Integral Plugged Blind via in Printed Wiring Boards

Abstract

Adoption of fine pitch and high lead count area array components in high-reliability applications such as military and space electronics demands high-density interconnect (HDI) features in multilayer printed wiring boards (PWBs). Usage of cap-plated fine vias that are plugged with dielectric material is one of the approaches to meet the circuit density requirements. In this article, a novel method called “integral plugged blind via” without secondary processing steps, such as curing and planarization, is described to realize HDI multilayer PWBs. In this method, through vias are epoxy plugged during the lamination of inner layer circuits in vacuum-assisted hydraulic press cycle. Plugged blind vias are formed as an integral part of the fabrication process. It is observed that there is no excess ooze out of resin material from the blind vias in the laminated substrate and blind vias are found to have a negligible dimple and no voids. This is attributed to the gasket effect due to the elastomeric property of specialty release films, which conform (follow) to the contour of plated through vias during the vacuum lamination cycle. Plugged blind vias are subjected to critical thermomechanical stress to evaluate interconnection integrity. It is found that there is negligible change in the resistance of vias and resin plugged vias are free from defects such as barrel crack or knee crack of plated through hole (PTH) or delamination in the cured plugged material. Robustness of these resin-filled blind vias is attributed to anchored plated barrel with the additional support provided by plugged material, which restricts the thermomechanical deformation of plated copper.