research proposal 2

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Additive Manufacturing

journal homepage: www.elsevier.com/locate/addma

Full Length Article

Selective electroplating of 3D printed parts

Kristin Angela,b, Harvey H. Tsangc, Sarah S. Bedairc, Gabriel L. Smithc, Nathan Lazarusc,⁎

a ORAU Fellowship Program at Army Research Lab, 2800 Powder Mill Rd, Adelphi, MD 20783, USA b Rochester Institute of Technology, 1 Lomb Memorial Dr, Rochester, NY 14623, USA c Army Research Lab, 2800 Powder Mill Rd, Adelphi, MD 20783, USA

A R T I C L E I N F O

Keywords: 3D printing FFF Electroplating Conductivity Composite materials

A B S T R A C T

Fused filament fabrication (FFF) 3D printers have been largely limited to thermoplastics in the past but with new composite materials available on the market there are new possibilities for what these machines can produce. Using a conductive composite filament, electronic components can be manufactured but due to the filament’s relatively poor electrical properties, the resulting traces are typically highly resistive. Selective electroplating on these parts is one approach to incorporate materials with high conductivity onto 3D-printed structures. In this paper, non-conductive and conductive filaments printed in the same part are used to enable selective electro- plating directly on regions defined by the conductive filament to create metallic parts through 3D printing. This technique is demonstrated for the creation of multiple distinct conductive segments and to electroplate the same part with multiple metals to, for instance, allow a magnetic metal such as nickel and a highly conductive one such as copper to be incorporated in the same part. Following the characterization of the process, a re- presentative 3D printed electrical device, a selectively electroplated solenoid inductor with low frequency in- ductance and resistance of 191 nH and 18.7 mΩ respectively was manufactured using this technique. This is a five order of magnitude reduction in resistance over the original value of 3 kΩ for the inductor before electro- plating.

1. Introduction

3D printing has become increasingly popular due to the ease of manufacturing intricate custom parts difficult or impossible to make by other means, and is in widespread use commercially, as in the aviation, aerospace, biomedical, and automotive industries [1]. The term 3D printing encompasses many types of additive manufacturing including stereolithography (SL) (using lasers to cure baths of liquid resin) [2], polyjet printing (using inkjet technology to create three-dimensional objects by using ultraviolet lamps to cure a polymer) [3], and selective laser sintering (using a laser to fuse metal and polymer powders). [4]. One of the most popular commercially available types is fused filament fabrication (FFF), sometimes called fused deposition modeling (FDM) [5]. In FFF, filament is extruded through a heated nozzle to print lines of melted polymer and build parts one layer at a time. FFF printers are easy to use and can be operated safely by a casual user, including ele- mentary and high school students for teaching purposes [6,7]; the printers and filaments are cheap [8] and multiple materials can be si- multaneously printed with multiple nozzles [9]. However, FFF does have its limitations. This type of 3D printing has a lower resolution than stereolithography and is limited to thermoplastic materials such as ABS

(acrylonitrile butadiene styrene) and PLA (polylactic acid) [5]. Ther- moplastic composite filaments have started to become increasingly available for use in FFF printers [10–13], allowing for a greater variety of electrically active parts such as antennas [14–17] and sensors [18] to be manufactured that could not have been made with thermoplastic alone.

It is possible to directly print a circuit [19] or an inductor [20] using both conductive and non-conductive filament with a dual extruder head but the conductivity of these state of the art conductive filaments re- mains poor in comparison to bulk metals [21,22], resulting in interest in the use of alternative means such as electroplating to add metal layers to 3D printed parts. Electroplating is a multipurpose technique used in many fields including jewelry and electronic manufacturing for the forming of metal layers and finishes [23]. Plating parts can improve strength by raising the Young’s modulus and making the part more durable [24–26], preventing corrosion or using for aesthetic reasons (gold or silver plated finish) [21]. By coating a part such as an antenna [15,16] or inductor [27] in metal, the conductivity can be improved for lower resistance. In electroplating, a current is run between two elec- trodes, a cathode and an anode, both which are submerged in an electrolyte bath. Metal ions transfer from the anode to deposit on the

https://doi.org/10.1016/j.addma.2018.01.006 Received 16 November 2017; Received in revised form 16 January 2018; Accepted 27 January 2018

⁎ Corresponding author. E-mail address: [email protected] (N. Lazarus).

