Wetting Dynamics Of Solders In SMT

The electronic industry is actively searching for Pb-free solders due to environmental concerns over Pb-containing solders. Lacking reliability data of wetting, many electronic companies will be reluctant to adopt Pb-free solders in the advanced products. At the same time, because solder wetting are thought to have a role in higher density and smaller cell and narrower spacing interval in microelectronics packaging, we did review our understanding of potential reliability issues of solder wetting and structure-property relationship. This dissertation presents a detailed analysis of reflow curves, based upon preheat factor and heat factor and cooling factor, in which is associated with the reliability of spot weld and is obtained optimizing reflow curves. The various analysis techniques, included metallographic microscopy (OM) and scanning electron microscopy (SEM) and electron spectroscopy (EDX) analysis, were employed. From these techniques, the whole interface course and mechanism of action are identified. By means of optical lever method and dynamic interferometry measurement, we conclude that interfacial stress in reflow soldering is measured at the first time. The results we obtained demonstrate that this technique will contribute to solder wetting technique in microelectronics packaging.This dissertation outlines the progress in solder wetting and research methods and main theory. Based on the above work, the purpose and significance of this dissertation have been pointed out. The program and the key problem of this dissertation have been reviewed too. Then, an improved analysis methodology to predict solder interconnect wetting was developed to reveal the causes of poor wetting during pip chip assembly and to provide solutions. The contact angle relaxation of spreading over time was measured in specially designed experimental setup for model development. As a result of experiment and model evaluation, the activity ranges of different solder were developed. An evaluation criterion of dynamic extension experiment can be devided into four sequent stages: solvent effusion and volatilization, solder paste pattern shrinkage, solder paste and solder ball stabilization, tin exudation. Reflow process parameter factors included preheat factor and heat factor and cooling factor. This dissertation showed that the growth of intermetallic compound (IMC) was formed faster at high heat factor, compared with the growth of IMC at low heat factor. With the aim to provide good wetting reaction, heat factor determining the change of the total volume of IMC was no more than 800-1000sec?°C. Likewise, there were significant difference among different preheat factor. Too big preheat factor can bring coked surface. The solder grains grew larger but fewer with cooling factor, indicating that a similar annealing reaction had occurred among them. A shear test method for solder joint in microelectronic packaging was proposed to analyse morphology and composition, by which three reflow factors can be factorively suppressed. In all samples, the fracture mainly occurred in the bulk solder. However, for the joint strengthen become lower, the fracture occurred in interfacial area between IMC and substrates. The best reflow profile of 63Sn37Pb and Sn3.5Ag0.5Cu were supposed, which is developed to use a new reflow process in practice. An optical system on the basis of substrate curvature technique had been constructed for real-time monitoring the annealing stress in the multilayer films. When a zero creep state reached, the measuring results was:γint of Ag(111)/Ni(111) at 450°C is 0.63 J/m2. It was the first time that optical lever method was used to provide instantaneous quantitative information on wettability of various configurations of components. This instrument had been used for the examination of reactive wetting on different combinations of solder alloys and substrates. 63Sn37Pb solder and Sn3.5Ag0.5Cu solder were kept in molten condition on Cu thin film/Si pad for different time periods ranging from 90 min to 200 min to render the ultimate interfacial reaction. The key interfacial stress of solder was measured separately under this condition. A kinetic analysis of the soldering reaction accompanied by the reaction is given. In insulation work, IMC serve as fast diffusion and dissolution paths of Cu in the solder to feed the reaction. Reactive wetting has an important role in measuring the interfacial stress on reflow soldering. Furthermore, the experiment was used MEMS (microelectromechanical systems) and demonstrate its capabilities, in which strobo scopic imaging, micro-interferometer, computer microvision were applied. It can be developed for measuring in-plane-rigid-body motions, surface shapes, out-of-plane motions and deformations of microstructures. Hariharan five-step phase-shift interferometry (PSI) algorithm and unwrapping algorithms were adoptted to measure out-of-plane motions. Interference of equal thickness was applied to analyse motions too. Wetting curves of the wetting force vs. time were obtained and used to extracted values for wetting time and maximum wetting force by MEMS equipment at the first time. In addition to an equation of wetting adhesion, the results was to evaluate the relative wetting performance among several selected lead-free alloys, which were considered to be the most viable candidates based on a combined performance properties. The performance properties of solder alloys under this study are in subsequence: 63Sn37Pb, Sn3.5Ag0.5Cu, Sn3.5Ag.