Digital fineline printing is one of the most important developments in additive electronics. Inkjet itself has come very far with excellent progress even towards R2R industrialization. However, inkjet has two limitations as a technology: (1) limited resolution, (2) limited ink viscosity range, and (3) limited throughput per print head. Here, we highly several digital and hybrid technologies that can overcome these limits.
XTPL- microdispensing with innovative non-Newtonian highly-loaded NP pastes
The first is the microdispensing technology developed by XTPL with nozzle diameters in the range of 0.5-12um. It demonstrates a unique combination of ultrafine line (few micron) 'digital' printing on flat and non-flat surfaces AND highly conducting (40% Ag bulk?) highly viscous nanoparticle (Ag, Cu, Au) pastes. Thus, this technology advances the art not just by extenting the resolution of digital printing beyond what inkjet achieves but also by enabling far more conductive and highly loaded conductive pastes.
The innovation here is not just the microdispensing machine, but also the unique non-Newotonian highly-loaded nanoparticle pastes. The AgNP pastes have high loading (>85 wt% in some cases), small particles (45nm), and are in ethylene glycol solvents. The pastes require relatively high sintering temperatures (250-300C) but offer high conductivity, e.g., 4.2 uOhm.cm for the ink with 80wt% loading.
In the first slide below you can learn about the microdispensing machine itself. It currently has a substrate size of around 50 mm x 50mm. The max print speed is around 10mm/s. The XY motor and Z motor controllers have accuracies of 2um and 0.5um, respectively.
In the next slide, we can see the types of structures that can be printed. In the benchmarking chart, it is demonstrated that they can achieve 2um linewidths with >40% bulk Ag conductivity, which outperforms other reports in the literature. In the right, one can see the types of structures being printed, showing that the printed structure demonstrate high aspect ratios.
Given the highly loaded nature of the pastes as well as the narrow nozzle, fear of constant clogging exists. In the next slide, it is shown that the non-Newtonian pastes can be printed through a 2.5um nozzle for long periods of periods, demonstrating the stability of the process. In the final slide, we showcase the structures which can be printed.
Electrohydrodynamic Printing: Breaking the Limits of Inkjet
EHD is one of the most exicintg developments in the additive electronics field. It offers several key advantages over traditional inkjet: (1) it can achieve lower resolutions beyond current capabilities of inkjet, (2) can handle a wider range of ink viscosities, and (3) can cover non-flat topographies.
EHD can print drops with <500nm diameters. It can reach 1-10um resolution, which standard inkjet would struggle to achieve. Furthermore, it can handle pastes with viscousities of some 1000 Cp. Given that the principle is based upon particles being pulled out by electrostatic force, whereas being pushed out by mechnical force as is the case with inkjet, the trajectory of the particles can be controlled, enabling one to print on 3D or non-flat topographies. These are all important technological steps forward.
In the second slide, you can see examples of printing lines, droplets, and other patterns, demonstrated by Enjet. In the case of lines, one can see L/S ranging from 2/2um to 80/80um, demonstrating both the versality and also ultrafine line printing capabilities of this technology. In the inset, digital printing over a non-flat surface is shown, demonstraing good step coverage.
A challenge for this technology is that it is slow. Most systems are R&D systems with a single head. However, companies are now developping industrial-scale mult-head printing. The embedded video shows a multi-nozzle array printing of 0.5-1um Ag NP lines by Scrona. This is a fantastic result because it shows a pathway towards industrial scale printing at ultra fine lines, beyond what inkjet achieves.
Final slides show some applications. The application space is in fact broad and expanding. The EHD can be used to digital print micropads for placement of ever shrinking microLEDs (is the die attach a particle free Ag ink??); it can be used to print quantum dots (QDs) onto ever shrinking microLED dies, enabling color conversation; it can be used to repair printed metallizations and wrap-around edge electrodes for microLEDs or repair TFTs post production; it can be used in semiconductor packaging to digital print inteconnects or shielding; etc etc
Join the TechBlick Innovation Festival (24 June- free | online) here to learn more. At this festival, Fraunhofer IAP will present the latest on EHD of QDs for display applications and DoMicro will present its perspectives on EHD printing, perhaps for die-first integration.