Additive Manufacturing 20 (2018) 164–172

Available online 09 February 2018 2214-8604/ © 2018 Elsevier B.V. All rights reserved.

T

cathode and form a metal coating. With many of the common 3D printing technologies using poly-

meric materials, electroplating the resulting parts requires deposition on plastics. It is possible to plate on non-conductive materials such as plastic but this involves preprocessing, for instance through the addi- tion of silver paint [26,28] or roughening, catalyzing and electroless plating[29–31] to turn the surface slightly conductive. Electroplating onto non-conductive surfaces using these approaches have been applied to 3D printed parts made using stereolithography, [25,32,33] laser sintering [25,34], and FFF [35–37] printed parts. The metal plating was used to improve strength [24,25] and conductivity [17]. The plastic part can also be dissolved after plating to leave behind the structured metal coating [34]. Both methods of rendering the surface conductive have limitations, with roughening/electroless plating causing the entire surface to become coated and silver paint requiring additional manual labor for patterning in order to achieve selective electrodeposition. There are laser activation printers that will pattern circuits onto parts after the application of silver spray paint that allows the circuits to be electroplated but these machine are expensive, costing hundreds of thousands of dollars [38]. There remains a need for a selective plating approach able to plate directly on 3D printed plastics, allowing com- plex, multi-electrode or multimaterial features to be made without additional patterning. This plating-based technique is intended as a complementary and potentially lower cost alternative to current hybrid manufacturing technologies for electrically active 3D printed parts, such as ultrasonic consolidation of conductive inks, photocurable con- ductive fluids as well as pneumatic deposition of conductive ink (as in the commercial printer the Voxel8) [39].

Recently, blanket electroplating directly on a molded piece of con- ductive composite was demonstrated [40]. In this paper we adapt this approach for 3D printing, demonstrating for the first time electroplating directly on printed FFF conductive composites. By simultaneous printing non-conductive thermoplastic and conductive composites on an FFF printer with multiple extruder heads, electrically isolated re- gions can be defined, allowing selective electroplating only in pre- defined regions. This same technique is then used to deposit two dif- ferent metals in different areas of the part. The plating characteristics and adhesion of the process are characterized, followed by the de- monstration of a complex 3D structure, a solenoid inductor with in- ductance of 191 nH selectively plated in copper. All the parts in this work were printed using a low cost printer (a Makerbot Replicator 2X) with commercially available filaments, bringing the price of this se- lective electroplating capability from hundreds of thousands of dollars for laser activation to two or three thousand dollars, within the reach of the hobbyist or university makerspace.

2. Material and methods

2.1. Production of 3D parts

The selective electroplating process developed here begins with 3D printing the part to be plated. A Makerbot Replicator 2X [41], a com- mercially available FFF printer, was used to print all the electroplated parts in this paper. FFF printers work by feeding a roll of thermoplastic filament through a heated end piece (Fig. 1) where the thermoplastic extrudes as a semi-liquid in thin lines. Several motors move the ex- truder head in the X, Y, and Z direction over the build platform to draw out each layer. The semi-liquid plastic lines cool quickly once deposited and solidify, binding to the layer underneath them. The Makerbot used here has two extrusion heads; for the prints in this work, the right head was used to print Makerbot MP03045 Leaf Green PLA filament [42] while the left printed the Proto-pasta CDP11705 Electrically Con- ductive Carbon Black filament [43]. The Makerbot prints in layers, switching material during each layer, printing PLA from the right head

followed by conductive composite from the left head before moving up to the next layer to repeat the process. This allows individual parts to have multiple overlapping materials.

