3D Printed Electronics for Printed Circuit Structures

Printed electronics is a familiar term that is taking on more meaning as the technology matures. Flexible electronics is sometimes referred to as a subset of this and the printing approach is one of the enabling factors for roll to roll processes. Printed electronics is improving in performance and has many applications that compete directly with printed circuit boards. The advantage of roll to roll is the speed of manufacturing, the large areas possible, and a reduction in costs. As this technology continues to mature, it is also merging with the high profile 3D printing. 3D printing is becoming more than just a rapid prototyping tool and more than just printing small plastic toys. Companies are embracing 3D printing as a manufacturing approach to fabricate complex parts that cannot be done using traditional manufacturing techniques. The combination of 3D printing and printed electronics has the potential to make novel products and more specifically making objects electrically functional. Electrically functional objects have the advantage of competing with printed circuit boards. Printed circuit structures will be a new approach to electronic packaging. It is the desire of many companies to reduce assembly processes, decrease the size of the electronics, and do this at a reduced cost. This is challenging, but the potential of printing the structure and the electronics as a single monolithic unit has many advantages. This will reduce the human touch in assembly, as the electronics and the object are printed. This will increase the ruggedness of the product, as it is a monolithic device. This will eliminate wires, solder, and connectors, making the device smaller. This has the potential to be the future of printed circuit boards and microelectronic packaging. This paper will show working demonstrations of printed circuit structures, the obstacles, and the potential future of 3D printed electronics. Introduction While 3D printing, as stereolithography (SLA), has been around since the early 1980s, it is has evolved considerably into many forms. For the purposes of this paper, fused filament deposition (FFD) also known as fused deposition modeling (FDM) will be considered. Only recently has FDM printing been joined by electronic printing to create 3D printed electronics. With this evolution in 3D printing, Printed Circuit Structures (PCS) can possess distinct advantages over Printed Circuit Boards (PCB). Many components that are present on a PCB can be integrated into a PCS. It has been shown that PCSs can contain fully embedded circuity such as antennas [1][2], lumped components [3], and even connectors [4]. Instead of creating a PCB to attach to an object, it would be possible to print the object with the circuitry as an integrated part of it. This printing method is made possible using a Direct Digital Manufacturing (DDM) machine which combines the use of multiple tool heads including a micro-dispensing pump, a heated extrusion head, a pick-and-place head, and a micro-milling, drilling, and polishing head. While PCBs require the use of many machines and require masking, PCSs can be completely automated as total fabrication is done in-situ on a single machine [5][6]. Although PCSs do have advantages, there are still several obstacles to overcome, namely speed of fabrication and strength of final parts. Fabrication Speeds FDM style 3D printing is notorious for being slow. This is mainly due to the low volumetric extrusion rates of conventional desktop 3D printers. There are many factors that determine extrusion rate. Some of these include nozzle diameter, nozzle temperature, bed temperature, X-Y movement speed, material, and even the extrusion motor. While all of these have an effect on the total amount of filament being extruded, the nozzle diameter is the main determiner of extrusion rate. Layer height, extrusion widths, and print speeds are all based on the nozzle diameter, therefore this is the facet of the printing process that stands to generate the most benefits from improving. The standard printing nozzle has a 0.4mm inside diameter. This allows for print speeds of up to 80-100 mm/s, depending on machine and desired print quality. Nozzle size can be increased, however, while this can shorten the overall print time, a decrease in quality will be seen. These decreases in quality can be things such as a rougher surface finish, rounded corners, and incorrect dimensions. Larger diameter nozzles are also limited when printing small objects as the small features can be problematic. While the quality-related downsides of selecting a large diameter nozzle are not attractive, they can be dealt with. A method of printing called “Spaghetti” printing was developed to greatly increase extrusion rates and decrease print times. This process involves printing filament out of a specially designed 1.75mm nozzle and then utilizing a milling head to give a high-quality surface finish where required