Research And Application Of Low Temperature Conductive Silver-based Pastes

Silver has the highest conductivity among metal conductors,and has a resistivity of 1.65×10^-8Ω·m.At the same time,due to the stable physical and chemical properties and excellent optical and electrical properties,silver-based conductive pastes have been studied and applied in many fields,such as membrane switches,electroluminescent devices and circuits,and so forth.However,the price and the sintering temperature of silver are relatively high,which cannot meet the requirements for the use of flexible electronic components.Therefore,the development of silver-based conductive pastes with low content of conductive phase fillers and high performance has good economic benefits and significance.In this paper,the micro-sized silver flakes were successfully prepared by liquid phase chemical reduction method,and the surface modification of silver powders was realized by surface modification agent.After the surface modification treatment,the silver powders were used as the conductive phase of the silver pastes.The properties of conductivity,printability,folding endurance,hardness and adhesion of conductive silver pastes cured at relatively low temperature were optimized by mixed binder phases.A low temperature curing conductive silver paste with excellent comprehensive properties was successfully prepared.The main results of this paper are as follows.（1）The micro-sized silver flakes with diameter of 1¹.5μm and thickness of 30nm were successfully prepared by one-step aqueous-phase chemical reduction method.With the increase of the concentration of reducing agent（FeSO₄·7H₂O）,the edges of the silver flakes gradually changed from the irregular curve to regular near linear shape.With the increase of reaction temperature and the content of polyvinylpyrrolidone（PVP）in mixed surfactants（citric acid,PVP）,the particle size of silver flakes decreases and the dispersion performance is optimized.The results show that the reaction parameters have great influence on the size and morphology of silver powders.（2）The micro-sized silver flakes prepared by chemical reduction method were successfully modified by strong electrolyte（sodium nitrate,sodium chloride）and organic compounds（succinic acid,1H-Benzotriazole）,and a low temperature curing conductive silver paste with better properties was prepared.The results show that the surface modification agent can replace the surfactant adsorbed on silver particles by ligand exchange,and the strong electrolyte aqueous solution can cause oxidative corrosion on the surface of silver powders and promote the dissolution of silver oxide on the surface.The size of silver powders treated by mixed surface modifiers（sodium nitrate and succinic acid）decreases with the increase of their concentrations,and when the concentrations are 1mol/L,the silver particles are connected to each other.Based on the above results,a self-sintering model was proposed.When the concentrations of sodium nitrate and succinic acid are 1 mol/L and the content of silver powders is 48 wt.%,the obtained silver paste has a resistivity of 1.08×10^-6Ω·m,and has excellent folding endurance and adhesion.（3）Low temperature curing conductive silver pastes with excellent properties were successfully prepared by different kind of binder phases,which includes polyester resin（PE）,thermoplastic polyurethanes（TPU）and polymethyl methacrylate（PMMA）.The results show that the conductivity,printability and folding endurance of silver pastes can be significantly improved by increasing the content of TPU.When the mass ratio of PE and TPU is 1:0.6,the resistivity of printed silver line is 1.08×10^-6Ω·m,and the change rate of resistivity after bending resistance test is 7.28%.With the increase of PMMA content,the hardness of silver paste is remarkably optimized,but the conductivity and printability are both reduced.When the mass ratio of PE and PMMA is 1:0.8,the hardness of the silver paste is63.6.In addition,when PE,TPU and PMMA are used as the mixed binder phases of silver pastes and the mass ratio is 1:0.6:0.3,the comprehensive properties of the silver pastes are optimal including electrical resistivity(2.62×10^-6Ω·m),folding endurance（change rate of resistivity is 39.16%）,hardness（35.46）,adhesion（5 B）.