| Popcorn failure in plastic electronic packages can be investigated by studying the unstable void growth of initially spherical or cylindrical void. During the soldering reflow process the entire plastic package is exposed to a temperature around 260°C. The moisture absorbed in the package evaporates due to high temperature and results in high internal vapor pressure in voids. Consequently, the porous material is stressed under combined vapor pressure and thermal stress. A representative material cell containing a single void is used to investigate void growth under vapor pressure. Hyperelastic material models, such as Mooney-Rivlin, neo-Hookean and Ogden's, are applied. The materials are considered as incompressible, isotropic, and homogeneous. The spherical void shows the instability of inflation pressure in some special cases. For thin-walled spherical void with the neo-Hookean material, cavitation pressure increases with wall thickness. When the thickness approaches infinity the material collapses at stable condition due to material strength. For any thickness of the spherical void with neo-Hookean materials, maximum inflation pressure is up to 2.5 times of shear modulus mu. The instability of spherical void with Mooney-Rivlin material depends on material property, coefficient of asymmetry, and thickness of the wall. It may collapse either due to pressure instability or material strength. The cylindrical void does not show any instability. Its inflation is stable as long as the resulting stress does not exceed the material strength. If the cylindrical void inflates under plane strain condition, coefficient of asymmetry, defined in Mooney-Rivlin model does not affect the solution. In both the spherical and cylindrical void deformation, the hoop stress and strain energy increase monotonically, even with the decrease of vapor pressure. |
