Study On Surface And Interface In Silver Filled Electrically Conductive Adhesives

Electrically conductive adhesives (ECAs), an alternative for traditional tin/lead (Sn/Pb) solders, have gained much attention owing to their environmentally friendly profits, lower temperature processing, finer pitch printing and more flexible and simpler processing over traditional tin/lead (Sn/Pb) solders. In addition, compared with lead-free solders, electrically conductive adhesive systems exhibit greater flexibility, creep resistance and energy damping. At the same time, ECAs have drawbacks such as high volume resistivity, unstable contact resistance under environmental aging conditions and limited impact resistance of the adhesive interconnections. In this paper, silver conductive fillers were prepared and characterized, the surface behavior of bright silver flakes and interfacial behavior in electrically conductive adhesives were studied, and methods for improving the electrical and mechanical properties of ECAs were also discussed.Spherical or sphere-like silver powders were prepared by a reduction process of the silver nitrate solution with hydrazine hydrate as the reducing agent. Influences of reaction condition and environmental parameters, on the size and shape properties of silver powders were studied, such as concentration of reactants, weight ratio of reducing agent to AgNO3, reaction temperature, adding speed of reactant, agitating speed and weight ratio of dispersant PVP to AgNO3 The optimum reaction conditions were determined: reaction temperature is room tempeture, concentration of AgNO3 is 0.35 mol/L, concentration of reductant is 2.1mol/L, adding time is 40min, agitating speed is 1000rpm, mole ratio of reducing agent to AgNO3 is 6:l , and weight ratio of dispersant PVP to AgNO3 is 0.9. Bright silver flakes were obtained by ball milling as prepared silver powders.Thermal properties of bright silver flakes before and after treatment by different solvents and chemicals were characterized by Differential Scanning Calorimetry (DSC) technique. Fourier transform infrared spectroscopy (FT-IR) was used to characterize the chemical structures of oleic acid and the trace amount of organic lubricants on the silver flakes surface. The morphology of silver flakes before and after treatment by a mixture of alcohol and diluted acid was observed by SEM. Elemental compositions of silver flakes before and after treatment were analyzed by EDAX. It shows that the organic lubricant on the surface of bright silver flakes is not the oleic acid physisorbed on the surface of silver flakes, but a chemisorbed lubricant layer, salt of silver with oleic formed during milling process. Efficiency of different solvents and chemicals on the removal of lubricant on the surface of silver flakes is different. To the best of our knowledge, it was the first time for us to find a method to fully remove the lubricant on the surface of silver flakes. A mixture of alcohol and diluted acid can fully remove the chemisorbed lubricant. The mechanism of the removal is that the H+ in the mixture of alcohol and acid firstly reacts with the salt, resulting in oleic acid and an inorganic silver salt, and the oleic dissolves in alcohol and the inorganic silver salt dissolves in water. The reaction processes until the full consumption of the salt, the chemisorbed lubricant on the surface of bright silver flakes.It was found that with the full removal of lubricants on the surface of silver flakes, the electrical conductivity is improved at the same weight filler loading. Compatibility of silver flakes filled epoxy system can be improved by adding coupling agents. It was also found that the added coupling agents have different effects on the electrical, aging and lap shear properties of ECAs. According to this study, the use of silane coupling agent KH-570 had a significant electrical conductivity improvement and lap shear strength increase before and after the aging test. But for the titanate coupling agent NDZ-401, the properties of the ECA have no significant change. Scanning electron microscopy studies revealed a strong adhesion/bonding between the filler and the matrix for coupling agent added ECAs.Adding some low melting point alloy in the fillers is helpful for the improvement of electrically conductivity of ECAs, in this study, the proper weight ratio of LMPAs in fillers is about 20%. The LMPA fillers melt and form metallurgical interconnections between the Ag flakes during the curing of ECAs.