The Wafer Level Chip Scale Package (WLCSP) is gaining popularity for its performance and ability to meet the miniaturization requirements of portable consumer electronics, such as cell phones. For the industry of electronic package, the package life of electronic products is deemed as the essential consideration in the operation period. In practice, electronic products are usually damaged due to a harsh mechanical impact, such as drop and bending. The solder interconnections provide not only the electronic path between electric components and printing circuit board, but also the mechanical support of components on the printing circuit board, so that the reliability of solder interconnection becomes an essential consideration for a package. In the thesis several parameters, including redistribution layer (RDL) material and thickness, passivation material and thickness, under-bump metallization (UBM) structure factors are discussed. A variety of WLCSP structures are investigated for solder joint reliability performance. In addition to the fatigue lives of the test vehicle, locations and modes of fractured solder joints were observed. It was found that wafer level packaging structure under drop clearly related with the characteristic life. The weakest point of solder ball was intermetallic compound (IMC), and wafer level packaging structure was the crack into the second passivation layer and UBM interface of the corner. WLCSP under temperature cycling test was done and observed the fracture only occurred at the solder ball near the package.

Continuous bending under tension (CBT) has been shown to increase room temperature elongation-to-failure (ETF) in various sheet metals past that of simple tension (ST). In commercially pure titanium (CP-Ti) Grade 4, up to 3x extended elongation over ST has been achieved. A greater understanding of deformation mechanisms in CBT would allow for its elongation-enhancing effects to be more fully exploited in HCP and other metals, creating potential for new forming strategies. While most of the extended ETF has been attributed to delayed localization via incremental deformation inherent to the CBT process, together with compressive stabilization and relaxation of mechanical strain fields, contributions of microscale components relating to damage evolution, defect structures, and slip system activity intrinsic to the process are also likely to play a role. CBT-induced cyclic bending/unbending stresses combined with applied macroscopic tension create complex through-thickness stress profiles, where differing hardening behavior is expected near the surfaces compared with the middle of the sheet. This work uses high resolution EBSD characterization of geometrically necessary dislocation (GND) density together with X-ray diffraction (XRD)-based evaluations of total dislocation density and in-plane digital image correlation (DIC) to provide an in-depth analysis of through-thickness dislocation development and associated hardening rates throughout the CBT process in CP-Ti Grade 4 sheet metal. It was found that dislocation density is relatively uniform across the sheet at lower cycles, increases in the sheet center at higher cycles, and eventually approaches saturation near failure. Namely, dislocation accumulation occurs more slowly in the ratcheting, bending/unbending portions of the sheet (i.e., near the surfaces) from cyclic load reversals, and develops faster in the central tensile portion, where dislocation density up to 1.43x higher than near the surfaces was observed. The fraction of 〈c+a〉-type dislocations stayed below ~27% within the sheet, decreasing with increased strain, suggesting that the texture evolves such as to favor 〈a〉-type slip. Indications of stronger texture evolution occurring in the ratcheting (cyclic) regions were observed, with central texture resembling that of a sample deformed in ST. High dislocation densities in the sheet center were found to precede significant central void accumulation, concentrating damage away from peak surface stresses, presumably contributing to delayed failure.

<p>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 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.</p> <p>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.</p> <p>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 whiskers and hillocks, intragranular breakup (IGB) with intrusions and extrusions and nanowhiskers (NWs) with diameters < 1 𝜇m were observed. Both increasing number of cycles and strain amplitude/rate promote defect formation for a fixed frequency, with the defect density being strongly frequency dependent.Vibration at low temperature and interrupting measurements for SEM characterization affected the relative densities. The density of larger surface defects is strongly influenced by interruptions while NW density is almost unaffected. </p><p>Both low resonant frequency and low T (223 K) promote IGB formation during cyclic bending due to large maximum strain amplitude and slower diffusion/creep at low T, respectively. Though the overall defect density for low T is smaller than that at room temperature (RT), the response of films is similar to that at RT, indicating the same mechanisms. The defect density decrease at low T is mainly determined by NW formation, and there is a transition from micro-sized surface defects to IGBs for cyclic bending at low T.</p><p>This research demonstrated that cyclic bending of cantilevers can be used to quantify the stress relaxation of tin films in an important stress regime for microelectronics and to develop defect mitigation strategies to improve the reliability of interconnects in electronic applications.</p>