Curing kinetics of anisotropic conductive adhesive film

Journal of Electronic Materials, Mar 2003 by Chan, Y C, Uddin, M A, Alam, M O, Chan, H P

Polymer-based conductive-adhesive materials have become widely used in many electronic packaging interconnect applications, such as chip-on-glass, chip-on-flex, etc. Among all the conductive-adhesive materials, anisotropic conductive adhesive film (ACF) is an attractive interconnect material because of its fine pitch capability. Anisotropic conductive-adhesive film is a thermosetting, epoxy matrix impregnated with a small amount of electrically conductive particles. During component assembly, the epoxy resin is cured to provide mechanical connection, and the conducting medium provides electrical connection in the z direction. The thermal cure process is critical to develop the ultimate electrical and mechanical properties of the ACF In this paper, the curing reaction of ACF was studied with a differential scanning calorimeter (DSC) under isothermal conditions in the range of 120-180 deg C. An autocatalyzed kinetic model was used to describe the curing reaction. The rate constant and the reaction orders were determined and used to predict the progress of the curing reaction. A good agreement is found between the proposed kinetic model and the experimental reaction-rate data. The reaction-rate constants were correlated with the isothermal temperature by the Arrhenius equation. The glass-transition temperature also has been studied as a function of cure degree and moisture absorption.

Key words: Anisotropic conductive adhesive film, curing, autocatalytic-- kinetic model, glass-transition temperature

INTRODUCTION

Recently, many novel electronic interconnect materials have been develop to miniaturize electronic components as well as reduce environmental contamination.1 Anisotropic conductive adhesive film (ACF) is one of the promising materials for the near future for such application.2 In fact, conductive adhesive joining technology has been used for many years in the area of hybrid technology, liquid-crystal display interconnect, and chip-on-glass technology. It is only recently that efforts have been made to develop new conductive adhesives for surface mount and low-cost, flip-chip electronics, volume manufacturing. Such efforts include the new formulation of ACFs without solvent and modified curing schedules.3,4 The physical, electrical, and mechanical properties of the cured conductive adhesives depend to a large extent on the degree of cure of the epoxy composition in the conductive adhesive.5 The glass-- transition temperature (Tg) is also dependent upon the curing profile. Thus, a full understanding of the cure mechanisms of conductive adhesives can provide tools to optimize the cure schedule so that residual stress in the conductive adhesive joints is minimal.5

Basic to such an optimal curing process is the need to first understand the physical curing mechanism and cure kinetics underlying the process and then be able to model the cure process accurately. This includes the determination of the mechanism or appropriate kinetic equation for the system being analyzed, and measurement of the reaction orders, activation energies, and frequency factor of the reaction. An accurate model not only helps to predict cure behavior for process design and control but also can be used to predict aging and degradation of thermosetting polymer systems and compare the cure behavior of different systems.6 To date, no studies have been reported on the cure-kinetics modeling of the anisotropic conductive adhesive system. As such, the objective of this project is to develop a method based on thermal analysis to elucidate the mechanism of the cure kinetics, which can be used to develop a generalized curing model for optimal curing of the anisotropic conductive adhesive system.

ACKNOWLEDGEMENTS

The authors acknowledge the financial support of the Hong Kong Research Grants Council (Project No. 8720003, CRC on Conductive Adhesive Technology for High Density Electronic Packaging).

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