Choosing the right conductive ink depends on a lot of different factors, including the equipment that’s available, the required performance, and the cost budget for the device being printed.
By Chris Wargo
Silver nanoparticles dispersed in an aqueous medium tend to have a slightly lower destabilization threshold. This allows the aqueous inks to cure faster than similarly sized nanoparticles stabilized in an organic medium, while still maintaining acceptable shelf life characteristics. As a result of the faster curing kinetics, aqueous nano-inks can be better suited to high speed printing processes than other conductive inks.
The thickness of a printed conductor is determined by several factors. The print method, particle size, ink rheology, and solids loading all determine the resulting dry-film thickness (DFT) of the print after drying/curing. The thickness of PTF inks tends to be much greater than that of nanoparticle inks deposited with equivalent printing conditions. This is the result of two factors: solids loading and particle size.
In general, PTF inks require a greater solids loading than nano-inks. This is because the ratio of conductive particle to binder must remain high for the cured film to maintain conductivity. There are also rheological issues that occur when diluting PTF inks with volatile solvents which also cause the inks to suffer in terms of their electrical properties if diluted too much. Particle size is also a limiting factor for minimum DFT. To be conductive, a flake-based ink must maintain several layers of flakes to ensure a properly formed conductive pathway. With flake thickness on the order of 1 μm, it becomes impossible to achieve a printed conductor less than a few microns thick.
In contrast, nanoparticle inks suffer from the inability to deposit very thick films. Because of the small particle size and lower solids loading required to maintain stable suspensions with a printable rheology, nanoparticle inks have difficulty depositing cured films more than 1 μm with flexography or 5 μm thick with screen printing. If the conductor interfaces with low-impedance circuitry requiring very low conductor resistances, then PTF is the better suited material because it can deliver a lower sheet resistance resulting from extremely thick laydowns. If thickness/resistance requirements are less critical, then nanoparticle inks can be used to print thinner traces that still meet the design specifications while saving material costs.
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