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Research Papers

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

[+] Author and Article Information
M. A. Neidigk

 Sandia National Laboratories, Albuquerque, NM 87185; Department of Mechanical Engineering, University of New Mexico, Albuquerque, NM 87131

Y.-L. Shen

Department of Mechanical Engineering, University of New Mexico, Albuquerque, NM 87131

J. Electron. Packag 131(1), 011003 (Feb 11, 2009) (8 pages) doi:10.1115/1.3068298 History: Received September 21, 2007; Revised April 22, 2008; Published February 11, 2009

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.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 7

Simulated variation of the maximum principal stress with temperature at an element in the region of highest tensile stress on the outside diameter of the ferrite core, during the entire thermal history for the case of viscoelastic analysis with the silicone coating

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Figure 6

(a) Simulated variation of the volumetric strain with temperature in a representative interior encapsulation element during the same thermal cycle before and after aging. (b) Simulated variation of the volumetric strain with time in a representative interior encapsulation element during the same thermal cycle before and after aging. Note: both figures apply to the case without silicone coating.

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Figure 5

Simulated variation of the maximum principal stress with temperature at an element in the region of highest tensile stress on the outside diameter of the ferrite core, during the entire thermal history for the case of viscoelastic analysis without the silicone coating

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Figure 4

Contour plots of simulated (a) hydrostatic stress in encapsulant and (b) normal stress components σxx, σyy, and σzz, and the maximum principal stress in the ferrite core after cooling to −55°C, in the case of elastic analysis with the silicone coating

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Figure 3

Contour plots of simulated (a) hydrostatic stress in encapsulant and (b) normal stress components σxx, σyy, and σzz, and the maximum principal stress in the ferrite core after cooling to −55°C, in the case of elastic analysis without the silicone coating

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Figure 2

Model geometry of the transformer assembly used in the finite element analysis. A representative region is shown with the finite element mesh.

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Figure 1

X-ray image of a failed transformer. Cracking in the ferrite core has occurred.

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