Printing was done here on an unheated build plate, with the plate covered with painter’s tape (Scotch Blue 2093EL tape, 3 M) for adhe- sion. The individual layer thickness was set to 0.2 mm, and the number of shells (outermost layers of the printed model forming the outer walls) was set to two. The extruder temperature for the non-conductive and conductive composite filaments were set to 91 °C and 104 °C respec- tively and all prints were done with 100% infill. The temperatures used depends on the melting temperature of the material, which is higher for the composite material due to the embedded particulate in the PLA [44].

2.2. Making electrical contact

Following printing, the parts were mounted in a custom 3D printed holder to establish electrical contact to the plating power supply (Fig. 2). A copper foil is clamped against the conductive composite region of the part, with the casing designed to prevent electroplating solution from making contact with the copper foil, allowing only the conductive region to be accessible during the plating process. The holders used in this work were printed on a higher end Stratasys FDM Titan, which is able to print to a finer resolution than the Makerbot to allow a better fit and minimize fluid leakage. The best method found to establish good contact between the foil and the conductive segment was to use raised patterns on the back of the casing piece. (Fig. 2) This provides higher pressure in the region to be plated. This was found to also produce the best results when selecting only certain electrically isolated segments to plate.

Fig. 1. MakerBot FFF 3D printer and extruder assembly.

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2.3. Electroplating setup

A standard copper sulfate bath used previously in our group for fabrication of microscale inductors [45,46] was used for electroplating here (Fig. 3). The bath contains 100 ml of 96% by weight sulfuric acid, 100 g of copper sulfate, and 400 ml of water. A mount was 3D printed that allowed the part to hang parallel to and 25 cm away from a larger copper foil piece in the bath. Keeping the anode and 3D printed part parallel to each other results in consistent distance between the cathode and anode across the part’s surface to obtain a more even copper de- position across the conductive PLA surface. The test piece was dipped into the bath to cover the section to be electroplated but the piece was kept high enough to keep the bare foil exposed above the holder out of the liquid to prevent plating on the contact electrode. A Dynatronix DuPR10-3-6 Pulse Power Supply was then used to apply current during the plating process. Following plating, the conductive region turns a uniform pink color showing consistent plating of copper (Fig. 2).

3. Results and discussion

In order to determine the salient features of the selective electro- plating method introduced in this work the following set of experi- ments, which are further detailed below, were conducted: the con- ductivities of the composite filament and electrodeposition rates were measured as a function of printing orientation; adhesion tests were conducted to assess the viability of this approach; selective electro- plating of arbitrarily placed features was proven; and an electronic part was fabricated and tested to prove that good electrical properties are possible.

Fig. 2. Encasing the test part and making electrical contact.

Fig. 3. Electroplating bath setup.

Fig. 4. Printing placement of depostion test pieces.

Fig. 5. Deposition thickness of copper: vertical print (top), horizontal print (bottom).

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3.1. Resistivity of conductive filament

Due to the layers inherent in FFF printing, the orientation of a part while printing can affect the build quality [47] as well as the resistivity, and the effect of orientation on plating behavior was therefore studied. Test rods, with 2 × 2 × 25 mm dimensions, were printed in two dif- ferent directions, one lengthwise (parallel to the build surface for the x/ y resistivity) and one perpendicular to the build surface (for the z di- rection resistivity). A 4 point resistivity measurement was then per- formed with a Keithley 2100 6 ½ Digit Multimeter on three rods for each orientation. The resistivity in the x/y direction was found to be 12.21 ohm-cm while the z direction was found to have a resistivity of 23.09 ohm-cm (with standard deviation 3.09 ohm-cm and 0.37 ohm-cm for the x/y and z orientations respectively).

