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Additive Reality: Printhead Selection or ‘Shop ‘Til You Drop’

The age of internet shopping released us from the hassle of moving ourselves from shop to shop to obtain the best deal on the most suitable product, which has resulted in an improvement to our quality of life. Still, such alleviation comes at a cost: dealing with almost perfect qualitative comparisons. Such comparison can be objective and final, thus it should be easy to get it right the first time. Still, we are left to judge what is good enough and what is better than good enough.

If inkjet tools could be found on an e-commerce site, fitting nicely into the category of “industrial and scientific," and a subsection of “additive manufacturing products,” aside from dimensions and weight, there would be several product specifications of which many would specify the jetting properties; these would basically detail the printhead(s) in the system. The knowledge needed for the right selection is how these specifications relate to the application at hand (for example, solder mask).

In my opinion, these are the most important properties of a printhead (PH):

• Native resolution, also known as nozzle per inch (NPI)
• Max frequency and driving voltage
• Minimum drop volume (assuming multidrop capabilities)
• Design for arraying, amount of nozzles and positioning compensation mechanism
• Heating and ink recirculation

In the following, each of these properties will be addressed and discussed. Before that, here is a short note on the dot per inch (DPI) unit: this is the common printing resolution unit. An image as a 2D object needs two resolutions. Both dimensions often share the same resolution, therefore only one is present. Inkjet printing identifies and implements the two resolutions (even when equal in value) in a different way: the cross-scan resolution is a combination of NPI and small steps movements. Its orthogonal equivalent, the in-scan resolution, imposes the relation between speed and jetting frequency.

A solder mask print benefits from printing resolution in the tens of microns. A resolution of 2550 DPI delivers pixels just below 10µm size. A printhead with a NPI of 50 would need 50 small steps (steps of 10µm) to achieve such cross-scan resolution. At every small step, the printhead coats part of the pattern over the full length of the substrate. Despite any high printing speed, the compensation of a low NPI comes at the cost of precious time. Another possibility is to stack and align multiple PHs to create a virtual printhead with higher NPI, though the mechanical tolerance will be tight. From the previous example, it’s less than five micrometer tolerance. If death and taxes are the few sure things in life, “exchanging a printhead” comes really close. Therefore, a service engineer will need skills, tools, and time to match these tolerances. In other words, the low-risk path is a head with a high NPI.

The electronics of a printhead is the interface between the digital input and the jetting output. These drive the mechanical actuators of the nozzles. The jetting frequency, when divided by the in-scan resolution is proportional to the print speed. A high maximum frequency is necessary for fast printing. One of the incoming tests for material qualification is of course its dampening properties to allow for the full range of available frequency. Another check to obtain the maximum working frequency is the inverse of the waveform’s period. The waveform is a train of voltage pulses that the actuators will convert to mechanical movement, providing kinetic energy to the liquid and subsequently ejecting a droplet from the nozzle orifice.  In very rough terms, the capability of a fast voltage rise (slew rate) or a high voltage enlarges the investigation space for suitable waveform. A good waveform delivers a drop that quickly travels to the substrate. Quick drops have short time of flight, which is necessary for high droplet placement accuracy.

The graphical industry has been pushing printhead manufacturers to add gray scale; the possibility of influencing the volume of the drop printed at each location. The gray scale is a way to assign a “scale level” to a defined waveform. The waveforms will differ in drop volume by a combination of multiple train pulses and tuned pulse amplitudes. The advanced user has the possibility to implement whatever relation between the scale and the delivered volume, within the limits of a usable working frequency. Nowadays many printheads support such configuration. Still, the minimum drop is the most often used parameter that characterize the printhead, since it tells about the nozzle geometry. Gray scale can painlessly transform a low drop volume, high precision printhead, to a still precise workhorse for great material output and therefore fast coating speed.

