tension. Hence, inkjet requires low viscosity inks, from 1 to 40 mPa·s.
The inks are available commercially or they can be synthesized in the
laboratory. Conductive inks used in inkjet-printing are therefore metallic
nanoparticles based inks, in a colloidal suspension, with a range of
particle size usually lower than 100 nm and a solid content below
50 wt.%. In order for them to become conductive, the sintering needs
to take place after printing. In addition, during printing a pre-sintering
step can be done, in order to increase solvent evaporation and promote
the coalescence of the nanoparticles, forming the conductive structures.
The thermal sintering [10] of metallic nanoparticles based inks is the
most common and simplest sintering technique. It usually requires
only an oven, with temperatures between 100 °C and 300 °C and long
sintering periods, over 1 h. The exposure to high temperatures for
long periods can result in some problems, such as microcracking [11],
due to substrate shrinkage. The microcracks will result in disruptive
and non-conductive printed structures. Another common effect is a
coffee-ring effect [11] where the sintering process starts at the edges
of the printed line, where the line is thinner and evaporation process
is faster. Thus, the movement of the nanoparticles towards the edges
produces a depression in the middle. This results in a reduction of the
thickness of the printed line and a possible reduction in conductivity.
All these effects can be controlled, for instance, by reducing the sintering
temperature and increasing the sintering time, in order to prevent substrate
shrinkage, control the solvent evaporation speed or using other
substrate materials with higher working temperatures