Development of electrically conductive adhesives a prospective alternative for tin lead solders

Abstract

Isotropically conductive adhesives (ICAs) are believed to be next generation interconnect materials and a viable replacement for tin/lead soldering technology. Continued efforts since last 15 years have resulted in the development of ICAs with improved properties, but some of the concerns and limitations still remain. Two major drawbacks of ICAs are poor impact performance and increase in contact resistance under elevated temperature and humidity. These drawbacks have been found to be caused by the presence of metals as conductive fillers. The primary focus of this work is to develop ICAs with improved properties by replacing metallic fillers with intrinsically conducting polymers and carbon nanotubes. The first part of the dissertation focuses on the use of polyaniline (PANI) as filler inside an epoxy matrix. Polyaniline is synthesized by conventional chemical polymerization technique and incorporated in the epoxy matrix prior to curing. The effect of PANI filler on the curing property of epoxy matrix is studied by Differential Scanning Calorimetry while the thermal degradation characteristics are observed by Thermogravimetric analysis. Bulk conductivity of the ICAs is measured by four probe technique. In order to characterise the ICAs for their impact properties they are subjected to drop test devised by National Centre of Manufacturing Sciences (NCMS), USA and the Lap shear test (ASTM D3163). Since these conductive adhesives have to be used in various service conditions, they are exposed to elevated conditions of temperature and moisture and their behaviour with respect to moisture uptake and loss of impact properties is studied for various time periods. Moreover, the diffusion of filler particles within matrix polymer is analysed by Scanning Electron Microscopy. In the following sections, the effect of nature and dimensions of conductive fillers is studied. Polypyrrole (PPy) is used as conductive filler because of its better conductivity and stability. PPy formed by dispersion polymerisation using dodecyl-benzene sulphonic acid as dopant has a very minute and uniform particle size and is thus easily diffused in the epoxy matrix. Hence lower concentration of filler is needed to establish good conductivity and this enhances the impact performance of ICAs. Geometric dimensions of the filler particles have a significant effect on conductivity and impact performance of the ICAs. Nanofibres of polyaniline are synthesized by interfacial polymerization technique and dispersed inside the epoxy matrix. The nanofibres show better diffusion and ICAs formed have good conductivity at lower concentration of filler. Similarly, carbon nanotubes (CNT) are incorporated as conductive filler. CNTs possess great dimensional stability and hence impart good impact strength to ICAs. Also, the conductivity increases appreciably at much lower filler loadings. The experimental data shows that the ICAs formed by using these non-metallic fillers have a great prospect of being used in electronic interconnections. The phase compatibility is greatly increased by using polymeric fillers and also the possibility of corrosion which is responsible for decrease in conductivity of metal filled ICAs is neglected. The filler-epoxy interfacial adhesion is very strong and hence these ICAs have very good impact properties. Lap shear fractures occurred by separation at the ICA-substrate interface rather than by cracking of adhesive. Percolation threshold is established at very low filler concentration. Incorporation of these fillers has almost negligible effect on the curing behaviour and thermal degradation of the resin. Accelerated aging seldom causes any significant effect on the impact properties of these ICAs. This research provides for a comprehensive understanding of replacing metallic fillers in currently available ICAs with conducting polymers or CNTs as fillers. These observations will be a useful tool for designing future ICAs. The improved reliability of these ICAs will be significant to overcome the shortcomings of available metal filled ICAs. Moreover, the use of conducting polymers as fillers in ICAs will further expand the already established world of conducting polymer applications.