult exceeds 1.0wt.%. Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE, alloy brazed joints creep parameters (stress exponent and creep activation energy) were determined; constant A is calculated. To arrive at a description of steady-state creep rate and the stress and temperature of the constitutive equations. Apparent creep activation energy of the correction of the test resulting Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE, brazed joints, draw a real creep activation energy. The results show that, under conditions of low stress, the Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE, brazed joints true creep activation energy for self-diffusion activation energy with the lattice of matrix Sn close so that its steady state creep rate under low stress by lattice self-diffusion rate control; The true creep activation of the Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE, brazed joints with matrix Sn dislocation under high stress conditions, pipe diffusion activation energy close , so that its steady state creep rate under high stress by dislocation pipe diffusion rate control. Therefore, by summing at a low stress, Sn-3.8Ag-0.7Cu and of SnAgCu-0.1RE soldering head, can be drawn: Optimization stress index were 6.9 and 8.2, true creep activation energy of self-diffusion and lattice activation energy close; under high stress, Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE, brazed joint optimization of stress index of 11.6 and 14.6, respectively, the real creep activation energy of dislocation pipe diffusion activation energy close to. Furthermore, Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE, brazed joints the master creep deformation mechanism the dislocation climb process. So as to arrive at the test temperature, and under stress conditions, the creep deformation of the Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE, brazed joints is controlled by dislocation creep mechanism. Specifically, the creep deformation dislocation glide and dislocation climb a result of the role and the main control mechanism for controlling the steady-state creep rate of dislocation climb. Under low stress, dislocation climb mainly through the matrix Sn lattice self-diffusion process control; under high stress, dislocation climb by dislocation pipe diffusion process control. Discussed the formation mechanism of the microstructure the SnAgCuRE solder alloy and their joints. Through the analysis of the interaction between the elements in the alloy, as well as the corresponding binary phase diagram, in SnAgCuRE alloy exists Ce-Sn eutectic reaction, La-Sn peritectic reaction, as well as Sn-Ag, Sn-Cu Total eutectic reaction. SnAgCuRE brazed joints microstructure SnAgCuRE of solder alloy microstructure, which is caused due to the cooling rate differences, and the Cu substrate to the dissolution of the solder. SnAgCu solder joint head of Rare Earth in creep resistance, the following factors: 1) the boundaries of dendrite formation RE-rich phase of the network will reduce dendrite interfacial energy sector, the activation energy increased dislocation climb, which reduce the dislocation climb rate. 2) the rare earth, can form a network structure at the grain boundaries, can be had barrier LT; WP = 5 GT; hinder the diffusion of atoms, inhibit grain growth and movement. 3) the addition of Ce, increasing the number of branches at the grain boundaries and the second phase particles, play a grain boundary strengthening effect. Rare earth elements in solid solution is small, most of the enrichment in the forefront of the solid-liquid interface in the liquid phase boundary layer, thus hindering Ag, Cu atoms through the interface

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