Modal finite element analysis of PCBs and the role of material anisotropy

Abstract

Printed Circuit Boards (PCBs) are epoxy resin-impregnated and cured sheets of counter woven glass fabric (e.g. FR4) laminated between thin sheets of Copper. The nature of the PCB is inherently anisotropic and inhomogeneous but previous modal FEMs of PCBs have assumed isotropic, anisotropic (transversely isotropic and orthotropic) material properties and shown good correlation with test data for specific scenarios [1-3]. This paper details part of a research program aimed at gaining a better understanding of accurately modeling PCB's dynamic behavior. New investigations into the impact of material anisotropy and, in particular, the effect of material orthogonal plane definition (Ex and Ey) on eigenfrequencies is analysed. A modal FEM of a JEDEC PCB is created, verified, and validated using well established theories by Steinberg and empirical data by others [4, 5]. The relative contributions of Ex, Ey and Ez on PCB eigenfrequencies is examined using a parametric modal FEM, analysing the role of material isotropy verses anisotropy. The impact of transversely isotropic material properties is also analysed for a typical JEDEC PCB. This analysis details the mesh density required for accurately modeling the PCB eigenfrequencies. The results show that a 100 % increase in Ez has only a 0.2 % difference in the eigenfrequency where as a 100 % increase in Ey has a 1.2 % difference in the eigenfrequency. The effect of orthotropic plane definition (alternating Ex with Ey) on the JEDEC PCB amount to a 7.95 % delta in eigenfrequency.