3.2. Characterization of copper deposition

Metal deposition during electroplating varies due to current density, time, and plating chemistry. Test pieces (Fig. 2) were designed with a consistent plating area of 20 mm2, with a plating current of 0.04 A in the copper sulfate bath, giving a current density of 0.002 A/mm2. Two sets of test pieces, one printed vertically and one horizontally (Fig. 4), were then plated for different amounts of time to determine the plating rate. Due to the resistivity difference from the filament orientation, the part printed flat parallel to the build plate (horizontal orientation) will have the z direction resistivity between the contact and the plating solution, while the vertically oriented piece will have the x/y resistivity. The results from plating test pieces for 2, 4, 6, and 8 h can be seen in Fig. 5; six measurements were obtained for each time and orientation, with the point and error bars corresponding to average thickness and variability respectively. The deposition thickness was measured with an Olympus LEXT OLS 4000 laser-confocal microscope by obtaining the height difference between the plated region and the surrounding PLA. The plating was approximately linear, with similar rates for both

Fig. 6. (A, B,C) CT scan of horizontal print: (B) top view of copper (C) cross section; (D,E,F) CT scan of vertical print: (E) top view of copper (F) cross section.

Fig. 7. Sample 1 for tape test.

Table 1 Tape Test Results.

Specimen Adhesion strength

Sample 1 4A Sample 2 4A Sample 3 4A

Table 2 ASTM Tape Test Scale [48].

Rating Scale for Tape Tests in ASTM D3359

5A No peeling or removal 4A Trace peeling or removal along incisions or at their intersection 3A Jagged removal along incisions up to 1.6 mm on either side 2A Jagged removal along most of incisions up to 3.2 mm on either side 1A Removal from most of the area of the X under the tape 0A Removal beyond the area of the X

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orientations (24 μm/h for the vertical and 17 μm/h for the horizontal prints). The significant error bars were largely due to the 200 μm printing resolution [41] of the Makerbot, and resulting roughness in the final print (which was particularly pronounced for the highly layered vertical print), as well as the slightly translucent nature of the PLA.

A Zeiss Xradia 510 Versa CT scanner was used to inspect two pieces, both plated 8 h but one printed vertically and the other horizontally (Fig. 6). The cross section scans (Fig. 6C and F) verify that the copper is forming conformal to the 3D print surface without any empty pockets forming as the copper builds up. From the top view (Fig. 6B and E), a number of gaps in the copper can be seen, especially on the horizontal print piece. The original part had evident gaps between some filament lines prior to plating due to imperfections in the print from the low end printer used, and these gaps are believed to result from the imperfect print rather than the plating process. In the horizontal print the com- posite filament was laid down by first outlining the square and then filling in the square with diagonal lines which can be seen in the for- mation of the copper. The vertical composite square was built sideways from horizontal lines making the composite square look striped which can be slightly seen in the formation of copper. The cross section also shows the unevenness of the filament under the copper, which fills in

the crevices, smoothing them slightly, with the copper thickness on the horizontal and vertical print ranging from 130 to 420 μm and 230–620 μm respectively. The individual grains can be seen in the pictures due to relatively large grain size. This result implies that a thermal, mechanical, or chemical smoothing process after printing will improve plating results.

To check the adhesion of the copper on the 3D printed part, a tape test was performed following ASTM standard D3395 [48]. Three piece were printed with a composite square with a 21 mm diagonal; this is slightly smaller than the 40 mm cuts in the standard to allow use of the holders and plating setup here. All the test samples were plated with a current of 0.1 Amps for 2 h resulting in a current density of 0.0022 A/ mm2. Two cuts were made in each piece forming an ‘X’ with a small angle between 30 and 45 ° (Fig. 7). A piece of scotch tape was placed central to the ‘X’ and pressed down to insure full adhesion was achieved. The tape was pulled off in a fast steady motion. On each piece results were similar (Table 1). There were small bits of copper stuck to the tape along the cut line. This falls under the ASTM standard of 4A, which is categorized by “trace peeling or removal along incisions or at their intersection” (Table 2). These tests show that the adhesion achieved with the composite filament is relatively strong and flaking is

Fig. 8. Test pieces (A) selective plating (B) plating with two metals (C) dome in casing for electroplating (D) side view of dome (E) piece before it was placed in chloroform (F) piece after it was placed in chloroform (G) top of via (H) bottom of via.

