Session: 10-01 Interactive Presentations

Paper Number: 100287

100287 - Effect of Isothermal Aging on the Evolution of Local Microscale Mechanical Properties Within Sac305 and Sac+bi Solders 

Mechanical characterization of Sn-Ag-Cu (SAC) solders is important for understanding the mechanical response and reliability of lead free electronic assemblies.  While SAC305 is often the standard alloy of choice in industry, adding of additional elements to form SAC+X solders is being considered to obtain improved mechanical properties and mitigate aging effects.  Prolonged exposure to high temperature can drastically affect the mechanical properties of SAC solder alloys by coarsening their microstructures.  However, it has been observed that adding Bismuth (Bi) to SAC alloys can reduce aging-induced degradations in their mechanical properties at low strain rates.  Therefore, mechanical characterization of both SAC305 and SAC+Bi solders subjected to isothermal aging is crucial for more accurate predictions of the reliability of the electronic assemblies.

In this study, the local mechanical behavior of SAC and SAC+Bi lead free solders exposed to isothermal aging has been explored using nanoindentation.  Solder specimens made of SAC305 and three SAC+Bi alloys with 1%, 2%, and 3% Bi were prepared using a reflow solidification technique with a controlled temperature profile.  Multi-stage polishing was done on each sample with a view to clearly exposing the beta-tin (β-Sn) dendrites and the Intermetallic Compound (IMC) phases within their microstructures.  After polishing, polarized light optical microscopy and Electron Back Scattered Diffraction (EBSD) techniques were used to identify the tin grains in each solder specimen as well as their crystallographic orientations. 

After preparation, the samples were then subjected to isothermal aging at T = 125 ^oC for 0, 1, 2, 5, 10, 15, and 20 days.  After each duration of aging, nanoindentation was performed on both the dendrite regions and the IMC particles in eutectic interdendritic regions between dendrites.  From the nanoindentation data, the modulus, hardness, and yield stress of the various microstructure phases in each solder alloy were extracted as a function of the prior aging conditions. 

For the SAC305 alloy, the mechanical properties of the β-Sn phase were found to be essentially independent of aging.  For the SAC+Bi alloys, the Bi goes into solution during aging, and the mechanical properties of the dendrite regions were found to be highly dependent on the percentage of Bi in the alloys as well as the aging time.  Using the measured mechanical property data for the various microstructure phases, we are working on developing micromechanics models for predicting the mechanical behavior of the bulk solders at the macroscale.

Presenting Author: Souvik Chakraborty Auburn University