Electrically conductive inks are a critical component in printed electronics. This article examines many of the formulations available and describes their characteristics.
Electrically conductive ink is widely used in industrial printing. Its composition, properties, processing or curing conditions, and cost may vary significantly depending on the application. In general, the ink is a mixture of binder, electrically conductive filler, and solvent with properly formulated rheology. After deposition as a film or as a line, the solvent dries out and both binder and filler are condensed, with filler forming a continuous or semi-continuous electrical conductor.
The conductor allows the current to pass through, or serves as an interconnect contact base to be connected with another conductor path, or serves as shielding against a radio frequency or electromagnetic field. In some applications, this conductor also serves as a protection layer against oxidation or wearing on top of another metal layer. Ink performance is determined by the type of binder, filler, and process, with the mechanical and physical properties dictated by the binder.
Binder
Glass frit is the prevailing binder of choice for applications where high temperature resistance or high reliability is required. The frit is blended with metal particles and solvent. The resulting formulation is called a thick-film ink. After being screen printed onto a ceramic substrate, the ink typically is dried at room temperature for five minutes, then at 150°C for 10 minutes, and finally fired for 30 minutes in air, with about 10 minutes at 850°C. After firing, the glass frit is fused into a glass binder and the metal particles sintered into a continuous metal conductor, which in turn is locked by the fused glass and bonded to the ceramic substrate.
The print resolution is about 125 μm for line width and spacing. However, a resolution of 40 μm may be achieved by using a new technology where the screen-printed ink is further photoimaged. For instance, a printed DuPont Fodel ink is dried for 10 minutes at room temperature, 25 minutes at 80°C, patterned by exposure under a 1-KW UV source (450-1200 mJ/cm2) for 30-75 seconds, spray developed with 1% Na2CO3 at 30°C, rinsed clean with water, then fired under air with a 60-minute cycle with 10 minutes at 850°C. Line/spacing resolution finer than 40 μm is a challenge for the industry.
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