International efforts to promote recognized standards for inkjet printing equipment intended for industrial manufacturing applications commenced for printed electronics (PE) a decade ago [1] while several standards for graphics printing equipment were developed before that time.

Several International Electrotechnical Commission (IEC) international standards or reports have been published [2–9] for PE inkjet printing equipment while further editions of International Organization for Standardization (ISO) standards for graphics printing equipment productivity, metrics, cartridge yield, color, and paper permanence have been released [10–15]. However, none are available for any other industrial (non-graphics) manufacturing application. Therefore it appears timely to review whether any of the existing PE standards could be applied elsewhere beyond PE.

The widening range of inkjet (printing) applications includes those for mass production as well as specialized uniquely fabricated products. Useful reviews include “Inkjet printing and 3D printing strategies for biosensing, analytical, and diagnostic applications” [16], “Inkjet printing for pharmaceutics - A review of research and manufacturing” [17], “Inkjet-printed flexible sensors: From function materials, manufacture process, and applications perspective” [18], “Recent developments in inkjet-printed OLEDs for high resolution, large area applications” [19], “Review of digital printing technologies for electronic materials” [20], “A review of inkjet printing technology for personalized-healthcare wearable devices” [21], “Additive manufacture of ceramics components by inkjet printing” [22], “Review—inkjet printing of metal structures for electrochemical sensor applications” [23], “Challenges, prospects, and emerging applications of inkjet-printed electronics: a chemist’s point of view” [24], “Inkjet metrology: high-accuracy mass measurements of microdroplets produced by a drop-on-demand dispenser” [25], “Ultra-high precision inkjet printing technology for display” [26], “Interfaces and pattern resolution of inkjet-printed organic light-emitting diodes with a novel hole transport layer” [27], and “Classifications and applications of inkjet printing technology: a review” [28]. Other papers, such as “Future, opportunities and challenges of inkjet technologies” [29], “Formulation, quality, cleaning and other advances in inkjet printing” [30], and “Inkjet printing quality improvement research progress: A review” [31], outline process improvements in the past decade that support the wider adoption of inkjet printing.

It should be noted that the main international standards for inkjet printing equipment that exist are those developed by the IEC [32] through national experts within its technical committee TC119 on the PE working group WG3. The TC119 committee was formed in 2012, and the status of its standards work projects has been reviewed subsequently in papers by various authors in 2017 (Takahashi [1]), 2018 (Hyodo & Sakai [33], Hoath [34]), and 2022 (Zapka, Chapter 71 [35]). The present paper provides the latest 2025 update.

Key considerations for inkjet-printed functional material within electronics are drop placement accuracy and drop volume reproducibility when roll-to-roll (R2R) printing of flexible electronics, variations in jetted drop speed, and/or arrival times at the moving substrate can additionally limit printing quality. By contrast, for inkjet-printed OLED pixels, drop velocity and volume variations can usefully be averaged out by using multiple droplets spreading into each preformed pixel pit. This application dependency gives clues as to which PE standard might or might not be particularly relevant to more general inkjet printing equipment designs and intended markets.

As inkjet technology development for advanced manufacturing has progressed [35, 36], suppliers of inkjet printheads and printing equipment incorporating them have mainly relied on performance information from the printhead manufacturers before integration into industrial printers. In most cases, this information has included statements of inkjet drop jetting speeds and drop volume (or drop weight), printing frequency ranges, temperature and humidity limits, and ink viscosity. The appropriate choice of printhead is made by the integrator and then tested for the intended application(s) of the printing machine. The end user may specify the printhead manufacturer for commercial reasons, however, but without agreed standards for such equipment, how can the equipment performance or indeed design be judged prior to purchase, design, or acceptance by integrators or end users?