as well as bringing the print into dimension. A speed experiment was set up comparing the printing speed of an ASTM D638 Type V tensile specimen printed using a 0.4mm nozzle and the “Spaghetti” printing approach. Two types of tensile specimens were printed; one with a 0° infill and one with a 90° infill from the horizontal. These samples were printed with no perimeters as this would throw off tensile testing. Both of these sample types are 1.0 mm thick and were printed with a nozzle temperature of 235° C and a bed temperature of 50° C. The perimeters seen in Figure 1 and Figure 2 are used to show dimensions. Figure 1: ASTM D638 Type V Tensile Specimen 0° Infill Figure 2: ASTM D638 Type V Tensile Specimen 90° Infill When printed with the 0.4 mm nozzle, each tensile specimen completed printing in 4 minutes and 24 seconds. When done with the Spaghetti method, each tensile specimen averaged 1 minute and 58 seconds total time with the actual 3D printing portion only taking 32 seconds. This experiment was repeated 25 times. Not only did the “Spaghetti” method complete the object more than twice as fast, the final surface finish from milling was far superior to that of the conventionally printed specimen. Figure 3: Spaghetti Printed Tensile Specimen Before and After Milling Figure 4: 0° Printed Tensile Specimen Using 0.4mm Nozzle Figure 5: Micro-Dispensed Conductive Paste on 3D Printed Substrate This benefits PCS greatly as printing the electronics portions requires a smooth surface for conductive material to be dispensed. Normally, for a conductive print to be successful, the FDM substrate layer needs to be printed with a nozzle as small as 100 microns to provide a smooth surface for the conductive material to be printed accurately and true to design. Although surface mapping is available, which enables contour printing, a smooth surface is preferred. This also provides an ideal surface when printing multiple thermoplastics onto one another. Figure 6: Steps of a Printed USB Device The smooth surface enabled the level placement of a USB chip using pick-and-place. Then conductive paste was dispensed to extend the pads of the USB from the chip. Next, a polycarbonate shell was printed around the perimeter of the circular portion to form a cup for an epoxy to be dispensed. Once cured, the epoxy was milled smooth. This is a small demonstration of multi-material, multi-process in-situ PCS fabrication. Strength of Printed Parts Printable fused filaments range from somewhat durable materials such as acrylonitrile butadiene styrene (ABS) to flexible thermoplastic elastomers (TPE) and even FST-rated ULTEM. These materials have their own strengths and weaknesses. Whether it is the high impact strength of ABS or the chemical and temperature stability of ULTEM, choosing the right material for a particular application can determine whether or not it is successful. However, FDM printed parts fall well short of the strengths of other fabrication methods such as injection molding [7]. This is due to the fact that the strength of 3D printed parts rely on layer-to-layer surface adhesion, adhesion of side-by-side printed lines, and print direction as well as the mechanical properties of the material itself. Another aspect of 3D printing that decreases overall strength of printed parts compared to bulk material properties is the presence of voids that are introduced into the part during the printing process. Figure 7: Cross Section View of Printed Layers with a 0.4mm Nozzle Figure 8: Cross Section View of "Spaghetti" Printed Layers In both of these examples (Figure 7 and Figure 8), voids can be observed wherever there is an overlap of side-by-side lines. This is due to the fact that the edges of printed lines are rounded. To combat voids, an overlap factor can be input into the generation of the print path of the object. This will squeeze the lines closer together, minimizing voids, to an extent. When a high overlap factor is specified, the material being squeezed together must go somewhere and thus it is displaced upwards. This will create a rough surface finish if on the top layer and could throw off overall height dimensions if allowed to compound throughout the print. When printed with the 0.4mm nozzle, many, albeit smaller, voids are introduced into the object. While the few voids that are present with the Spaghetti printed layer are larger, the total void area of the parts printed with the 0.4mm nozzle is greater. These voids contribute to the ~10% difference in ultimate tensile strength for the printed specimens (Figure 9). Figure 9: UTS Comparison of "Spaghetti" and 0.4mm Nozzle Tensile Specimens The print direction plays a major role in the strength of printed parts [7][8]. It is common practice when printing structural parts to have layers printed in the direction of force that the final part will experience. To exemplify why printing in the direction of force is important, tensile specimens were printed in the direction of force (0° specimens) and