High density multilayer substrates using patterned microvia interposers

Printed wiring boards and IC packaging technologies are continuing to dovetail in the form of high density substrates with increasing complexity and finer dimensions. Double sided circuits with lines and spaces of less than 2 mils and vias down to 1-3 mils are being achieved in both rigid and flexible laminates. Despite these impressive geometries, lamination of several of these double-sided circuits into multilayer constructions is required to produce the necessary density within reasonable dimensions. One approach is to use an adhesive interposer with patterned microvias. These vias can either be filled with a conductive composite material or can be penetrated by conductive posts formed directly on the circuit pads. The composite material discussed is an organic-metallic (Ormet(R)) paste that creates electrical connection between the two circuit pads by alloying with the pad metallization via a mechanism known as transient liquid phase sintering (TLPS). The partially sintered network that is formed provides a reliable path for electrical conduction, comparable to cermet materials, but at the laminate compatible processing temperatures. The structures and performance of high density multilayer substrates fabricated by lamination of double-sided flexible circuits interleaved with patterned interposers and conductive composites in various configurations are presented. Examples of the structures are given along with alternative uses for this type of interposer such as connection of high density circuit patches to standard low density printed wiring boards to provide localized high density for chip attach.