Processing
1. Competitive Advantage in Processing
The processing advantage of PChem’s nanoparticle inks enables more conductive, higher resolution, and in general higher performance devices to be manufactured faster with less metal.
Printable conductors or conductive inks have been used for decades in the manufacture of wafer-based photovoltaic solar cells, windshield defrosters, medical test strips, and membrane touch switches. The screen printable inks used for these applications are generally referred to as polymer thick film (PTF) systems. PTF inks are based on large metal flakes or spherical particles that are dispersed in a solvated polymeric “vehicle” that after the carrier solvents are evaporated sets into a conductive film. The polymer binds the particles to each other and the substrate (see Figure 1 below). The metal particles are coated with oxide, organic ligands, surfactants, and polymeric binder thereby creating gaps in the film. So while these particle are in contact with each other, there exist non-metallic, highly resistive gaps between the particles that are highly resistive junctions leading to volume resistivity significantly greater than the bulk metal used (10-20 times the bulk metal)[1]. Metallic nanoparticles offer the possibility for eliminating these highly resistive gaps by the process of sintering.
Metallic nanoparticles are inherently unstable and will agglomerate without proper stabilization. There are many methods to stabilize metal nanoparticles. Most methods tend to over-stabilize the nanoparticle; that is, the stabilization specie on the surface of the nanoparticle requires a lot of energy to be removed, thus exposing the metal surface of the particle so that it can be utilized. More energy translates to higher temperature and longer processing time.
The patented PChem technology optimizes the stabilization of the nanoparticles and allows milder process conditions (lower temperature and shorter process times) to be utilized. Under the “right” process conditions, the nanoparticles will coalesce when the solvent evaporates, producing a network of particles that then sinter, forming a continuous metallic network or film, like a foil. It is this sintering process that results in significantly improved conductivity. Again, Figure 1 below depicts current paths in conventional PTF ink films compared to metallic nanoparticle ink films.
Figure 1. Comparison of conduction mechanisms in PTF inks and PChem’s nanoparticle inks.
PChem developed its proprietary silver nanoparticles with the primary process goal to enable thermal sintering at or below 150 o C with very short residence times. In fact, PChem’s silver nanoparticle inks can be sintered in seconds at temperatures other nanoparticle and PTF inks would require minutes to cure effectively (e.g., 100-120oC). Figure 2 below shows the superior curing performance of PChem’s nanoparticle compared to the competition. The sintering rate of PChem inks is orders of magnitude faster at the same temperature using the same heat transfer mode.
Figure 2. Comparison of cure kinetics of PChem’s flexographic ink to Competitive inks.
