Evaluation of Aging Induced Microstructural Evolution in Lead Free Solders Using Scanning Probe Microscopy

The changes in solder mechanical behavior that occur during isothermal aging are a result of the evolution of the SAC solder microstructure. The most well-known and widely observed changes are coarsening of the Ag
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Sn and Cu
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Sn
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intermetallic compounds (IMCs) present in the eutectic regions between beta-Sn dendrites. Some researchers have proposed empirical models to describe the growth of these secondary phase particles as a function of aging temperature and aging time, and related this growth to mechanical property changes. In most of the prior investigations, microstructural changes during aging have been observed by comparing two different solder joints subjected to different aging conditions. Thus, the comparisons made were qualitative in nature (e.g. average IMC particle size) since the two microstructures were from different samples and could not be directly compared. In the current work, we have developed new procedures to capture solder microstructure while the sample is being heated inside an oven (in-situ aging study). The heating stage and scanning probe microscopy (SPM) facility within a nanoindentation system were utilized to achieve the goal. The sample was kept within the nanoindentation system and exposed to a high temperature aging using the heating stage present in the instrument. In particular, aging was performed at T = 125 °C for up to 26 hours, and the topography of the microstructure of a fixed region (10 × 10 microns) was continuously scanned using the SPM system and recorded after one hour time intervals. This process generated several images of the microstructure as the aging progressed. When placed together sequentially, these images have been used as frames to create an experimentally recorded movie of the microstructural evolution in SAC solder joints exposed to aging. Image analysis software was utilized to quantify microstructural changes (total area, number and average diameter of IMC particles, etc.) with respect to aging time.