Mechanical Engineering

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    COMPARISON OF INTERCONNECT FAILURES OF ELECTRONIC COMPONENTS MOUNTED ON FR-4 BOARDS WITH SN37PB AND SN3.0AG0.5CU SOLDERS UNDER RAPID LOADING CONDITIONS.
    (2010) Gregory, Patrice Belnora; Barker, Donald B; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Electronic circuit boards can experience rapid loading through shock or vibration events during their lives; these events can happen in transportation, manufacture, or in field conditions. Due to the lead-free migration, it is necessary to evaluate how this rapid loading affects the durability of a leading lead free solder alternative (Sn3.0Ag0.5Cu) assemblies as compared with traditional eutectic lead based solder Sn37Pb assemblies. A literature review showed that there is little agreement on the fatigue behavior of Sn37Pb solder assemblies and Sn3.0Ag0.5Cu solder assemblies subjected to rapid loading. To evaluate the failure behavior of Sn37Pb and Sn3.0Ag0.5Cu solder assemblies under rapid loading conditions, leadless chip resistors (LCR), ball grid arrays (BGA), small outline integrated circuits (SOIC), and small outline transistors (SOT) were subjected to four point bend tests via a servo-hydraulic testing machine at printed wiring board (PWB) strain rates greater than 0.1/s. The PWB strain was the metric used to evaluate the failures. The PBGAs and LCRs were examined with both Sn37Pb and Sn3.0Ag0.5Cu solders. There was no significant difference found in the resulting test data for the behavior of the two solder assembly types in the high cycle fatigue regime. PBGA assemblies with both solders were also evaluated at a higher strain rate, approximately 1/s, using drop testing. There was no discernable difference found between the assemblies as well as no difference in the failure rate of the PBGAs at this higher strain rate. The PWB strain was converted to an equivalent solder stress index using finite element analysis. This equivalent stress index value was used to compare the results from the LCR and BGA testing for Sn37Pb and Sn3.0Ag0.5Cu. Independently generated BGA data that differed with respect to many testing variables was adjusted and incorporated to this comparison. The resulting plot did not show any significant differences between the behaviors of the two solder assemblies under rapid loading outside of the ultra low cycle fatigue regime, where the assemblies with Sn37Pb solder outperformed the assemblies with SnAgCu solder.
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    Evaluating the Susceptibility of Electronic Components Assembled with Leaded Solder to Flexural Failures, with High Rate Considerations
    (2008-11-20) Watkins, James Robert; Barker, Donald B; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Multilayer ceramic capacitors (MLCCs) are subjected to high strain rate flexural loading via drop tower and air gun tests, with PWB strain rates ranging from 1/s to 10/s. Three MLCC part sizes, three different manufacturers, and standard and flexible termination parts are included in the study. Standard termination capacitors failed via the well documented flex crack failure mechanism. However in all cases this crack followed a vertical path not typical of this failure mechanism. Flexible termination capacitors failed via a newly discovered failure mechanism involving delamination in the end cap metallization between the silver filled epoxy and the nickel-tin plating. For size 1206 parts, this delamination was seen in both end caps, and the part detached from the test board. For size 0603 parts, this delamination occurred in one end cap, while the opposite end cap fractured though the ceramic in a manner similar to a flex crack. Size 0603 parts also failed via the vertical flex cracks documented in standard termination parts. All of the documented failures of MLCC devices at PWB strain rates of ≥ 1/s occurred at maximum PWB strain values greater than an order of magnitude lower than those seen in lower strain rate testing. This rate dependency of MLCC part failures has vast implications for products intended for high rate environments. Additionally, when the PWB strain rate was increased along with PWB maximum strain, flexible termination capacitors performed worse than their standard termination equivalents. This brings to issue the role of these next generation parts in portable consumer electronic devices as well as other designs with high rate implications. Ball grid array (BGA) devices are subjected to four point bend tests via a servo-hydraulic testing machine at PWB strain rates ≤ 0.1/s. The resulting BGA data is found to adhere reasonably well to the Coffin-Manson low cycle fatigue relationship. Independently generated BGA data that differs with respect to many testing variables is plotted alongside the experimental data. The high correlation of the data set indicates the possibility of creating a BGA mechanical failure model that is independent of failure site, package type, and test specifications.