Matthew Neidigk

Predicting the Effect of Underfill Filler Volume Fraction on Solder Fatigue Life and Residual Stress for Surface Mount Components Using Nonlinear Viscoelastic Analyses

Glassy thermoset polymer underfills are commonly used for reliability enhancement in modern electronics. By adding filler to the polymer, underfill mechanical properties, such as bulk and shear moduli and coefficient of thermal expansion, can be altered. Addition of underfills can affect the solder reliability and component failure during dynamic environments. By modifying the nonlinear viscoelastic simplified potential energy clock model, a generic computational tool was created for analyzing filled polymers. Together with a unified creep plasticity model for solder and the Coffin-Manson fatigue criterion, solder fatigue life for underfilled surface mount components was investigated for various underfill filler materials and filler volume fractions (FVFs) using finite element analyses. By creating models of representative components with very different geometries, the effect of adding an underfill and increasing the FVF of hard and glass micro-balloons (GMB) fillers was analyzed. For a large stiff component, the addition of an unfilled underfill reduced the localized tensile stress in the component. Underfill filler volume fractions greater than 10% for hard filler and 15% for GMB filler resulted in a positive effect on the fatigue life. The results were different for a small flexible component. The addition of an unfilled underfill slightly increased the localized tensile stress in the component, but a positive effect on the fatigue life was still demonstrated if the underfill FVFs were greater than 15% for hard filler and 30% for GMB filler.

Nonlinear Viscoelastic Finite Element Analysis of Physical Aging in an Encapsulated Transformer

The generation of thermal stresses is a major cause for mechanical failure in encapsulated electronic components. In this study numerical modeling is employed to analyze thermal stresses in a high-voltage transformer encapsulated with filled epoxy. The transformer assembly consists of materials with an extremely disparate range of thermomechanical properties. The thermal histories considered mimic those in the operational condition. It is found that, upon thermal cooling from elevated temperature, the ceramic core can be under local tensile stress although it is entirely surrounded by materials with much greater coefficients of thermal expansion. The unique aspect of this paper originates from the fact that the volume shrinkage of the viscoelastic encapsulant during physical aging contributes to an increase in stress over time, thus increasing the tendency of fracture. This counter intuitive result (stress increase due to nonlinear viscoelastic physical aging) can now be predicted using constitutive models recently developed at Sandia National Laboratories. When a silicone coating between the core and the encapsulation is included, the stress is significantly reduced. The modeling result is shown to corroborate with the actual performance of the transformer.

Cyclic Loading Experiment for Characterizing Foam Viscoelastic Behavior

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.

Analysis of Laser Weld Induced Stress in a Hermetic Seal

Laser welding of glass-to-metal electrical connectors is a common manufacturing method for creating a hermetically sealed device. The materials in these connectors, in particular the organic glass, are sensitive to thermal induced residual stress and localized heating. An analytical laser weld model is developed that provides simulation and analysis of both thermal and mechanical effects of the welding process. Experimental studies were conducted to measure the temperature at various locations on the connector. The laser weld is modeled using both surface and volumetric heating directed along the weld path to capture the thermal and mechanical response. The weld region is modeled using an elastic-plastic weld material model, which allows for compliance before welding and stiffening after the weld cools. Results from a finite element model of the glass-to-metal seal are presented and compared with experimental results. The residual compressive stress in the glass is reduced due to the welding process but hermeticity is maintained.

Packaging Strategies for Printed Circuit Board Components Volume I: Materials & Thermal Stresses

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.