A micromechanical analysis is presented to obtain the effective macroscale orthotropic thermomechanical behavior of plain-weave fabric reinforced laminated composites based on a two-scale asymptotic homogenization theory. The model is based on the properties of the constituents and an accurate, three-dimensional simulation of the weave microarchitecture, and is used for predicting the thermomechanical behavior of glass-epoxy (FR-4) woven-fabric laminates typically used by the electronics industry in Multilayered Printed Wiring Boards (MLBs). Parametric studies are conducted to examine the effect of varying fiber volume fractions on constitutive properties. Nonlinear constitutive behavior due to matrix nonlinearity and post-damage behavior due to transverse yarn failure under in-plane uniaxial loads is then investigated. Numerical results obtained from the model show good agreement with experimental values and with data from the literature. This model may be utilized by material designers to design and manufacture fabric reinforced composites with tailored effective properties such as elastic moduli, shear moduli, Poisson’s ratio, and coefficients of thermal expansion.
Failure of plated-through-holes (PTHs) due to thermomechanical stresses is a well established cause of failure of multilayer printed wiring boards (MLBs). This paper uses the finite element method (FEM) to examine the nature of the stress distribution within the PTH structure when the MLB is subjected to thermal loads. Guidelines are laid out for realistic modelling of material properties and boundary conditions in the FEM model. Parametric studies are conducted to study the qualitative effect of several geometric parameters on the critical stresses in the PTH. Both traditional glass-epoxy (FR-4) MLBs and highly anisotropic Kevlar-polyimide MLBs are examined. Differences in behavior observed between the two materials underline the pitfalls in extending the standard design thumbrules for standard FR-4 MLBs to other MLB materials. The purpose here is to provide guidelines for the reliable design of PTHs. Actual fatigue life predictions are deferred to a later paper.
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