LIFT Processs: digitization of screen and stencil printing
LIFT or laser induced forward transfer is a process enabling digital non-contact deposition of highly viscous conductive pastes and even adhesives and solder. This is in contract to inkjet which digital prints low viscousity inks.
The principle of operation is demonstrated in the first slide. A transparent film is coated uniformly with a thin layer of paste. When laser pulses hit a spot on the film, if the paste is correctly formulated, it will detach and land on the substrate. As such, this technique opens the way to print without mask or nozzles patterns of various viscous materials on any substrates.
The second slide shows the various materials that could be printed. The table is from IO Tech, suggesting that a wide range of off-the-shelf materials can be LIFT printed. It is of course not as straighforward as this since many parameters need to optimized, e.g., laser fluence, pulse rate, distance of film to substrate, print speed, coated film thickness, shear thinning properties of the paste, target substrate, etc, etc
I include some printed patterns from literature. These are printed straightlines using PV metallization pastes, showing that narrow linewidths as well as very high aspect ratios can be achieved. In one example, a linewidth of 65um is achieved. Note that this is wider than the state of production in screen printing of PV pastes (34um)
In subsequent slides, you can see various demonstrations. In these examples, solder paste is LIFT printed, adhesives are deposited, or packaging interconnects are fomed (in this example a linewidth of 20um is claimed, although we have not seen verification yet). Finally, you can see that also Ag and Cu nanoparticle inks can be formulated to be compatible with LIFT. In general, these examples demonstrate the LIFT can go where inkjet can not.
In general, LIFT is an intersting technology. The production is not yet fully commercialized despite the principle of LIFT being well established for some years. The latest efforts are aimed at creating industrial-scale R2R machine able to print multi-materials. It will be an interesting space watch, especially if it indeed succeeds in printing fine linwidths using viscous pastes digitally (without mask or nozzle) but at high speeds.
Join the TechBlick Innovations Festival (24 June 2022 | FREE | Online) to hear from Keiron Printing Technologies, a start-up in Eindhoven developing and commercialising a novel LIFT machine.
R2R: Jet selective metallization for industrial level production
JetMetal Technologies has developed a novel process which is able to spray jet metallize surfaces with select area control. In this technique, two water-based components are sprayed onto a surface and via a redox process under atmospheric temperature and pressure conditions a thin layer of pure metal (in this case mainly Ag) is formed. The thickness can range from 10nm to 5um but is most typically a few hundred nanometers.
This process is thus a bridge between high-throughput painting process and thin and controlled plating deposition. The formed lines are close to pure Ag, and thus offer high conductivity. Indeed, JetMetal Technologies suggests that they can reach 85-90% of bulk Ag conductivity on a smooth PET substrate when the sprayed coating is 500nm thick.
Both the thinness and high conductivity are clearly differentiated from traditional particle-based pastes and inks because such traditional inks are typically 20-30% bulk Ag conductivity when applied on low-T PET substrates. Furthermore, with the exception of inkjet or particle-free inks, the printed thickness levels are typically in the few micrometer ranges. Like printed inks and pastes, the jet metallization will also need to prove adhesion to different substrates.
A challenge for any spraying or jetting process is the ability to achieve selective area metallization. JetMetal has developed a hybrid process in which a dielectric ink is first printed (Screen, inkjet, gravure, etc) to act as a mask. The jet metallization then applies the Ag, metallising the exposed parts and (this is crucial) removing the masking ink at the same time. Therefore, no lift-off or similar process will be required. We would imagine that the process needs to be carefully controlled so that the right metallization thickness is achieved and the masking ink is fully dissolved at the right time so no residual ink is sprayed.
As shown in the slides below, JetMetal has a S2S screen printing machine in-house (400x400mm with >50um resolution) as well as a R2R pilot jet metallization line (400m width, <3m/min web speed).
Multiple applications are showcased in the slides.
RF Antenna: a thin and highly smooth (Sa<20nm) layer is deposited achieving 90% bulk Ag conductivity with >50um resolution. The properties are shown in the slide
PI based heater with with 50nm homogeneous Ag layer
A metal mesh with 150nm linewidth and >90% aperture acting as a semi transparent thin film heater
a thermoformed 3D circuit with <1000% elongation. This is interesting because in their case they elongate the masking ink first and then metallize the 3D shape using the jetting process. Thus, the elongation of particle-filled conductive inks will not be the limiting factor
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