The mist-based dispensing method is adept at putting micron-sized lines on non-planar surfaces.

Aerosol Jet technology is a fine-feature material deposition solution used to directly print functional electronic circuitry and components onto low-temperature, non-planar substrates.

Aerosol Jet printing functions on the principle of generating a mist through atomization of a nanoparticle colloid solution. Nanoparticle materials liquids, or inks as they are generally called, are made up of nanoparticle solids, solvents and organic binders and even some polymer resins, which are included to make the ink perform on different substrates. These inks must have a certain viscosity and particle sizing in order be printed with aerosol jet. That viscosity is generally less than 500 centipoise (cP), and the particle size must be under 100nm.

In Aerosol Jet, the liquid ink exits either pneumatically or with ultrasonic energy. This generates mist droplets ranging from 2 to 5µm. Then, a carrier gas, which in almost all cases is pure nitrogen, carries that mist through the print head. Inside the print head is a nitrogen sheath that collimates and accelerates the beam through a converging nozzle. What comes out is a beam of material made up of very small droplets that exits the nozzle at high velocity. This allows it to be anywhere from 1 to 5mm off the surface of the part and print right over the nonplanar substrate, so that the surface does not have to be flat. Because the nozzle is so far from the part, it can be moved in patterns to create circuitry on the surface of the part, regardless of the flatness.

Circuit definition. The amount of mist coming through the nozzle defines the metric of output we use, called mass output. Mass output is how much dry material is deposited per unit of time, which is normally in milligrams per minute. The specified print speed or nozzle surface velocity, and a given mass output, establishes the specified thickness.

The line width is controlled by the size of the nozzle orifice. Nozzles sizes can range from 100µm up to several millimeters. These control the feature size. Each nozzle has a range of feature sizes that it will work with. For example, a 300µm nozzle will be able to print from 30% to 50% of its nozzle diameter. Thus, a 300µm nozzle will print from about 90 to 100µm up to around 140 to 150µm.

The dispense rate is dependent on the ink, its viscosity and the particle size. The solvents in it control how much it dries out during printing, and can affect things like line edge quality. The ability to print very fine features is directly related to the type of ink used.

Just like different ink is used to create fine features, the same machine is not used to print all types of circuits. While Aerosol Jet can dispense a range of types of materials, its primary function is adding circuitry to 3-D injection-molded components, or antenna interconnects on semiconductor die to replace wire bonds with printed structures. It is also used for sensor printing and many other unconventional applications. 

Creating functionality. Aerosol Jet is like a dispensing machine. Users develop a process, which results in a printed feature width and line. Then a structure is designed around that target printed feature. The process recipe becomes the definition or the resolution of the printing capability, and the entire structure gets built around that resolution.

But Aerosol Jet lives in a world of below 200µm features, which is rare for dispensers. It prints noncontact and on nonplanar surfaces, so it can do things other dispensers cannot. It is focused on applications that are very small such as wire bonds and antennas. The nonplanar conformal circuitry is what really enables the Aerosol Jet market.

Antennas in the context of 2-D PCBs are fairly straightforward, and most manufacturers aren't going to use Aerosol Jet technology for that. But for a very complex device such as a handset or a cellphone, which has multiple layers of materials that signals pass through, getting the signals out to the outermost corners of that handset are critical. 3-D antennas have been in almost every single phone sold for the past decade. In most cases, the antenna is digitally designed onto a 3-D CAD object and printed in a matter of minutes. There’s no plating involved. There are no extra steps. The antenna is designed into the structure, simulation is performed, and then it is physically tested under traditional methods. Under these circumstances, Aerosol Jet is used for 3-D printed antennas.

Configurations. Aerosol Jet technology enables high-resolution electronic circuits and components to be printed on 2-D and 3-D surfaces from a wide range of materials. The technology is packaged into a variety of platforms. Inline systems are for printing patterns on substrates with components on them. The state-of-the-art can print at or below 200µm feature sizes.

Flexible systems can come in a three- or five-axis configuration. With the right software, these can handle low- or medium-volume production. There’s a print engine that can be integrated into automation lines for high volume production. This is for specialized production use cases, because generally speaking, Aerosol Jet is not used for PCBs but rather for printing on unique shapes or objects and giving them functionality by adding circuitry. These could be 3-D objects that are large or small and have varying surface quality.

When specifying Aerosol Jet, most customers start with a material in mind or a problem they are trying to solve. One of the first questions often is, can you print this material? The supplier then tries to identify whether they have a material in-house or if one must be formulated. Aerosol Jet is not as ubiquitous as screen printing, where a single vendor may have catalogs of materials. That said, there are a large number of material vendors. We help find the right material for the application that meets the necessary conductivity or dielectric properties, adhesion and environmental performance, as well as the form factor of the application.

Buyers also typically want to know the print times, because that directly correlates to the economics of the purchase. Aerosol Jet is an additive technique, and material must be added everywhere it goes, which directly affects the cycle time associated with printing each part. Repeating such small features means the material usage rate is low. For a typical process, less than 10ml of ink will result in eight hours of printing time. Yield, then, is determined by the number of parts that can be printed in that 8 hr.

Ed.: Aerosol Jet is a trade name of Optomec.

Bryan Germann is product manager of the Aerosol Jet product line at Optomec (optomec.com); bgermann@optomec.com.

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