Laser-assisted Direct Write for aerospace applications

Abstract

Particulate silver inks are finding increasing use in the direct manufacture of electronic circuitry, for example in Direct Write (DW). Laser curing of these inks as opposed to traditional oven-curing methods is advantageous for reasons of speed and the protection of nearby heat-sensitive components, and particularly for the construction of circuitry on conformal surfaces and ‘on the fly’ modification or production. To fully appreciate the benefits provided by using a laser-based process, it is important that the final-cured component has electrical and physical properties that are comparable to, if not better than, those that are produced using the conventional technique. Currently, in some single pass curing techniques, cavities are formed in the track due to rapid boiling off of solvents that leads to a higher resistivity and surface roughness than that of oven-cured tracks. While mass loss is similar, overall density is decreased through porosity of the tracks. By going from a single pass curing process to a multiple pass process and controlling the power regime, it is possible to improve the track quality by allowing the gradual release of solvent vapour from the ink and achieve lower resistivities than that of oven-cured samples and a surface roughness that is comparable. This incremental process improves settling of the silver particulates leading to a resistance of the cured inks that is lower than that achievable through oven curing. This creates the possibility of fabricating a surface-mounted or -embedded damage sensor without thermally affecting the substrate. The effect of substrate type thermal conductivity on the laser curing speed of silver inks is investigated. It was found that a noticeable factor in determining the resulting resistance of the conductive track was the substrate thermal conductivity. Both CO2 and Er:YAG laser radiation was investigated. Although CO2 lasers are in widespread use, the increasing demand for portability and ease of integration into existing DW systems favours fibre delivered laser radiation such as that from an Er:YAG fibre laser. This source was also investigated here to determine if fibre delivered processing could be achieved.