High Performance Conductive Adhesives for Lead-Free Interconnects
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0968-V07-01
High Performance Conductive Adhesives for Lead-Free Interconnects Yi Li and ChingPing Wong Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA, 30332
ABSTRACT Tin-lead solder alloys are widely used in the electronic industry. With the recognition of toxicity of lead, however, electrically conductive adhesives (ECAs) have been considered as one of the most promising alternatives of tin-lead solder. While silver is the most widely used conductive fillers for ECA, silver migration has been the major concern for the high power and fine pitch applications. In this paper, a novel approach of using self-assembled monolayers (SAMs) passivation has been introduced to control the silver migration in nano-Ag ECAs. The protection of silver nano particles with SAMs reduced the silver migration dramatically and no migration was observed upon application of high voltages (up to 500 V) due to the formation of surface chelating compounds between the SAM and nano silver fillers. Unlike other migration control approaches which sacrifice electrical performance, the SAM passivated nano Ag fillers also enhanced the electrical conductivity and current carrying capability of adhesive joints significantly due to the improved interfacial properties and high current density of those molecular monolayers. The joint resistance of the SAM incorporated nano-Ag conductive adhesive could be achieved as low as 10-5 Ohm (the contact area is 100 ×100 µm2) and the maximum allowable current was higher than 3500 mA. As such, a fine pitch, high performance, non-migration and high reliability adhesives are developed for potential solder replacement in high voltage, high power device applications. INTRODUCTION Tin-lead solder alloys are widely used in the electronic industry. They serve as interconnects that provide the conductive path required to achieve connection from one circuit element to another. There are increasing concerns with the use of tin-lead alloy solders in recognition of hazards of using lead. Electrically conductive adhesives (ECAs) have been considered as one of the most promising alternatives of tin-lead solder. ECAs consist of a polymeric resin that provides physical and mechanical properties such as adhesion, mechanical strength, impact strength, and a metal filler that conducts electricity. ECAs offer numerous advantages over conventional solder technology, such as environmental friendliness, mild processing conditions (enabling the use of heat-sensitive and low-cost components and substrates), fewer processing steps (reducing processing cost), low stress on the substrates, and fine pitch interconnect capability (enabling the miniaturization of electronic devices). Therefore, conductive adhesives have been used in LCD (liquid crystal display) and smart card applications as an interconnect material and in flip-chip assembly, CSP (chip scale package) and BGA (ball grid array) applications in replacement of solder [1-4]. Silver (Ag) filler is the most attractive choice among all the conductive fi
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