Mechanism And Control Method Of Contact Micro-dispensing High-viscosity Adhesive

The micro-dispensing of high-viscosity adhesives(>1 Pa·s)is less affected with the material and structural size of parts.It has many advantages including no requirement of high temperature and pressure,and weak interior.stress.Thus,it is suitable for connecting micro parts and has a wide range of application requirements in many fields such as microassembly.With the development of devices towards integration and miniaturization,higher requirements have been placed on the volume and repeat accuracy of micro-dispensed adhesives.Restricted by the high-viscosity flow resistance and physicochemical characteristics of adhesives,the currently frequently-used injection-based liquid dispensing method cannot meet the micro-dispensing requirements of high-viscosity adhesives.Moreover,the research on the micromanipulation mechanism of high-viscosity liquid is not enough.The theoretical model that describes the micro transfer and flow of high-viscosity liquid is lacking.It is urgent to conduct in-depth research on the theoretical model and develop a new micro-dispensing method for high-viscosity adhesives.In response to the key issues in the micro-dispensing of high-viscosity adhesives,the following researches have been carried out:(1)In the micro-dispensing problem of high-viscosity adhesives caused by injection-based liquid dispensing method,a method for pre-controlling liquid dispensing that is controlled by the initial distance between microinjection needle-tip and liquid-bearing substrate was proposed.Combined with the constitutive model of non-Newtonian fluid,dynamic mesh and interface tracking,the calculation method of contact micro-dispensing high-viscosity adhesives was established.Then,the micro-dispensing process including the extruding,stretching and breaking was calculated,and the dispensing experiment device controlled by the initial distance was set up.According to the device,the effect of initial distance(45～145 μm)on the dispensing process of adhesives with nano-liter level was experimentally studied.Combining theory and experiment,it was found that the non-linear relationship between the variation of initial distance and the amount of dispensed adhesives is the result of surface tension and viscous flow resistance dominating at different stages.According to the research results,combined with laser triangulation measurement and image feature extraction technology,the precise automatic micro-dispensing equipment controlled with the initial distance were developed for the adhesive seal of micro inertial sensors.(2)Based on the microfluidic template technology,a micro droplet-patterning method was established.A microfluidic chip containing the predetermined microdroplet patterns was manufactured.Combining microfluidic injection and silanization,the anti-adhesive boundaries of polygonal patterns were obtained.Based on the anti-adhesive properties of boundaries,combined with the energy conservation of droplets spreading and viscous dissipation,the critical volume model for controlling the spread of transferred droplets was established.Then,the criterions were set to confirm the droplet-volume limitation of overflowing across anti-adhesive boundaries.Using ultraviolet curable adhesive and negative photoresist as samples,the effects of apparent viscosity,surface tension,polarity and polygonal geometry on the spreading of dispensed droplets were studied.When viscous force and surface tension dominated the droplets movement,it was found that the anti-adhesive boundaries can effectively restrain the spreading of droplets.Moreover,smaller area,more edges of polygon,and more non-polar groups of adhesive were helpful to improve the droplet-patterning capability in the polygons.It provided new ideas for shape control of micro adhesive droplets.(3)To eliminate the influence of flow resistance in the injection-based liquid dispensing method,a method of micro-transfer-printing liquid based on the force-feedback of squeeze-film adhesive damping that is suitable for micro-dispensing of high-viscosity adhesives was proposed.Based on the squeezing flow of viscous fluid between two parallel plates,considering the influence of surface tension,a mathematical model that describes the variation of adhesive droplets sizes and squeeze-film force between transfer stamp and substrate was established in the squeezing process.It provided a theoretical basis for analyzing the mapping relationship between the squeeze-film force and the transferred droplets amount.Combining the loading of Coulomb electrostatic force and micro force-feedback,the method and experimental device that are used for micro-dispensing of liquid had been formed.On this basis,the threshold of squeeze-film force(1-5 mN)was selected to trigger the stretching and breaking of liquid-bridge.The effect of squeeze-film force threshold on the transferred amount was analyzed.It was found that the decrease of squeeze-film force threshold is approximately linear to the decrease of transferred amount.Consequently,the high-viscosity adhesive droplets at a level of 10 picoliters were realized.