Process Development for Additive Fabrication of Z-Axis Interconnects In Multilayer Circuits

Digital printing technologies are rapidly gaining importance for manufacturing electronic devices with their extremely low fabrication cost and be able to print on flexible and conformal substrates such as polyimide rather than conventional FR-4. One of the technologies, Aerosol Jet, works well when it comes to the wide selection of substrates and materials to be able to print. AJP process utilizes aerodynamic focusing to focus a collimated mist of Nano-particles onto a substrate using an inert carrier gas known as sheath gas, such as nitrogen. To account for the wide array of materials, the process further breaks down to two different types of atomization: UA for low viscosity materials (1 to 5cP) which utilizes ultrasonic waves to atomize; and PA for low and high viscosity materials (1 - 100°CP) which utilizes an inert carrier gas to atomize, thereby suspending the Nano-particles into air which travels through a mist tube and gets deposited. Aerosol-jet printing process requires the control of multiple parameters simultaneously for a fine print such as atomizer flow rate, sheath flow rate, nozzle size, print speed, base temperature, UA current, print height (distance between nozzle exit and the substrate), number of passes. Furthermore, for desired pattern, the print process requires a toolpath file to follow which can be generated from a 2D CAD model, implying there is no specification of the line width. The desired line width is obtained by changing the above-mentioned parameters accordingly, which can be time consuming since each material is of a different viscosity and varying metal and solvent composition. In this paper, a detailed approach for making a 2 layer and 5 layer micro-via using metallized silver Nano-particle ink and a dielectric adhesive is shown. Sintering of the nano-particle ink is one of the most important manufacturing step that governs the electrical and mechanical properties of the 3D printed micro-via. Prior data shows that sintering time and temperature have a big effect of the resistivity, shear load to failure and micro structure of the silver nano particles. In this paper, the most appropriate sintering profile for the conductive lines was found by progressively tracking the resistivity for varied candidate sintering profiles. For the best sintering profile obtained, progression of resistivity, shear load to failure and microstructure growth was monitored at each sintering step.