As part of a surveillance effort, a 30-year-old solder joint that exhibited one third the required strength was identified. A failure analysis ensued, to identify the root cause of the low strength, the objective being to determine whether a materials aging mechanism was to blame. 50Sn-47Pb-3Sb solder was used in structural solder joints for a nickel component that was fabricated over 30 years ago. This alloy was likely chosen for its improved creep strength compared to binary Sn-Pb solder, though a reduction in “wettability” is reported to accompany the improved mechanical properties. Wettability refers to the ability to coat and react with a substrate surface, and is the basis for producing a high quality, metallurgical bond in a solder joint.
During fabrication, a number of units were pulled to failure. If these units did not meet the strength requirement, the lot was scrapped. In 2016, a surveillance effort to investigate current joint strengths after ~30 years of aging was initiated. While average strength values of the units decreased, a few units fell below the required strength value, and one unit exhibited very low strength compared to the others. A failure analysis was performed to explain this strength reduction.
Non-destructive and destructive characterization methods were used to examine and compare the low strength failure with higher strength failures as well as untested units. Fracture surfaces, failure locations, solder volume, porosity levels, solder hardness, solder composition, and solder microstructure were evaluated.
Optical and electron microscopy methods were used to initially examine the solder fracture surfaces, as received. Untested, high strength, lower strength, and the lowest strength units were then sectioned for viewing longitudinal and transverse orientations of the joints for further metallographic analyses.
Wavelength dispersive spectroscopy (WDS) was used for composition and microstructure evaluation via a scanning electron microscope (SEM)/electron probe microanalysis (EPMA), and microhardness testing was performed to compare the joints’ mechanical properties.
Measured mechanical properties of aged bulk solder were compared with literature data to quantify the extent that aging contributed to strength degradation.
Results and Discussion
Hardness testing, SEM imaging, and wavelength dispersive spectroscopy (WDS) reveal similar microstructures between untested, high, and low strength joints. These similarities indicate that the joints were all aged in similar conditions and confirm the solder composition to be the same. Hardness values hovered around a nominal 11 Vickers.
WDS maps show similar phase morphology and distribution throughout the solder joint between an untested, high, and low strength joint. Fractographic and metallographic analyses identified 3 failure modes within the solder joints: 1) shear ductile; 2) porosity-assisted; and 3) interfacial. Higher fractions of porosity-assisted and interfacial failure likely contributed to low strength.
Fractographic and metallographic comparisons between the high and low strength tested units suggest that the prevalent, local porosity-assisted and interfacial failure modes contributed to the low-strength failure. While other units exhibited some interfacial failure and porosity-assisted failure, the conditions were not as extreme as that exhibited in lowest strength unit.
It is plausible that localized porosity aligned near a stress path contributed in regions 1 and 3, while rapid, interfacial failure occurred at region 2. Uneven loading likely resulted after the interfacial failure. Possible artifacts from testing are not presented or discussed.
The source of porosity could have been due to an undocumented process variable during the soldering procedure (i.e. inappropriate type/amount of flux). Poor wetting may have also resulted from inappropriate type/amount of flux in addition to joint disturbance prior to solidification, porosity accumulating at the interface, and/or improper cleaning methods, therefore facilitating interfacial failure.