FORMATION AND EVOLUTION OF TIN SURFACE DEFECTS DURING CYCLIC MECHANICAL LOADING

Xi Chen
2020
Stress relaxation in tin films can result in microstructural changes visible on the surface, referred to as "surface defects," and can include whisker and hillock formation, cracking, nucleation of new grains, and grain growth. Sn whiskers are of particular concern for microelectronics reliability in which Sn whiskers growing from component surface and cause catastrophic short-circuiting. While prior research has identified the conditions and mechanisms for surface defect evolution during aging
more » ... lution during aging and thermal cycling, the response of tin films due to mechanical stress, especially high frequency vibration, is not fully understood. In practical terms, high frequency vibration is an important source of mechanical stress generation in microelectronics for automotive and aerospace applications. This research, based on high frequency vibration of cantilevers, adds to the existing mechanisms for stress relaxation process in metal thin films, not just for tin films, as well as proposed new mechanisms for such processes. In the first study, the piezoelectric drive of small atomic force microscopy (AFM) cantilevers vibrated at resonance are used for high frequency cyclic bending experiments. Intermetallic (IMC) formation as well as initial film morphology and thickness (corresponding to surface grain size) all influence the response of tin films for cyclic bending. A laser doppler vibrometer (LDV) system was used to identify the real-time strain along the cantilever during cycling, suggesting that the small strains are responsible for the limited nucleation and growth for defects though the defect density increases with the number of cycles and strain distribution along the cantilever. In the second study, the effect of larger strains on defect evolution was determined using vibration of larger cantilevers at resonance as a function of number of cycles, frequency, temperature, and whether the vibration was continuous or interrupted for SEM characterization of defect type and density. In addition to typical micro-sized whisker [...]
doi:10.25394/pgs.12709355 fatcat:lfzg2uws7jfelnllydz34xoi6y