The Effect Of Silver Nanostructure On Electrical Performance Of Conductive Adhesives

Electrical conductive adhesives(ECAs) are the key materials for the electronic packing, electrodes and circuit/device interconnect which are used in various fields of electronics, such as the surface mount technology and sensors. As electronic components are driven towards miniaturization, multifunctions and flexibility, the metallic nanomaterials as conductive fillers are intensively studied both in domestic and abroad. The silver nanoparticles filled in conductive pastes have always been a focus of research in this field. The positive or negative effects of silver nanoparticles as filler in conductive pastes are not been answered yet, and there are issues needed for further clarification. In this thesis, spherical silver nanoparticles, silver nanowires and silver nanoplates were prepared by the traditional liquid reduction method and solvothermal method, respectivley. The isotropically conductive adhesives(ICAs) filled with the home-made nano-silver alone or co-filled with micro-sized silver flakes as hybrid fillers were fabricated. Furthermore, the electrical properties of ICAs were investigated in term of the morphology, loading ratio and surface properties and sintering behaviors of silver nanoparticles, and the mechanism of the conductive behavior was discussed.Firstly, silver nanoparticles with size 30 to 50 nm were synthesized by tranditional liquid phase reduction method. Silver nanowires with an average diameter 450 nm, length 20 to 70 ?m and silver nanoplates with diameter 100 to 300 nm were synthesized by solvothermal method. Glutaric acid was used as nano-silver surface modification agent in ethanol solution and the different aspect of nano-silver after treatment on the sintering behavior were analyzed. Morphologies and sintering behaviors of the latter two types of nano-silvers were not greatly changed by glutaric acid. The carboxylic acid groups on the surface of nanosilver are not physical adsorption but chemical bonding. The sintering temperature of these three nano-silvers decreased after surface treatment.Secondly, the effects of different curing procedures on the electrical properties of ICAs were investigated. The ICAs conductivity was improved by low temperature and long time curing. There influence of high curing temperature is very strong for the low silver content of ICAs. The thermal post-heating treatment processes had no obvious effect on the electrical properties and microstructures of silver conductive adhesives which had been well cured.Finally, the ECAs filled with nano-silver alone or co-filled with micro-sized silver flakes as hybrid fillers were studied. The ECAs curing behavior was optimized where the formation time of conductive path was shorted and the curing temperature was declined by silver nanowires as conductive filler. When the concent of silver nanowires was 35 wt%, the volume resistivity of ECAs cured at 168 ℃ reached 9.48×10-4 Ω·cm, and cured at 300 ℃ was down to 8.21×10-5 Ω·cm. Incorporating silver nanoparticles into ECAs filled with silver nanowires, the formation time of conductive network was extented, and resistance was increased. When the content of silver nanowires and nanoparticles was 65 wt%, the resistivity of ECAs cured at 180 ℃ only reached 1.37×10-4 Ω·cm, and cured at 300 ℃ reached 9.60×10-5 Ω·cm. But its conductivity was further improved with higher curing temperature and optimized surface treatment. While silver nanowires were added into the ECAs filled with micro-sized silver flakes, on time of conductive network was shorted and the curing temperature was decreased. There was a significantly impact of higher temperature on the volume resistivity. When the content of silver was 75 wt%, the resistivity of conductive adhesives cured at 250 ℃ reached 6.72×10-5 Ω·cm, which is only 0.5 resistivity of the ECAs filled 75 wt% micro-sized silver. ECAs volume resistivity was increased at low temperature curing by using silver nanoparticles or nanoplates and micro-sized silver flakes as conductive filler. Its resistance could be reduced by using glutaric acid and high curing temperature. On the other hand, the high temperature curing can make the silver nanoparticles inter-connected to form conductive paths perfectly, thereby offering better conductivity.