Session: 10-01 Interactive Presentations

Paper Number: 100288

100288 - Mechanical Property Evolution in Thermally Cycled Sac+bi Lead Free Solders 

Solder joints in electronic assemblies are frequently exposed to thermal cycling environments in their service life or during accelerated life testing where temperature variations occur from very low to high temperature.  Due to the CTE mismatches of the assembly materials, cyclic temperature leads to damage accumulation due to shear fatigue in the solder joints.  In addition, dwell periods at the high temperature extremes will cause thermal aging phenomena and additional microstructural evolution and material property degradation.  Further aging effects can occur during the ramp periods between the low and high temperature extremes.

In our recent papers, the mechanical behavior evolutions occurring in SAC305 and SAC+3%Bi (SAC_Q) lead free solders have been characterized for up to 20 days of exposure to four different thermal profiles including isothermal aging, slow thermal cycling, thermal shock, and thermal ramping. In the current investigation, we have extended our prior study to examine several different SAC+Bi solder alloys with various bismuth contents.  In particular, a family of SAC+Bi alloys with 1%, 2%, and 3% Bi were studied with different thermal exposure profiles. The primary objective of this study was to determine how much bismuth is needed in the lead-free alloy to mitigate mechanical and microstructure evolutions during thermal exposures.  Uniaxial miniature bulk specimens were prepared for the three SAC+Bi alloys using a controlled reflow profile.  After fabrication, the samples were then preconditioned by thermal exposure under stress-free conditions for various durations up to 100 days.  After preconditioning via thermal exposures, the samples were investigated to characterize their mechanical and microstructural evolutions.

For all of the alloys, the degradations for the slow thermal cycling exposure were surprisingly higher than those for isothermal aging.  Increasing the Bi content of the SAC+Bi alloy led to increased mitigation of thermal degradation effects for all of the exposure profiles.  Reduced microstructural evolution in the SAC+Bi alloy samples was found to be the major reason for the improved resistance to mechanical behavior changes.

Presenting Author: Mohammad Al Ahsan Auburn University