Several open-cell flexible foams, including aged polyurethane foams, were mechanically characterized over a temperature range of 40 to 20 °C. Quasi-static compression was performed to obtain the stress-strain behavior of the foams. The stress-strain relation is nonlinear, but typically there is a small range of linear behavior initially. Compressive cyclic loading at different amplitudes and frequencies of interest (20–60 Hz) were applied to measure foam’s hysteresis properties, i.e. stiffness and energy dissipation. The cyclic characterization includes foams with different amount of pre-strains, some are beyond the initial linear range as occurred in many applications.
Decisions on material selections for electronics packaging can be quite complicated by the need to balance the criteria to withstand severe impacts yet survive deep thermal cycles intact. Many times, material choices are based on historical precedence perhaps ignorant of whether those initial choices were carefully investigated or whether the requirements on the new component match those of previous units. The goal of this program focuses on developing both increased intuition for generic packaging guidelines and computational methodologies for optimizing packaging in specific components. Initial efforts centered on characterization of classes of materials common to packaging strategies and computational analyses of stresses generated during thermal cycling to identify strengths and weaknesses of various material choices. Future studies will analyze the same example problems incorporating the effects of curing stresses as needed and analyzing dynamic loadings to compare trends with the quasi-static conclusions.
Legislated requirements and industry standards are replacing eutectic lead-tin (Pb-Sn) solders with lead-free (Pb-free) solders in future component designs and in replacements and retrofits. Since Pb-free solders have not yet seen service for long periods, their long-term behavior is poorly characterized. Because understanding the reliability of Pb-free solders is critical to supporting the next generation of circuit board designs, it is imperative that we develop, validate and exercise a solder lifetime model that can capture the thermomechanical response of Pb-free solder joints in stockpile components. To this end, an ASC Level 2 milestone was identified for fiscal year 2010: Milestone 3605: Utilize experimentally validated constitutive model for lead-free solder to simulate aging and reliability of solder joints in stockpile components. This report documents the completion of this milestone, including evidence that the milestone completion criteria were met and a summary of the milestone Program Review.
Encapsulation of high voltage transformers can be a difficult undertaking. Stresses arise due to the coefficient of thermal expansion (CTE) mismatch of the components. Due to the viscoelastic nature of the encapsulation, these stresses can change over time. Excessive tensile stress in the ceramic cores results in cracks which can affect the performance of the transformer. The transformer that is the subject of this paper performed well after manufacturing and an initial thermal cycle; four years later however, the same transformer failed during the heat-up portion of a similar thermal cycle. X-rays revealed a large crack in the ceramic core. This paper summarizes the elastic and nonlinear viscoelastic finite element modeling that was done in support of the failure investigation and redesign of the transformer. In both the elastic and viscoelastic finite element models, the maximum principal tensile stresses at the low temperature condition of the thermal cycle exceeded the estimated ultimate tensile strength of the core material. At room temperature, the models predicted that the maximum principal tensile stresses were sufficiently high to produce subcritical crack growth in the core material. The viscoelastic model indicated that the core could experience a significant increase in stress due to physical aging of the encapsulation. Modeling stresses compared well to the cracks found in the failed transformer. The final design utilized a silicone coating applied to the interior surfaces of the cores. The coating acts as a stress relief layer that decouples the high CTE encapsulation from the ceramic core. The addition of the silicone coating resulted in a significant stress reduction. X-rays of transformers made with the silicone coating reveal no cracks in the cores.