Authored By:
Steve Thomas
Conductive Inkjet Technology Ltd.
Cambridge, England
Summary
Inkjet printing is of great interest in the field of electronics manufacture because its digital nature negates the need for physical tooling. A wide variety of active and passive materials are currently being investigated for use in inkjet printed electronics. These include semiconductors, light emitters and photovoltaic materials as well as dielectric materials and straight forward conductors. The range of conducting materials that can be printed is somewhat limited by the constraints of inkjet printing. Ideally, particle sizes should be below 1 micron and the viscosities and surface tensions of the fluids need to be tailored to the particular printhead being used. Regardless of these limitations, various technologies are now being implemented in the production of circuit boards, interconnects and antennas by inkjet printing.
The properties of these inkjet printed circuits do not currently mimic traditional PCB materials - in particular, the sheet resistances of inkjet printed materials tend to be significantly higher than traditional copper clad laminate and the minimum feature sizes are somewhat larger than state of the art semi-additive plating. However, inkjet printed circuit technologies are a still finding many applications which are particularly well suited to their properties and the digital nature of their application.
Conclusions
Electronics manufacture by inkjet printing is now a reality with various technologies being available for production. Although carbon inks are unreliable and conducting polymers provide very poor conductivity, nano-particle silver inks and catalytic inks for print and plate conductors provide a viable production route with useful levels of conductivity and mechanical properties. In the case of print and plate conductors, their similarity to traditional PCB materials means that standard downstream processes like ENIG, immersion metals and OSP can be easily applied.
The wide variety of materials properties required for printed conductors rules out the use of desktop-type inkjet printers which have been optimized for aqueous inks in graphics applications. The industrial inkjet printheads which are more compatible with these materials properties tend to operate at lower resolution but do offer the advantages of wider materials compatibility and allow the exploitation of UV curable and solvent-based inks as well as the aqueous materials used in desktop technology. The use of UV curable ink technology offers many advantaged in speed of process, image quality and surface adhesion and is of particular use in reel-to-reel applications.
A significant difference between inkjet printed features and features produced by more traditional photolithographic means is the discrete features sizes obtained from the relatively low resolution of inkjet printing. Although this can appear as severe pixilation in binary inkjet images, the use of greyscale printing can significantly reduce this effect. In certain circumstances the effects of surface tension can be used to smooth edges out completely.
Current commercial Raster Image Processing (RIP) software is optimized for the graphics industry and while it will process greyscale images, the results are designed to best fool the eye rather than fill space most accurately. RIP software for materials deposition is currently under development and will provide the final key in the ability to fully exploit inkjet printing for the rapid and efficient production of electronics
Initially Published in the IPC Proceedings
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