| Surface mount technology has many useful features, such as component-intensive, low cost, easy to component integration, product stability, easy to automate production and so on. It is a core of electronics manufacturing processs that elecronic components are fixed and encapsulated in the circuit board, of which solder paste is the most widely used. Solder paste dispensed on the circuit board is the first step of the entire SMT production process and is the most critical and important simultaneously, which affects the entire production line efficiency and performance and reliability. There are many defects in the traditional screen solder paste printing process, for example, too much or too little of dispense amount, uneven amount, printing graphic dislocation, depression, breakpoint, graphics iciles, slump, pollution, solder balls bridging and other defects. Piezoelectronic driven droplet ejection technology has high accuracy and fast response. Combined droplet jet dispense technology, this paper proposes solder paste jet dispensor driven by a piezoelcetronic stack. In the paper, the jet theory fo two-phase flow solid-liquid is analyzed and the various factors that affects solder paste jetting is studied. According to the theoretical analysis, prototype is manufactured and many testing is done and the main contents of the paper are as follows:1. the non-contact droplets forming principle analysis of solder pasteFrom solder paste composition, it can be seemed that solder paste is a liquid-solid two phase mixture. Compared with the ordinary glue paste, there is soft metal alloy materials in the solder paste. So there is no contact with the striker and the nozzle and the gap has to exist when solder paste is jetted. The forming principle and process of solder paste jetting is analyzed. And the paper describes the use of piezoelectric material that can realizes higher frequency and higher precision solder paste droplet jetting.2. The basic characteristic of solder paste flowFrom the two-phase fluid flow pattern to start, considering the concentration of solder paste, solder paste flow body can be seen as a homogeneous Bingham fluid flow. Reynolds number of the homogeneous solid-liquid two-phase Bingham plastic flow body is analyzed, from which it can be determined that the motion is laminar flow when solder paste flows in the very small wee pipeline. On the basis of shear stress distribution in the diameter direction when homogeneous solid-liquid two-phase fluid flows in the pipeline, the velocity distribution in the diameter direction can be obtained when solder paste flows in the pipeline. Resistance equation of solder paste flow is derived and the relationship between the resistane and the average flow velocity, pipe diameter, limited viscosity is also discussed.3. fluid dynamics theory and simulation analysis of solder paste droplet jettingSolder paste droplet jetting states of three work process stage are analyzed. The relationship of the impact inertia force, the tensile viscous forces and the surface tension is obtained from the analysis of solder paste droplets strees state, from which the solder paste droplet ejected condition is achived. According to the solder paste jetting theory and the structure proposed by the paper, different structures are simulated using FLUENT, which influences the jetting results. By the simulation results and focus revelation principle, a kind of striker with concave spherical structure is proposed in the paper. Different radii of concave spherical of the striker are simulated with mixture model using FLUENT, from which velocity of the nozzle center of different structure can be obtained and the optimal radius is chosen at the particular scenario of this article. Based on the distribution of pressure on the interior of the nozzle, the nozzle channel structure is optimized. With this structure, different factors for example the nozzle diameter, motion time of the striker and the glue pressure on solder droplet ejection are simulated.4. Dynamics analysis of the droplets jetting structure for solder pasteWorking principle and the main structure of the microjet valve driven by piezostack are introduced in the paper. Firstly, the lever magnification structure is checked, from which we can verify its reasonable. Secondly, the stability of the striker is analyzed, from which its stablity is inversely proportional to the length of the striker. Finally, the entire leam amplification system is simulated with Matlab and Ansys and it is designed resonable.5. Performance analysis of the microjet system driven by piezostackKinds of testing instruments and equipments needed in the solder paste droplet injection system are prepaired. The output characteristic of the the amplified structure and the striker is tested for static test and dynamic test, from which we can get the displacement output of the striker. And compared with the displacement of the piezostack at different voltages, actual structural displacement magnification is obtained. When the driven voltage is 100 V, the displacement output of the piezostack is 31.61μm and that of the striker reaches 202.464μm. In the dynamic test, we can observe the lifted time and fall time of the striker, from which we can deduce that the maximum theoretical frequency is 769 Hz.6. Experiment testing of the solder paste jetting system driven by piezostackChange the factor, for example, the gap between the striker and the nozzle, the driven voltage, the valve opening time, the driving gas pressure, and different solder paste jetting results will present. The experiment tests show that the gap between the stiker and the nozzle is smaller, the greater of the jetting strength and the the greater of the injeted droplet quality. The larger of the driving voltage, the greater quality will be got. When the striker totally rises to the highest point, the influence of the valve opening time will get much smaller. Similarly, the influence of the driving pressure is also limited. But if the long-term stability of the jetting, appropriate parameters will be needed. When driving voltage 80 V, nozzle 0.1mm, valve opening time 2.0ms, driving pressure 0.6MPa, the gap between the striker and the nozzle 10μm, the solder paste droplet ejection repeatability is tested and the results show that the repeatability error is within 3.2%.
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