Modeling of the rotary-screw-driven dispensing process

Abstract

Fluid dispensing is a process used to deliver fluid materials to targets such as substrates, boards, or work-pieces in a controlled manner. This process has been widely used in electronic packaging industry for such processes as integrated circuit encapsulation (ICE) and surface mount technology (SMT). The most important parameters needed to be controlled in this process are the flow rate of fluid dispensed and the profile of fluid formed on a target. The modeling and control of such a process involves different engineering disciplines including mechanical, control, software/hardware, and material sciences. The present research is aimed to carry out a comprehensive study on the modeling of the rotary-screw dispensing system, in which a motor-driven screw is used to deliver fluid materials. At first, characterization of the flow behavior of fluids used in the electronic packaging industry is addressed. Under the assumption that the pressure applied to feed the fluid material has reached its steady state value, a steady state model is then developed to represent the flow rate of fluid dispensed in the rotary-screw dispensing process. On this basis, by taking into account the fluid compressibility and the fluid inertia, a dynamic model is developed to represent the dynamics of the flow rate, which is critical if the amount of fluid required to dispense is very small. Experiments conducted on a typical commercial dispensing system of DS-500 (provided by Assembly Automation Limited, Hong Kong) were used to characterize the flow behavior of the fluid dispensed based on the model developed. The method of identifying the flow behavior from dispensing experiments, rather than a rheometer, allowed us to eliminate the massive measurements needed in the use of rheometer. To validate the steady state model, simulations were carried out in Matlab and the results were then compared with the experimental results obtained. It is shown that the simulation results are in close agreement with the experimental results. Based on the dynamic model developed in this study, simulations were carried out to investigate the effects of operational parameters, such as temperature and fluid properties, on the flow rate of the fluid dispensed. In addition, the inconsistency in the fluid amount dispensed was also investigated by using the dynamical model. It has been shown that for dispensing small amounts of fluid, the dynamics of the flow rate dominates the process and that in this situation, the amount dispensed can be predicted by using the dynamic model developed and, in contrast, the use of the steady state model, which is commonly adopted in industry, can result in a large error in the model prediction. Based on the dynamic model, a new approach is developed to integrate the model into the design of fluid dispensing system. This approach could be used not only to evaluate the existing dispensing systems, but also to design new dispensing systems.