The mechanical properties of lead free solder joints in electronic assemblies are constantly degrading when exposed to isothermal aging.  As shown in our prior papers, these changes lead to dramatic reductions in reliability of lead free electronic assemblies subjected to aging. The root cause of the changes in solder joint mechanical behavior is the evolution of the SAC solder microstructure that occurs during aging.  The most well-known and widely observed changes are coarsening of the Ag~3~Sn and Cu~6~Sn~5~ intermetallic compounds (IMCs) present in the eutectic regions between beta-Sn dendrites.  In the most prior studies, it has been impossible to measure the changes that occur in IMC size and spacing since observations for different aging conditions (e.g. different aging times) were made using different solder joints.  Thus, the comparisons made were necessarily qualitative in nature since the two microstructures were from different samples, and thus could not be directly compared in a quantitative manner.

 

In our recent paper, we demonstrated new procedures that removed the limitations of prior studies on the effects of aging on lead free solder microstructure.  In particular, rather than using several different solder joints, we examined the microstructural changes that occurred in small fixed regions of a single solder joint sample after several durations of aging.  Scanning Electron Microscopy (SEM) was used to examine the aging induced microstructural changes, and nanoindentation marks were added to the cross-section at certain locations to facilitate locating the fixed regions of interest in subsequent microscopy observations.  This approach allowed us to visualize and quantify phenomena such as change in shape (growth/shrinkage), coalescence, and migration of the IMC particles, as well as the coarsening of beta-Sn dendrites and growth and merging of Sn subgrains.

 

In the current work, we have extended our prior studies to examine microstructural evolution in several other SAC+X lead free alloys that have shown the potential to mitigate aging effects.  Solder joints have been extracted from SAC+X assemblies and subjected to aging exposures up to 2000 hours at several elevated temperatures (T = 75, 100, and 125 C).   After each aging time, we were able to quantitatively analyze the evolution of Ag~3~Sn and other IMCs with aging. Specifically, the aging induced changes in number of IMCs, total area of all IMCs, average area of a particle, average diameter of a particle and the normalized diameter were quantified in the fixed regions.  In addition, uniaxial stress-strain and nanoindentation tests were also carried out on the SAC+X specimens before and after aging to characterize the corresponding degradations in mechanical properties with aging temperature and aging time.  Finally, the measured mechanical behavior changes were correlated with the observed microstructure evolutions in the SAC+X solder joints.


Aging Effects in Sac+x Solders Exposed to High Temperatures

Division / Sponsors: 
Electronic & Photonic Packaging Division (EPPD)

Paper Proceedings: https://asmedigitalcollection.asme.org/InterPACK/InterPACK2020/volume/84041

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