DEVELOPMENT OF CONDUCTIVE ADHESIVES: A PROSPECTIVE ALTERNATIVE FOR Sn/Pb SOLDERING

Abstract

Electronic interconnections have been dominated by tin/lead solders since decades and they are widely used even today. However, as the environmental awareness increased the toxicity of lead has received universal attention and its overall adverse effect on human health is being closely monitored. Even small quantities of Pb can lead to damage of brain, nervous system, liver and kidneys when ingested. Disposal of electronic waste especially printed circuit boards containing lead is a major concern globally, because lead accumulation in water sources due to continued dumping of this waste is severely affecting the biosphere. Hence, world attention has been brought towards eliminating or minimizing the use of lead in electronic interconnections. Legislation and policies have been proposed in Europe and United States to ban or limit the use of lead in solders. In July 2006, the use of several chemicals and agents was restricted in the European Union according to the RoHS (Restriction of hazardous substances) directive. Lead that is commonly used in solders was among these substances. However, such efforts haven’t yet been effective due to lack of a suitable alternative for lead based solders. Therefore, great efforts are being made to develop lead free and environmentally sound interconnect bonding technology as an alternative to tin/lead solders. The first alternative developed is lead-free solders, which are essentially low melting temperature metals and metal alloys. In Japan & European Union, commercial industry has already acceded to lead-free solders. However, there are some marked limitations associated with this technology, which have not allowed it to flourish in the global market. The other alternative is electrically conductive adhesives (ECA’s). ECAs provide an environment friendly solution for interconnections in 2 electronic applications. Another major reason for the interest in conductive adhesives is the requirement of increasing miniaturization and integration, which leads to smaller passive components and more complex IC components. Continuing improvement in adhesive technology has enabled ECAs to replace solder in many electronic assembly applications. ECAs offer several potential advantages over conventional solder interconnection technology including finer pitch printing, lower temperature processing and more flexible and simpler processing. Also, ECAs have greater flexibility, creep resistance and energy damping as compared to lead free solders, which can reduce the possibility of failures that occur in lead-free solder interconnections. Therefore, ECAs and among them isotropically conductive adhesives (ICA) are perceived as next generation interconnect materials for electronic packaging. However, compared to mature soldering technology, conducting adhesive technology is still in its infancy and some limitations and drawbacks do exist. Main limitations of current ICAs like limited impact resistance, unstable contact resistance and poor mechanical strength in various climatic and environmental conditions are major obstacles preventing ICAs from becoming a general replacement for solders in electronic applications. A better understanding of the properties like impact failure and loss of conductivity due to corrosion has been attributed to presence of metallic fillers in ICAs. So, there is an immediate need for replacement of these metallic fillers with some more reliable filler materials. This need has to be addressed by identifying or developing the proper materials suitable for application as fillers in ICAs. This study identifies the already established non-metallic conductive materials as probable fillers for developing ICAs which may be used as prospective alternative to tin/lead soldering. The main objective of this dissertation is to incorporate organic conjugated conducting polymers and carbon nanotubes as fillers in an anhydride 3 cured epoxy matrix and study the composites so formed for their use as ICAs. The optimization of filler concentration and its effect on overall properties forms the core of this research work. Incorporation of a filler phase inside the matrix decreases its impact performance. Hence proper filler loadings have to be used so that the resulting impact performance of these materials is suitable enough for them to bind the electronic components to the substrates. Currently available ICAs use metals especially silver as filler. Presence of this metallic phase inside the matrix polymeric phase leads to phase separation and hence adhesion at filler-epoxy interface is weak. This is responsible for poor impact properties of metal filled ICAs. Moreover, the filler loading in these ICAs is very high (up to 80%). Only a small portion of shear strength of polymer matrix can be retained. This study also attempts to address this limitation by incorporating organic conducting polymers inside the matrix. Since, there is an improved compatibility between the two phases, better adhesion with filler is expected. Although, CNTs show comparatively lesser compatibility, yet their effect on impact performance is substituted by their high electrical conductivity because of which very small filler loadings are needed for effective conductivity establishment. Also, metal filled ICA bound interconnections are susceptible to oxidation or corrosion due to the presence of dissimilar metals at the interface. It leads to decrease in conductivity of the joints when exposed to moist service conditions. By replacing metals with highly stable conducting polymers and CNTs, environmentally stable conductive adhesives are formed. The overall properties of the ICAs developed are a direct function of the effective diffusion of filler particles within the matrix. If fillers are properly distributed, conductivity is established smoothly due to the formation of infinite conductive channels. Also, it leads to uniform distribution of stress under impact. 4 PANI synthesized by conventional chemical polymerization when used as filler and it shows difficulty in mixing due to large particle dimensions. But in case of PPy synthesized by suspension polymerization, the particle size is uniform and small. Hence, lower filler loadings produce good conductivity and the ICAs developed have better impact properties. Similarly, when nanofibres of PANI are used, conductivity improves and impact strength is enhanced. Due to good dimensional stability and small size, CNTs when used as fillers impart good conductivity and improved impact performance to ICAs. Hence, it is established that dimensions and uniformity of filler particles has an appreciable impact on the overall performance of ICAs.