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66 The PCB Magazine • September 2017 surface to well within the requirements of dis- pensing circuitry. Once done, the USB chip was precisely positioned on the smooth surface us- ing a pick-and-place head. Next, a conductive material was dispensed beginning at the pads of the USB chip and fin- ishing at the end of the tab on the printed base. Again, the smooth surface here benefits the dis- pensing process by creating a uniform height across the entire area. This allows for higher consistency when dispensing materials as this process requires a constant "Z-gap" or distance between nozzle and substrate. A polycarbonate shell was then printed around the perimeter of the circular portion to create a barrier used to hold a dispensed epoxy. Once the epoxy cured, the milling head was brought back in to give a nice, smooth finished surface. This is a small demonstration of multi-material, multi-process in-situ PCS fabrication. Strength of Printed Parts Printable fused filaments range from some- what durable materials such as acrylonitrile bu- tadiene styrene (ABS) to flexible thermoplastic elastomers (TPE) and even FST-rated thermo- plastic polyetherimide (PEI) resins. These mate- rials have their own strengths and weaknesses. Whether it is the high impact strength of ABS or the chemical and temperature stability of ther- moplastic polyetherimide (PEI) resins, choos- ing the right material for a particular applica- tion can determine whether or not it is success- ful. 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 relies on layer-to-layer surface adhesion, adhesion of side-by-side printed lines, and print direction as well as the mechanical properties of the ma- terial itself. Another aspect of 3D printing that decreases overall strength of printed parts com- pared to bulk material properties is the presence of voids that are introduced into the part during the printing process. In both of these examples shown in Figures 7 and 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 3D PRINTED ELECTRONICS FOR PRINTED CIRCUIT STRUCTURES Figure 6: Steps of a printed USB device.