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Fig. 9. (a) Diagram of tunnel pre-plating, (B) 3 mm long tunnel cross-section; (C) copper under tunnel (D) tunnel test piece.

Fig. 10. (A) via close up; (B) side of via wall.

Fig. 11. 3D Electroplating motor setup for inductor.

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expected to be minimal without active scratching.

3.3. Selective plating

By designing multiple conductive segments that are electrically isolated, different segments can be plated at different times. A rectan- gular test part was designed with two electrically isolated squares (Fig. 8(A)). By making electrical contact to only one square, the part could be completely submerged in the bath and electroplated, with copper depositing only on the contacted square. This same method also allows multiple materials to be electroplated onto the same 3D printed part. As a demonstration, a test part was made combining copper, a high conductivity metal, with a magnetic material (nickel), which would be valuable for making a device such as a magnetic-core in- ductor. A nickel sulfamate bath [49] was used to plate the second metal. A non-conductive PLA rectangle containing six separate con- ductive PLA lines with 1.5 mm width and spacing was designed. The piece was first plated in the copper solution with electrical contact made with three of the six lines, followed by plating of the remaining three lines in the nickel plating bath, resulting in alternating copper and

nickel lines with no overlap (Fig. 8(B)). 3D shapes can be designed and plated as well. A dome with two composite lines forming an “x” was designed (Fig. 8(C) and (D)). The dome was electroplated the same way as the flat surfaces aside from curing the copper anode to follow the arc of the dome across its surface.

Selective electroplating can also be performed on the interior of 3D printed parts without it being necessary for the composite filament to have a direct line of sight with the copper anode, which is another advantage here over laser activation plating methods [38]. Various length tunnels consisting of arches of non-conducting PLA, each creating a tunnel with a 1 mm by 1.5 mm cross section, were designed above conductive lines (Fig. 9). The part was then plated using the plating setup, again with an anode foil placed 25 cm from the 3D printed part. Even though there was no longer a direct path to the conductive PLA region, copper was plated successfully under the full length of every tunnel on the test piece, as verified by a resistance measurement along the length of the tunnel with maximum resistance of 0.3 Ω showing successful plating, with maximum demonstrated tunnel length 9 mm. (Fig. 9 (B)).

For applications where having polymer in the final part is un- desirable, for instance if the part is intended to be exposed to higher temperatures, past work on electroplating of 3D printed parts has de- monstrated dissolving the plastic to leave only the plated metal [32]. For the selective plating process demonstrated here, a similar process is possible using chloroform to dissolve the PLA to leave behind plated copper. A test part (Fig. 8(E)) was dipped half in the chloroform. The PLA and composite filament dissolved where exposed to chloroform, leaving only the plated copper layer (Fig. 8(F)). This method can be used to create air bridges and different electrical components. The PLA filament was completely soluble in the chloroform while the composite filament took longer to dissolve and wasn’t completely soluble, leaving behind a residue of the conductive particulate. With further develop- ment/improved solvent selection, the carbon black could likely also be dissolved to obtain cleaner electroplated copper parts.