To help establish common criteria for equipment used in printed and flexible electronics manufacture, the IEC [32] dedicated a Technical Committee (TC119) and its Working Group (WG3) to focus on creating and publishing international standards for such printing equipment and the measurement methods used. Other TC119 working groups consider material testing, (e.g., ink properties). Following the last surveys [33, 34] of then published standards and explanation [2, 3] of optical assessments for measuring inkjet droplets, further IEC standards [4, 5, 7, 8] and a technical report [6] have been (or are being [9]) published while the ISO and IEC have become more closely aligned [37]. It seems very timely to revisit the status and explanation of the IEC standards for an advanced inkjet conference audience and to explore their wider applicability for inkjet printing.

This paper examines the measurement basis for each published standard for PE equipment for inkjet printing to help explore whether there could be wider applicability to general or specific equipment for non-PE applications. Discussion of some details within the existing PE standards could assist others to make their own conclusions as to whether to follow any PE standards and/or prompt the creation of alternative inkjet printing standards or measurement methods [38, 39].

Most of the PE standards for inkjet printing equipment have focused on the outcome of the jetting process, the printhead being of the drop-on-demand (DoD) type where each ink drop forms after ink is jetted from a nozzle in response to a drive waveform applied to the channel behind the nozzle. Drops moving from the DoD inkjet printhead towards the deposition location are the initial outcome of the jetting process and, irrespective of the application, are the prime focus. Figure 1 shows a shadowgraph of a transparent UV analogue ink being jetted from a multi-nozzle inkjet printhead at speeds sufficient for generating long ligaments (“tails”) and trailing satellite drops reaching locations typically 1 mm below the nozzle bank (visible at the top of the figure).

The DoD drops of interest in applications are almost invariably (but not exclusively) those reaching the deposition location. It is assumed that they have attained a single spherical shape even if they are formed of more than one fluid body by coalescence during the flight from the DoD nozzle plane. Satellite droplets are often smaller than the main drops, but if still separate at the deposition location are of importance where drop placement is critical (e.g., printed electronics).

Figure 2 shows a schematic (not to scale) of simultaneously jetting nozzles in an array, exaggerating drop speed and size differences in flight towards the deposition location/substrate. These differences are unintended in practice as they can spoil the final deposition outcome, especially if the process involves relative movement between the printhead and the substrate.

For establishing standards and for other purposes, what is measured must be carefully specified to avoid confusion: individual drops or average drops? The results of individual drop measurements can be averaged after analysis, whereas the measured average of drops does not find variations but may be simpler to measure (e.g., for stable printing) than using individual drop measurements.

Figure 3 shows some image outcomes for different shadowgraph lighting scenarios. These correspond to the following: (a) simple strobe lighting with similar drops nominally arriving at the same location and superposed on the recorded image; (b) single flash lighting for recording an individual drop in the image; and (c) double flash lighting for recording a moving drop at two different times in the same image. Scenarios (b) and (c) are associated with more advanced drop watchers than (a) as they allow analyses of individual drop sizes and speeds rather than of average drop images. The variable drop positions shown in Fig. 3(a) are exaggerated but would be merged in an image with added background for each drop shown, adding to difficulties for the image contrast.

The DoD drop formation process takes a small but finite time after the drive waveform start trigger. This arises from the motion of the ink meniscus that spans the DoD nozzle in response to the drive waveform and the later ejection of a jetted drop separated from the residual ink meniscus [40]. In academic studies of jetting inkjet printheads [41], jetted drop separation or first emergence of the meniscus has been adopted as the time zero for jetted drop speed measurements. In industrial systems, the drive start trigger is often taken as the time zero to exploit synchronism with substrate (or printhead) motion control timing for reliable drop location with stable inkjet printing.

Figure 4 shows the schematic representations of two unreliable jetting scenarios: (a) depicts a “failed” inkjet nozzle having no jetted ink owing to an excess of fluid across the exit (a “flooded” nozzle), resulting in a missed deposition (not shown); (b) depicts one of the “first drop effects” immediately following a “latency” period without jetting, showing the successive jetted drop locations below a single “active” nozzle after the travel time designed to reach the substrate location (shown by the dashed line) for continuous jetting through the nozzle. “Failed” nozzles can arise from (ink) particle residues, stray satellite drops, and/or nozzle plane wetting from other nozzles if the driving waveform is not optimum. “First drop” effects could occur after the evaporation of ink components.

