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September 2017 • The PCB Magazine 65 Figure 4: 0° printed tensile specimen using 0.4 mm nozzle. Figure 5: Micro-dispensed conductive paste on 3D printed substrate. cess involves printing filament out of a special- ly designed 1.75 mm nozzle and then utilizing a milling head to give a high-quality surface fin- ish 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 0.4 mm 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 print- ed with no perimeters as this would throw off tensile testing. Both of these sample types are 1.0 mm and were printed with a nozzle temperature of 235°C and a bed temperature of 50°C. The pe- rimeters seen in Figure 1 and Figure 2 are used to show dimensions. When printed with the 0.4 mm nozzle, each tensile specimen completed printing in four minutes and 24 seconds. When done with the spaghetti method, each tensile specimen av- eraged one 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 com- plete the object more than twice as fast, the fi- nal surface finish from milling was far superior to that of the conventionally printed specimen. This benefits PCS greatly as printing the elec- tronics 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 noz- zle as small as 100 microns to provide a smooth surface for the conductive material to be print- ed accurately and true to design. Although sur- face mapping is available, which enables con- tour printing, a smooth surface is preferred. This also provides an ideal surface when print- ing multiple thermoplastics onto one another. Creating PCS incorporates multiple process- es and materials, though everything is done on one system. Take, for example, Figure 6. This small demo shows the steps to create an embed- ded USB device. First, a base to hold the USB chip was printed out of ABS plastic, one of the most common 3D printing materials. To accommo- date the printing of the circuitry, which will ex- tend the pads of the USB chip to the edge of the device, a smooth surface was desired. The mill- ing head was selected to accomplish this task. With around one micron of runout, the milling head is able to selectively smooth and flatten a 3D PRINTED ELECTRONICS FOR PRINTED CIRCUIT STRUCTURES Figure 3: Spaghetti-printed tensile specimen before and after milling.