Interfacial Debonding in PWBs

Abstract

AbstractGlass fiber reinforced epoxy laminates have been widely applied in the electronic packaging industry as multilayer printed wiring boards (PWBs). Previous investigations indicate that, the electric insulation between printed through hole (PTH) and conductive lines can be lost if the boards are subjected to certain environmental conditions [1,2,3]. The insulation breakdown is caused by copper filament formation along the glass fibers once the filament bridges any two nearby circuittraces, such as a PTH and a power plane, or between two PTHs. The conductive filament formation is a two step process: path formation and electrochemical reaction. The path formation is caused by degradation of the glass-epoxy interface, which provides the copper ion migration path for the second step. Humidity and temperature are the key factors leading to interfacial degradation. Usually, humidity is of more concern because water absorption helps the path formation. Among board materials, epoxy resin, as the matrix material, is more susceptive to moisture absorption than glass fiber or copper [3,4]. Operating temperature that changes the degree of moisture absorption [5,6], and thermal cycling that enhances water absorption, and stress induced from thermal-mismatches [7], are also considered as the important factors to the interface degradation.Epoxy/glass interfacial debonding can be introduced near PTHs by thermal cycling [8]. The interfacial debonding between glass fibers and the epoxy matrix near PTHs accelerates the electromigration process. In order to study the interfacial debonding mechanisms, thermal cycling tests of PWBs were conducted and the dynamic process of interfacial debonding was characterized using environmental scanning electron microscope (ESEM). This paper presents the experimental results with discussion of the possible interfacial debonding mechanisms.