Another possible application of this technique is the creation of a 3D printed circuit board. Printed circuit boards (PCBs) are an important and widely used technology in electronics manufacturing, with a market size of tens of billions of dollars every year [50]. In a PCB, layers of metal used for electrical interconnect are patterned on insulating layers used for mechanical support and isolation to create a laminated board [51]. While it is possible to make a single layer PCB, multi-layer and double-sided boards allow far more complexity [51] but also re- quire the capability for connection between layers to allow crossover of electrical traces. The thicknesses plated in the work here roughly cor- respond to between 2 oz. and 4 oz. printed circuit boards according to the convention in the industry (referring to ounces of copper per square foot of board, with 1 oz. of copper equaling approximately 35 μm thick copper layers) [52]. The possibility of creating vertical vias and un- derpasses was therefore also investigated. A via structure consisting of a hole surrounded by conductive PLA was uniformly plated to join se- lectively plated copper traces on the front and back (Fig. 10). This technique was then used to create an underpass, allowing two plated copper lines to cross using two plated vias (Fig. 8(G) and (H)). The part in Fig. 8G and H was plated in three steps: first plating of the straight line (vertical in the image), followed by plating of the top of the hor- izontal line (Fig. 8G), and then finally plating of the underpass (Fig. 8H). The composite filament lines are separated from each other with PLA plastic 1 mm thick.

3.4. Inductor

As a demonstration of the creation of a functional 3D electrical device, an air core solenoid inductor with an outer diameter of 21 mm was designed and created using this selective electroplating technique. When plating the solenoid inductor, a motor was used to rotate the solenoid to produce an even coating around the whole piece. To avoid

Fig. 12. Plated solenoid: Black sections are the composite filament, with the outer surface of these sections consistently pink in color indicating uniform coating of copper.

Fig. 13. Inductance (top) and resistance (bottom) of solenoid inductor following plating.

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twisting of the wires, the motor was oscillated back and forth with an AC input from a function generator (Fig. 11). Once a solenoid was successfully plated, (Fig. 12) an Aglilent 4294A Precision Impedance Analyzer was used to find the inductance and the resistance, averaging 191 nH and 18.7 mΩ respectively from 10 kHz to 100 kHz (Fig. 13), a five order of magnitude reduction in resistance over the original re- sistance of 3 kΩ for the unplated inductor. By assuming a thickness of 150 μm and a bulk resistivity of copper to be 17.2 nΩ m [22], the ex- pected resistance is calculated to be 11.3 mΩ, reasonably similar to the measured value. The modest variation can be attributed to the clamping design and unevenness of the surface, causing certain parts to not plate evenly causing black composite spots to be seen at times along with the copper length. In initial plating, the region where the foil was clamped failed to plate effectively due to imperfectly applied pressure in that area, resulting in a black patch. The solenoid was then re-mounted to more consistently apply pressure there followed by a final plating run without turning on the motor to plate this region. This is an im- perfection in our current clamping setup rather than an issue with the technique, and with an improved clamp design the solenoid would plate evenly.

4. Conclusion

In this paper it is shown that it is possible to electroplate selectively onto FFF printed parts to improve electrical properties and print true electromechanical systems. Using a conductive filament and a non- conductive filament, a part can be electroplated straight from the 3D printer using the conductive filament to map where the metal deposi- tion will occur. Different isolated conductive segments can also be added to plate separately to allow the addition of multiple plating materials on the same part. As a demonstration, a complex 3D shape, a solenoid inductor, was printed with a resistance of 3 kΩ. After plating the resistance dropped to 18.7 mΩ and an inductance of 191 nH. With alternative 3D-printing-based techniques, manual labor, preprocessing parts, or a machine worth hundreds of thousands of dollars are required compared with a low cost hobbyist 3D printer as used here. Going forward this work could also be used in higher quality FFF printers to develop higher resolution parts. One limitation in our current approach is the need to clamp relatively near the region to be plated to obtain reasonable resistances during plating; this is largely due to the rela- tively resistive filament used here. This is not a fundamental limitation; custom filaments could be made or other less resistive conductive fi- laments could be obtained commercially [20] that could be used in this selective plating process, allowing simpler plating of complex features.

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