Mechanics of cracking and delamination in 3D-printed microelectronic films

Integrity of thin films with micron-sized thickness is of high importance to the microelectronics industry. The mechanisms and mechanics of the cracking of such films made by conventional methods such as chemical and/or physical vapor deposition are well understood. Printed electronic techniques have recently emerged that allow films of various functional materials to be fabricated on-demand via nanoparticle printing followed by sintering. The failure mechanisms in such cases, however, are strongly influenced by the fabrication technique and the material used, but this dependence is not well understood. In this research, we study the mechanisms of cracking and delamination of Aerosol Jet (AJ) 3D nano-printed gold films on ceramic substrates. The layer-by-layer film fabrication allows the determination of effect of each process step (printing, drying, and sintering) on film stress and failure. We show that film cracking occurs only during the film drying phase (i.e., immediately following printing), and is relatively independent of the underlying substrate. We also show that the most significant factor affecting cracking is the glass transition temperature (T_g) of the binder in the printed film (which is removed during sintering but is present during printing and drying); with drying-induced capillary stress giving rise to the classic ‘mud-cracking’. In other words, if printing is done close to the T_g of the binder, the system becomes unusually strain tolerant and cracking can be avoided. As expected, the delamination is found to be a function of the film-substrate interface interaction. Finally, we develop failure mechanism maps for printed electronic films and determine the thickness below which reliable films can be fabricated. This work lays the foundation of engineering strategies for the reliable fabrication of electronic films via 3D printing.