Printing methods are typically cost effective for creating precision electronic circuits. Traditional printed circuit board manufacturing use chemical etching of copper clad substrates to create circuit patterns. This subtractive method is excellent for mass production. However, additive deposition of conductive materials in many cases is more suitable for rapid prototyping and customization of circuit designs.
Conductive particles such as silver flake or nano particles can be mixed into fluids and printed with conventional printing techniques. For large particles screen printing and coating processes are most suitable. Whereas, nano particles can be deposited using piezo inkjet print heads.
Nano particles can be fabricated such that low temperature sintering can occur. Electrical conductivity can be significantly increased after sintering particles to create a conductive network with properties similar to solid metal. Silver nano particles can be functionalized so that sintering can occur at room temperatures. Room temperature curing of inks is a big advantage of inkjet printing.
Inkjet printers often use a cartridge for holding ink. Some cartridges also have integrated printheads. Other printers such as Brother desktop printers, only have an ink reservoir in the cartridge, and the print head is integrated separately in the printer.
The advantage of ink cartridges is the ease of ink replacement without directly handling liquids. The cartridge can be inserted and remove without concern for spilling or contamination. Also the shelf life of the ink is longer in the sealed cartridge. A cartridge is often stable for many months even installed in the printer. For long term storage refrigeration can be used to further reduce evaporation.
Basic inkjet printing of conductive inks requires a suitable ink viscosity, ink formulation, curing strategy, and compatible substrate.
- A viscosity less than 10cP is typically required for inkjet print heads like micro piezo common in desktop printers.
- Aqueous ink formulations are common for desktop printers. Additional polymers and functionalization are typically added to properly disperse conductive particles.
- Room temperature fast air cure is most typical curing method for desktop printers.
- Not all substrates can be used without special surface treatment. Inkjet inks are typically aqueous. Therefore, some absorption of water into the substrate is necessary to properly cure the ink. Adhesion of the remaining solids is also determined by substrate surface properties.
Advanced inkjet printing can involve multiple materials, special substrate surface treatment, multi-stage curing such as convection, infrared, ultraviolet, high intensity light, and laser processes.
- Other materials such as dielectrics, adhesives, and other functional layers can also be printed. These materials enable more sophisticated circuit design.
- Substrates can be coated with ink reception layers or plasma treated for improved ink curing and adhesion. Some engineered substrates can be purchased for direct printing. Other techniques are performed as a separate process immediately before printing.
- Multi-stage curing enables the use of more diverse inks and solids. The choice of curing depends on the particle combination of ink and solid as wells as the compatibility and temperature limit of the substrate. Some inks need additional energy during curing to eliminate the volatile liquid. Other inks require additional energy to promote sintering of solids. Some substrates like polyimide are stable at higher temperatures. While, PET, polycarbonate, and even paper substrates have more limited temperature constraints. Some engineered substrates have sensitive coatings or laminate films that can be damaged by high energy so high temperature curing inks are not possible.