Contact Resistance and Metallurgical Connections Between Silver Coated Polymer Particles in Isotropic Conductive Adhesives

Sigurd R Pettersen,Shijo Nagao,Helge Kristiansen,Zhiliang Zhang,Susanne Helland,John Njagi,Jianying He,Katsuaki Suganuma

doi:10.1007/s11664-016-4498-1

Abstract

Recently, there has been an increasing interest in silver thin film coated polymer spheres as conductive fillers in isotropic conductive adhesives (ICAs). Such ICAs yield resistivities similar to conventional silver flake based ICAs while requiring only a fraction of the silver content. In this work, effects of the nanostructure of silver thin films on inter-particle contact resistance were investigated. The electrical resistivity of ICAs with similar particle content was shown to decrease with increasing coating thickness. Scanning electron micrographs of ion milled cross-sections revealed that the silver coatings formed continuous metallurgical connections at the contacts between the filler particles after adhesive curing at 150°C. The electrical resistivity decreased for all samples after environmental treatment for 3 weeks at 85°C/85% relative humidity. It was concluded that after the metallurgical connections formed, the bulk resistance of these ICAs were no longer dominated by the contact resistance, but by the geometry and nanostructure of the silver coatings. A figure of merit (FoM) was defined based on the ratio between bulk silver resistivity and the ICA resistivity, and this showed that although the resistivity was lowest in the ICAs containing the most silver, the volume of silver was more effectively used in the ICAs with intermediate silver contents. This was attributed to a size effect due to smaller grains in the thickest coating.

Highlights

As lead-based solders are banned in a growing range of products due to environmental concerns, Sigurd R
As the electrical measurements were conducted on the same samples pre- and postET, this large decrease in both spread and average resistivity for the samples with the thinnest coatings suggest that a significant structural change occurred in these samples during the environmental treatment
When the silver flakes of conventional isotropic conductive adhesive are replaced by micron-sized polymer spheres coated with tens to hundreds of nanometers of silver, the result is a complex composite system with a large range of factors influencing the material properties

Summary

Introduction

As lead-based solders are banned in a growing range of products due to environmental concerns, Sigurd R. Isotropic conductive adhesives (ICAs) are emerging as a promising alternative in electronic interconnects because of advantages such as lower processing temperatures and fewer processing steps compared to lead-free solders.[1] Conventionally, ICAs comprises micron-sized silver flakes embedded in an adhesive matrix, normally a thermosetting epoxy system. Even with the large surface-to-volume ratio of silver flakes, the amount of silver must be relatively large to achieve sufficient electrical conductivity; typically a volume fraction of 25–30% is required, corresponding to a weight fraction of 70–80%. This large fraction of precious metal severely affects the cost-efficiency of ICAs, and increases the impact of undesirable material properties such as brittleness and thermal expansion mismatch between the components

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Discussion

Conclusion