Can DoD inkjet printing ever be considered stable? First drop effects on drop speed and/or drop volume from a DoD nozzle are often found after a (latency) period of non-jetting [25, 42], which disappear once the jetting frequency increases beyond a certain level. Such effects are inherent [43] in piezo-DoD printheads, but the use of non-jetting channel drive waveforms, jetting waveform compensation methods, and nozzle cleaning may alleviate such unwanted changes in drop speed and volume. Avoidance of first drop effects in printing applications without heavy duty cycles on the DoD channels requires special measures, which might include nozzle cleaning, using “spitting” areas prior to production printing, and ink reformulation to avoid evaporation at nozzles.

The DoD jetting and drops are observed in flight by using optical means such as drop watchers [44] that record images that are synchronized (but fixed-time-delayed) with the applied drive waveform. Two-dimensional (2D) images are typically shadowgraphs of the drops, using short light flashes to reduce any drop motion blur. The images are recorded at high digital resolution to permit image processing software to extract drop locations and contours [45] for the analysis of speed and volume. The jetted liquid drops rapidly assume near-spherical shape in flight, which aids image analysis.

Recorded 2D images may correspond to single or double light flashes: the former is used for stable printing (assuming reproducible drops) and the latter is used for finding the speed of a single drop.

For the highest accuracy (especially for drop contour/size determination), the drop watcher image plane should remain properly focused on the drop motion throughout the region of interest. As jetted drops generally move in three-dimensional (3D) space and not just in 2D space, 3D directional information may be obtained using two orthogonal drop watchers simultaneously viewing in the nominal jetting (z-) direction and a sideways (x- or y-) direction. The 3D imaging is needed for single-nozzle printheads; as one sideways x-direction of a single 2D drop watcher image may well contain the major deviation from the nominal jetting direction; this 2D x-direction is often conveniently parallel to an array (row) of multiple nozzle outlets, ignoring any residual deviation.

Figure 5 schematically represents shape measurements on images of inkjet drops in flight or of jetted deposits on a planar substrate, using software to determine some key parameters of interest in applications: the diameter D and/or the area A, and the circularity C of the shape.

Typical drop travel (throw) distances for DoD inkjet printing equipment designs are 1 mm [35, 44], so a common industrial guideline used to quote the drop impact speed adopts the time taken to travel from a somewhat shorter distance to a slightly longer distance in the formula for this speed:

Travel time differences are usually far more precisely known than the drop location differences [41]. The location of the drop is determined from either the center or the leading edge of the 2D image [42], with the center usually having higher precision. The image plane magnification and pixel calibration factors need to be established before reporting the jetted drop impact speeds in meter/second.

The printhead is always held stationary and upright during measurements for standards purposes although it might be moved sideways across the optical field of view of the drop watcher for channel selection purposes. Drop watchers are often used to monitor jetted drops simultaneously from several inkjet printhead nozzles, for straightforward visual drop (speed) comparison purposes, but this compromises the available image resolution for drop volume determination. Alternatively, average drop volume could be estimated by using drop weighing rather than shadowgraph methods applied to single drops. Inkjet ink properties such as liquid density, surface tension, and viscosity under jetting conditions are beyond the scope of this paper.

Drop placement accuracy relies on optical scanning after drop deposition on suitable media. Usually, the media are moved under a fixed printhead with multiple inkjet nozzles jetting, with rigid media on flatbeds and flexible media on R2R stations. In contrast to jetting conditions used for in-flight measurements for standards purposes, most inkjet deposition arrangements necessarily include realistic airflows between the printhead and the substrate [46].

Work has already progressed on standardizing alternative means to assess inkjet drop speed [39], weight and volume [9], and potentially jetted drop direction, with plenty of opportunities for further technology experts to join the IEC TC119 working groups. Applying one or more of the existing PE equipment standards to applications beyond PE seems timely. Perhaps another international group working under ISO should consider developing general inkjet standards based on the existing published work.