Morphology changes in solder joints--experimental evidence and physical understanding

The microstructures of solders in microelectronic components, lead containing as well as lead free, change over time. This is first of all due to comparatively high homologous temperatures which occur during reflow as well as during operation of the component. Moreover, because of the intrinsic thermal mismatch between the various materials that constitute the package substantial mechanical stresses and strains will arise and assist the process of microstructural change. In this paper we will, first, briefly provide experimental evidence for such microstructural change and how it relates to the solder bulk as well as to the various interfaces and passivation materials that are used when a solder joint is formed. Second, we will review state-of-the-art modeling techniques that allow to simulate such changes of microstructure provided certain material parameters are known. For this purpose we will set up all equations and then provide information on all the material parameters required for a numerical solution. We will present computer simulations based on this theoretical framework and study the influence of the material parameters on coarsening and aging and, in particular, examine the impact of mechanical stresses and strains. Finally we will address difficulties and challenges involved, experimental as well as modeling ones.