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Research Papers

Ohmic Curing of Three-Dimensional Printed Silver Interconnects for Structural Electronics

[+] Author and Article Information
David A. Roberson

W. M. Keck Center for 3D Innovation,
The University of Texas at El Paso,
El Paso, TX 79968
e-mail: droberson@utep.edu

Ryan B. Wicker, Eric MacDonald

W. M. Keck Center for 3D Innovation,
The University of Texas at El Paso,
El Paso, TX 79968

1Corresponding author.

Contributed by the Electronic and Photonic Packaging Division of ASME for publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received May 15, 2013; final manuscript received March 30, 2015; published online April 17, 2015. Assoc. Editor: Paul Conway.

J. Electron. Packag 137(3), 031004 (Sep 01, 2015) (8 pages) Paper No: EP-13-1041; doi: 10.1115/1.4030286 History: Received May 15, 2013; Revised March 30, 2015; Online April 17, 2015

Ohmic curing was utilized as a method to improve the conductivity of three-dimensional (3D) interconnects printed from silver-loaded conductive inks and pastes. The goal was to increase conductivity of the conductive path without inducing damage to the substrate. The 3D via/interconnect structure was routed within 3D polymeric substrates and had external and internal sections. The 3D structures were created by the additive manufacturing (AM) process of stereolithography (SL) and were designed to replicate manufacturing situations which are common in the fabrication of 3D structural electronics that involve a combination of AM and direct write (DW) processing steps. The photocurable resins the 3D substrates were made of possessed glass transition temperatures of 75 °C and 42 °C meaning that a nonthermal method to increase the conductivity of the printed traces was needed as the conductive inks tested in this study required oven cure temperatures greater than 100 °C to perform properly. Ohmic curing was shown to decrease the measured resistance of the via/interconnect structure without harming the substrate. Substrate damage was observed on thermally cured samples and was characterized by discoloration and scaling of the substrate. Resistance measurements of the via/interconnect structures revealed samples cured by the ohmic curing process performed equal or better than samples subjected to thermal curing. The work presented here demonstrates a method to overcome the thermal cure temperature limitations of polymeric substrates imposed on the processing parameters of conductive inks during the fabrication of 3D structural electronics and presents an example of overcoming a manufacturing process problem associated with this emerging technology. An ink selection process involving characterization of the compatibility of inks with the substrate material and the use of different inks for the via and interconnect sections was also discussed.

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References

Figures

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Fig. 5

(a) Photograph of experiments where CB102 used as vias and E1660 used as interconnects on ProtoTherm™ substrates and (b) the graphical results. Placement of the samples in the photographs corresponds with identifiers in the graphical results.

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Fig. 6

Resistance versus time plots for ohmic curing cycles carried out at increasing applied current values for the hybrid E1660/CB102 conductive path printed in ProtoTherm™ 3D structures

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Fig. 4

(a) Photograph of experiments where E1660 was used as vias and interconnects on ProtoTherm™ substrates and (b) the graphical results. Placement of the samples in the photographs corresponds with identifiers in the graphical results.

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Fig. 3

(a) Schematic of the scaffold support used to facilitate the 3D printing of complex geometries. (b) Photographs showing an example of the “scaling” substrate damage imposed by curing a ProtoTherm™ structure for 30 min at 138 °C on a substrate built on a scaffold support compared with a substrate cured at the same temperature, built on a polycarbonate sheet. Note the lack of scaling on the substrate built without a scaffold support.

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Fig. 7

(a) Photographs of experiments involving substrates that were 3D printed from WaterShed™. Note the yellowing of the substrates associated with thermal curing at temperatures above 46 °C and (b) graphical results of the thermal cure process. Placement of the samples in the photographs corresponds with identifiers in the graphical results.

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Fig. 8

(a) Photographs of experiments involving WaterShed™ substrates where E1660 used to print interconnects and CB102 was used to print vias and (b) graphical results of the thermal and ohmic cure processing. Placement of the samples in the photographs corresponds with identifiers in the graphical results.

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Fig. 9

Example of tailoring the applied current based on wattage to cure process outliers from the thermal precure process for (a) E1660 conductive trace printed in a ProtoTherm™ substrate and (b) combination E1660/CB102 conductive path printed in a WaterShed™ substrate

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Fig. 10

Typical resistance versus time behavior for an ohmic curing cycle. The first cycle at a given applied current causes a sharp drop in resistance. The subsequent cycles are not able to cause further significant curing effect. In this case the applied current was 4 A.

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Fig. 2

Examples of the process issues encountered when attempting to use DuPont CB028 ink on 3D structures: (a) dewetting of the ink in the via structure of a ProtoTherm™ structure; (b) ejection of the ink during the thermal curing process in a WaterShed™ structure; and (c) dewetting of the ink in the via portion of a WaterShed™ structure

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Fig. 1

The test structure used in this study which featured internal vias and external interconnects

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Fig. 11

(a) Optical micrograph showing evidence of charring of DuPont CB102 in a ProtoTherm™ substrate and (b) backscatter SEM micrographs showing charring of DuPont CB102 near the via/interconnect interface of a dual E1660/CB102 interconnect/via structure on a WaterShed™ substrate

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