Adhesion and toughening mechanisms at underfill interfaces for flip-chip-on-organic-substrate packaging

The flip-chip-on-organic-substrate packaging technology utilizes a particul ate reinforced epoxy as the underfill (UF) to adhere the chip to the packag e or board. Although the use of underfill encapsulation leads to improved r eliability of flip-chip solder interconnections, delamination at various in terfaces becomes a major concern for assembly yield loss and package reliab ility. In spite of their importance, the adhesion and fracture behaviors of the underfill interfaces have not been investigated until recently. Consid erable controversy exists over the effects of underfill formulation and the adhesion and toughening mechanisms of the interfaces. The present work foc uses on investigating the effects of several key variables on the interface adhesion strengths for UF/chip and UF/organic substrate systems. These var iables are underfill organosilane content, filler particle content, rubber particle content, surface morphology and chemistry of the chip and organic substrates, The approach of this study is to measure the effect of these va riables on the interfacial fracture energy using the double-cantilever-beam (DCB) techniques. The results demonstrate that the underfill interfacial a dhesion and fracture characteristics are controlled by several distinct but competing mechanisms, such as formation of primary bonds, crack-pinning by glass fillers, debonding of glass filler from epoxy matrix (defect formati on), and cavitation and shearing induced by rubber particles. Fundamental u nderstanding of the interfacial adhesion and toughening mechanisms can prov ide guidance for developing new processes and materials to enhance interfac ial adhesion and reliability.