Sn Whisker Nucleation and Growth: Fundamental Mechanisms Controlling Where, When and Why Whiskers Form on Sn Coatings

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TECHNICAL SUMMARYSn whiskers are a serious reliability problem in electronics manufacturing. Whiskers have been implicated in failures of pacemakers, nuclear power plants, satellites and other critical systems. Recently, the removal of Pb from manufacturing (for environmental reasons) has made the problem more severe since Pb had been used as an alloying element to prevent whisker formation. In previous work, whiskering has been shown to increase with the layer stress and finite element analysis (FEA) has been used to model how the intermetallic (IMC) growth at the Sn-Cu interface leads to stress in the layer. The new work will focus on the microscale features that determine why whiskers start to grow at specific sites, i.e., which grains will form into whiskers/hillocks and how is this influenced by the magnitude of the stress. Scanning electron microscopy/electron backscattering diffraction (SEM/EBSD) will be used to monitor the surface repeatedly. These images will be used to create videos of how whiskers/hillocks grow out of the surface. Correlation with the underlying grain structure will show which grain configurations are most likely to form whiskers. Synchrotron microdiffraction studies at the APS synchrotron will be used to measure the deviatoric strain as well as the grain structure in real time as whiskers form. Results of these studies will be used to determine what factors control where whiskers nucleate (e.g., local stress concentration, weak oxide, IMC accumulation, recrystallization, horizontal grain boundaries, etc.). The data will be interpreted using finite element analysis simulations, with the measured microstructure serving as a critical input for the calculations. Additional measurements during thermal cycling and under corrosive environments will be used to determine how these factors influence whisker formation. The knowledge gained from this work will be used to develop mitigation strategies (using layer structure and composition) to suppress whisker formation. NON-TECHNICAL SUMMARYTin whiskers are long thin filaments that grow out of tin coatings used in electronics manufacturing. These whiskers have caused failures in pacemakers, nuclear power plants, satellites and other critical systems. The problem has become more serious recently due to the removal of lead from manufacturing for environmental reasons. Lead-tin alloys had been used for the last 50 years to suppress whiskering and there is currently no alternative that prevents whisker formation as well as lead. The focus of this research is to understand why whiskers form where they do in order to develop schemes to prevent their formation. Stress in the layer is believed to be the driving force for making whiskers, but that does not explain why they form at specific sites on the surface. The researchers will use advanced characterization tools (electron microscopy) to repeatedly monitor the tin surface to observe the whiskers when they start to form. Videos made from these images will show how the whiskers grow out of the surface from individual sites. This technique also measures the orientation and shape of each tin grain in the layer which will enable the whisker formation to be correlated with the characteristics of the sites where they form. The researchers will also use a unique X-ray diffraction tool with microscopic spatial resolution to determine the stress in the individual grains in the layer. These measurements will provide input for mathematical modeling of the surface to understand how the stress develops in each grain and how that leads to whisker growth. Future work will include studying how humidity and temperature influence whisker growth. The results of these studies will be used to develop processing schemes to make layers that are resistant to whisker formation.