The application of computerized processing techniques to manipulate and enhance the flow of non-Newtonian viscoelastic thixotropic materials in a production environment

The flow process discussed takes advantage of the complicated, thixotropic nature of an RTV silicone paste using digital processing techniques and computer control to accomplish the desired processing result. The objective was to generate a barrier dam on hybrid circuits in substrate form using a silicone paste. The dam would later be filled in. The paste has to be precisely extruded through a geometrically complicated nozzle to produce the dams on up to 48 separate circuits per substrate simultaneously. The combination of a complicated flow path and a non-Newtonian, thixotropic material produced an inconsistent result. Evaluation of the material indicated that it exhibited stable upper and lower Newtonian regions in viscosity vs. shear rate plots. However, in the region between these two, the viscosity showed a very strong dependence on both shear rate and time at shear. Results showed that process conditions were imposing shear rates on the material which corresponded to viscosities that were highly dependent on both shear rate and time at shear. Since the volume of material extruded was fixed by the geometry of the dam and flow rate through the nozzle restricted by mechanical limitations, the shear rate as a result of flow was essentially fixed for the process.<
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