"Surface Precipitation and Growth of Bismuth Particles in Sn-Ag-Cu Solder Joints" 

John Wu, MSE PhD Candidate 

Advisor: Professor Nikhilesh Chawla

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Abstract:

Since the widespread use of Pb-free solders, many different types of elements have been added into solders to form alloys of ideal mechanical behavior. Alloying Bismuth (Bi) to the commonly seen Tin-Silver-Copper (SAC) solder systems have been especially of interest, as the addition of Bi is able to improve the overall mechanical properties and lower the melting point for more flexible assembly processes. It has been previously reported that low amounts of bismuth added to SAC305 solders would result in a precipitation effect on the polished surface. However, the growth rate and mechanisms of these precipitates at the early stages of surface exposure have not been carefully observed. The approach in attempt to explain early stage bismuth precipitation was achieved by re-polishing already precipitated surfaces. The act of re-exposure and resetting the precipitation cycle allowed calculation of precipitate growth. The results of precipitate size relation to time indicated that precipitation was strongly driven by surface diffusion. A parametric correlation between growth rate and growth mechanism is presented.