One single printhead, despite its perfect match with the application, will not be able to match manufacturing needs alone. The number of nozzles in a printhead is a first indicator of its readiness for manufacturing. However, the key to high-throughput inkjet relies in assembling several printheads to further increase nozzle count and possibly cover the full substrate with an array of printheads. This printhead stacking is easier and less printhead intensive when a high NPI is available. A well-designed manufacturing tool will have an assembly of several printheads designed for straightforward maintenance. Additionally, the presence of tools to allow a reciprocal alignment well within the desired pixel resolution is crucial. The supplier of the inkjet manufacturing tool is responsible to implement the printhead alignment with a reliable method. Such method ensures that an exchanged printhead will work perfectly with the others from the first drop.

Finally yet importantly is the heating and recirculation. The previous column on materials detailed the fluidity of the ink. Present solder mask inks need a jetting temperature slightly higher than room temperature. This ensures control on the ink viscosity (jetting at room temperature might bring a seasonal shift in jetting quality). A worthwhile design would consider the amount of power needed to stabilize the temperature of the flow of liquid used at print speed conditions. Anything else than the chosen jetting temperature will lead to inconsistency in drop speed and therefore drop positioning. Most printheads have a convenient integrated heater, good for low output. Other printheads do not have such complexity and the manufacturer simply specifies the viscosity range. An external heater will have to take care of the right liquid temperature at all times.

A recirculating unit is a perfect match to an external heating unit. The flow of liquid is fixed and, by far, higher than the output of ink through the printhead. Therefore, the heater works in a steady state, ensuring a good jetting temperature at all times. This advantage, despite being relevant, should not be the only reason to integrate a recirculation in the system. Regardless of your best effort in feeding ink to a printing system, air bubbles can occur at any place in the ink system, and the smaller the cavity where these bubbles are, the greater the problem, from meniscus pressure instability to nozzle clogging. A recirculation system ensures a continuous absence of this common trouble. The benefit of such a unit outweighs the pain of occasional purging to deal with already compromised manufacturing.

My preferred five properties of a printhead and their description above provide you with a basic knowledge for a fair judgement. The takeaway of this column is that a good printhead selection leads to a reliable manufacturing tool. The inkjet tool supplier, as seen in the first column, designs an accurate ensemble of components. This task is almost independent from the reliability of the components. Still, a manufacturing tool that cannot operate continuously misses completely the point of its existence.

This column on printheads and their selection cannot close without a final insight on the importance of drop volume. A previous paragraph dismissed it quickly as “tiny means accurate,” the best way to come back to the subject is to add the knowledge of contact angle described in my last column and drop position accuracy, from the first column. In Figure 1, you will see the profile of drops for three volumes that are typical for printheads used for solder mask applications. In each frame of the animation, a different contact angle with the surface produces the drop profile. Additionally, dashed, the normalized sum of a thousand drops, or in other terms the convolution of the drop profile and the positioning accuracy, shows the relation between drop location and a given placement accuracy. The picture sequence, after few repeats, will show you that the drop positioning affects all volumes in the same way at the same angle. Still its effect is relatively lower at small angles.

Figure 1: An animation of the profile of drops for three volumes that are typical for printheads used for solder mask applications.

More striking is the quick drop diameter decrease when moving from 15 to 45 degrees. Designing a printing process with robustness in mind dictate to avoid this low angle range. Beyond the 45-degree angle, the diameter reduces at a slower pace, giving a low marginal reward to the refined pre-treatment effort. Obtaining and maintaining a high and homogenous contact angle on the substrate surface is a tricky task. Furthermore, the risk of a misplaced drop is not nihil; actually, it is relatively bigger at higher angles. Such a situation would quickly destroy the delicate equilibrium of a liquid wall at 90 degrees and cascade into additional misplacement of the droplet compared to the target position. A contact angle between 45 and 60 degrees offers a reasonable process window for feature fidelity. This gives ground to the simple and straightforward interpretation of the benefits of a small nozzle volume.

My next column will focus on how the chosen printhead creates the desired pattern and thickness on a given topography.

Luca Gautero is product manager at SUSS MicroTec (Netherlands) B.V.