Substrates
The main substrates for PE are polymer films, paper/nano paper, glass/flexible glass, wafers, metals and textiles. The substrates used for graphic printings are less than ideal for printed electronic applications, due to their higher roughness in comparison to polymer films. However, higher thermal stability of paper, in comparison to some of the films, makes it more attractive for use. Therefore, there is a need for more engineered paper substrates, specifically for PE applications.
Surface smoothness and substrate cleanliness are essential for PE. Unevenness can cause the surfaces of printed conductive traces to be uneven. This could create breaks and shortages in the circuitry. The substrates have to be thermally and dimensionally stable, and chemically compatible, in order to withstand the printing and sintering processes of manufacturing. The glass transition temperature (Tg) of the substrate—the temperature at which the substrate starts deforming— has to be compatible with the processes as well.
Printing Method
Flexographic printing is used for flexible packaging significantly more than gravure in North America and South America (see Figure 4). Therefore, the purpose of this project is to formulate a water based nano Ni conductive ink for flexo printing.
In general, inks are categorized according to their solvent component: water based, solvent based, UV/EB based, soy based, etc. Solvent
based inks often contain enough organic solvents and volatile organic compounds (VOCs) to be a concern environmentally. Water based inks can also contain small amounts of VOCs with fewer environmental concerns. They are less flammable than solvent based inks, therefore they are easier to store and use. Water based inks have advantages over solvent types, like their low evaporation rate, which allows them to be more stable on the press without drying. They supply constant viscosity in the ink fountain and doctor blade chamber.
The basic components of a conductive ink are functional pigments that are used to give the needed electrical property; binders to help ink to stick to the substrate; additives to add the needed performance of inks, such as plasticizers, waxes, wetting agents, defoaming agents, buffers and vehicle/carrier that are used to deliver ink onto the substrate. However, in functional ink formulation, the use of binders and additives should be as minimal as possible, since they negatively affect the functional properties.
Sintering Methods
Most of the conductive inks must be sintered after they are printed on a substrate. This can be accomplished by means of multiple processes like drying, curing, reactive chemistry and annealing. Curing consists of heating the inks to volatilize vehicle components. This allows the particles to contact each other. Sintering is the process of growing the particle grain that takes place during curing. It impacts the material structure as ink particles change the grain. It impacts the conductivity as the ink layer becomes more interconnected upon formation of particle bridges, and removal of solvents and resins.
The main types of sintering are microwave heating, electrical, spark plasma, laser and photonic. The method used in this research was photonic curing—a high temperature thermal processing of thin films using pulsed light from a flash lamp on substrates—courtesy of a NovaCentrix PulseForge 1200. Photonic curing allows material to be cured faster than traditional thermal processes, without raising the temperature of the substrate or subsequent printed layers. Photonic energy is pulsed into the sample at about 1 millisecond intervals at the 200-nm. to 1,500-nm. output range. Figure 5 shows an example of photonically cured copper based ink. The system is able to cure 0.5-μ. to 5-μ. ink film thicknesses. Sheet resistances as low as 20 milliohms/ sq. for silver and 150 milliohms/sq. for copper are reported for photonic curing.
Phase 1: Rheological Analysis
Rheology is defined as the science of investigating flow and deformation of matters. The Advanced Dynamic Stress Rheometer AR2000 was used to study the viscosity behaviors of inks in response to shear and temperature. Four different inks were used for the experiment:
- Sun Chemical water based flexographic regular printing ink
- NovaCentrix HPN-DEV conductive nickel flake paste for screen printing
- DuPont CB200 conductor copper paste ink
- Sun Chemical CXT-0346 flexographic conductive silver ink
A 20-mm. parallel plate was selected for the device geometry, as it is usually used for high viscosity, high solid materials containing large particles. A shear rate was applied starting from 0.1 to 500 s-1 at 23 degrees Celsius for steady a state flow test. A Peltier heating plate was used to raise the ink temperature from 20 degrees Celsius to 60 degrees Celsius at 100 s-1 constant shear rate during evaluation of the temperature effect on flow behavior.