Microstructural Study of Lead Free Solder Joints Subjected to Mechanical Cyclic Loading Under Various Conditions

A common method of failure in electronic packages is the failure of solder joints. Solder joints subjected to thermal cyclic loading usually fail due to fatigue failure. This is brought about by the shear forces introduced by the different Thermal Expansion Coefficients (CTE) of the package components. Previous researches have shown that recrystallization assisted cracking plays a significant role in the fatigue failure of these solder joints. In this paper, the authors seek to observe the changes in the microstructure of SAC305 solder joints subjected to mechanical cyclic loading under various conditions. The test specimens were 3x3 BGA array of SAC305 joints sandwiched in between two FR-4 PCBs (Printed Circuit Boards). The specimens were held in epoxy preforms and polished to expose the solder joints. The samples were then extracted and some of these were aged at 125 degrees C for 10 days. Both the aged and non-aged specimens were then isothermally mechanically cycled at either room temperature or at 100 degrees C. The authors then proceed to observe the differences in the microstructure due to mechanical cycling of the joint under different conditions, namely, aging, or high temperature mechanical cycling, or a combination of both. The micrographs before and after the cycling were captured by an optical microscope. The other objective of this study was to also observe if recrystallization played a part in the ultimate failure of the joints when subjected to shear mechanical cycling for each of the conditions.