Screen Printing - August/September 2016 - 35

Opposite, 3D printing could potentially find a home in producing printed circuits with features between 10 and 100 microns, filling a gap between screen printing and photolithography. Photo © Yoram Reshef, courtesy of Nano Dimension performance requirements, and the customer's budget for equipment. Here are the technologies that are most applicable to 3D printing for electronics applications. The machines that most people associate with 3D printing use fused deposition modeling (FDM), in which a filament of material is melted and deposited onto a surface in layers. The printers are relatively inexpensive, ranging from tabletop machines for hobbyists to more substantial equipment for industrial use. FDM printers primarily print various plastics, although the filaments can be infused with metals or other materials. Selective laser sintering (SLS) uses powders as the raw material. As the name implies, a laser sinters (melts) the powders to fuse them in precise locations, avoiding the need to heat the entire part to produce the desired mechanical strength. SLS is versatile and can work with polymers, metals, ceramics, and even exotic materials such as food-based powders, but the machines are expensive and best suited to high-end applications. Many high-end 3D printers use stereolithography (SLA), in which a UV laser is used to selectively solidify liquid resins made from photosensitive polymers. The process can print highquality objects with high resolution, and, historically, came with a cost to match. The newest generation of printers is much less expensive, with models that are even accessible to hobbyists. Inkjet printing is a common technique for high-resolution 2D printing, but it can be used in a 3D context by printing multiple layers on top of one another. Unlike FDM and SLS, however, inkjet cannot be considered a true 3D approach, as it is best suited to applications where the thickness (or height, z) is much less than the length and width (x and y) dimensions. Aerosol Jet printing, a process patented by Optomec, is related to inkjet printing and has been touted as a 3D-printing technique because of its capability of printing features onto FIGURE 1. Fixtures that test whether hardware is working properly are a perfect application for 3D printing because they are highly customized and produced in very low volumes. top, a 3D printer creates conductive traces for a fixture that will test whether the robotic fingers manufactured by RightHand Robotics are functioning properly. bottom, the fixture lights up, indicating that the part is working correctly. Images courtesy of Voxel8 curved surfaces rather than only to planar substrates. Like inkjet, however, it does not truly print 3D structures. The process generates a mist from liquid raw materials and focuses the aerosol stream of material to deposit droplets onto the substrate. An integrated laser sinters the deposit, similar to the process in an SLS machine, without heating the underlying substrate. The Aerosol Jet process can create features as small as 10 microns, printed over steps and contours. The Importance of Materials 3D printing has historically been focused on printing a variety of plastic materials. When it has been extended to printing metals, it usually involves bulk components for aerospace and similar applications. High-end printers can create very detailed structures with micron-scale resolution, but the materials are not tailored for producing reliable electronics. For this reason, the focus of 3D-printed electronics is not to print complex circuits, but rather to take advantage of applications that encompass both mechanical and electrical function and require conductive traces to make the electrical connections. This is similar to the evolution of 2D-printed electronics, where early commercial products relied on fairly low-resolution conductive traces in applications that were not extremely demanding. The sweet spot for 3D printing is micron-scale resolution, with features between 10 and 100 microns in size. This is the realm where screen printing struggles to achieve sufficient performance, while photolithography is overkill. In order to meet this demand, manufacturers are developing silver inks designed specifically for 3D printing. For many product designs, the inks need to be dispensed onto curved, not flat, surfaces, so they need to be viscous enough to hold their shape while not being so viscous that they cannot be dispensed. They also need to cure at low temperatures, august/september 2016 35

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