Modeling of Spreading Behavior of UV-Curable Dielectric Ink from its Rheological Characteristics

In the 2.5D packages, panel-level redistribution layer interposers without through silicon via has been drawn great attention due to cost-effectiveness and productivity. The conventional production method requires photo-imageable dielectric patterning, which involves high process cost and material consumption due to the exposure process. As a solution, inkjet printing using UV-curable ink has the advantages of freedom of design, cost-effectiveness, and mass customization, as well as being able to immediately form a complex 3D profile at room temperature on a micro-scale. In order to obtain a target radius of the 3D feature using inkjet, it is important to analyze the droplet spreading behavior of the ink. The ink applied to inkjet should be a low-viscosity liquid. In the past, it was difficult to measure spreading of low-viscosity liquids because of their fast spreading speed, but with the recent development of measuring methods, it has become possible to measure them. However, there is still a lack of research on spreading of low-viscosity-organic liquids, which led to this study. In this work, we developed the 3D profile measurement system using a contact angle meter and an ultra-high-speed camera, followed by the calculation of the tendency of the droplet profile. The variable was controlled by the viscosity and the contact angle of a dielectric ink which can be hardened by UV irradiation. The measured droplet spreading was converted to 1 pl, the volume ratio of the inkjet printing scale, and the spreading time was obtained by adjusting the capillary time scale and converted into the time-dependent behavior of the drop jetted from the actual inkjet nozzle. We have established a calculation model for diameter through regression analysis of droplet spreading of several cases, which can be largely divided into inertial stage and viscous stage in the order of time; 1) In the inertial stage, the potential and the kinetic energy of the drop are converted to the surface energy, and the surface tension acts as the driving force, 2) The viscous stage is led by the viscous dissipation of the wedge flow around the contact line of the droplet, and the surface energy acts as the driving force. The constant of the trend line of spreading for each stage was regressed as a function of the viscosity and the contact angle of the ink. Therefore, based on this model, the spreading behavior of the droplet after jetting can be predicted through the analysis with the solution properties of the ink.