Graduation Year

2020

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Electrical Engineering

Major Professor

Arash Takshi, Ph.D.

Committee Member

Sylvia Thomas, Ph.D.

Committee Member

Jing Wang, Ph.D.

Committee Member

Sarah Ying Zhong, Ph.D.

Committee Member

Theresa Evans-Nguyen, Ph.D.

Abstract

The current project focused on studying an innovative and novel method, called hydrogen evolution assisted (HEA) electroplating, for rapid and localized electrochemical copper deposition that can be applied for integration of electronic circuits to printed circuit boards (PCBs) and fabrics for developing smart textile (e-textile) and advanced wearable electronics. This method has shown a great enhancement to the deposition rate of copper compared to the conventional galvanostatic electroplating method, opening new venues for patterning conductive circuit layouts on fabrics and for the direct integration of devices into fabrics. While due to the slow growth rate and low mechanical flexibility of copper, the conventional electrodeposition has not been much studied for wearable electronics, our new HEA electroplating method addresses both issues.

Our preliminary studies on HEA electroplating showed the feasibility of growing copper laterally across a 1 mm gap between two copper cathodes (~4 mm wide) that were patterned on a FR-4 printed circuit board (PCB) substrate by the usage of an acidic electrolyte made of H2SO4 and CuSO4. The lateral growth was achieved by applying an electric field of ~1 V/cm between the two cathodes while the average voltage between an anode (i.e. copper electrode) and each cathode was of 1.25 V. The large voltage difference between the anode and the cathodes resulted in concurrent reduction of copper at the cathodes and evolution of hydrogen (i.e., HEA).

As a part of this research the effect of H2SO4 and CuSO4 concentrations on the lateral growth speed was studied. Incrementing the acid concentration in the aqueous solution to 1.5 M (H2SO4) and using the HEA electroplating process at room temperature showed a direct relation in the acceleration of the deposition rate reaching a speed of 110 μm/s. Despite the porous structure, the HEA electroplated copper showed a similar conductivity as solid copper and a remarkable mechanical strength suitable for use as interconnects on an electronic circuit.

In HEA, concurrent to the copper reduction, hydrogen bubbles are generated at the cathode resulting in a porous copper layer. However, the application of constant voltage does not allow bubbles to leave the surface of the electroplated electrode resulting in a non-uniform copper growth which in turn results on an unpredictable micro and nanostructures. To address the problem, we have studied the effect of superimposing a low frequency triangle shape AC to the DC potential to allow the hydrogen bubbles leaving the surface. The results clearly show that by controlling the AC and DC voltages around 100 mHz AC, a fast copper layer with fern shape nano structure can be achieved.

Further, the feasibility of adopting HEA electroplating method for patterning an electronic circuit on fabrics has been tested for copper deposition on a few different samples of fabrics. To provide the conductive path for the electroplating, first, the desired pattern was applied as a template on the fabric using a conductive ink including multiwalled carbon nanotubes (MWNTs). It was found that the type of the fabric affects the copper growth rate. The fastest copper growth rate was achieved on 1000 Denier Coated Cordura Nylon sample, while 100% Virgin Vinyl sample showed the slowest growth rate. Further study of the morphologies using the scanning electron microscopy (SEM) technique showed that the material being used as substrate has a direct relationship with the morphology of the electroplated copper as well as the scan rate and the potential range being applied.

The observed low resistance of the electroplated copper is a promising sign that HEA electroplating can be used for rapid copper deposition on various substrates. The produced hydrogen bubbles from the HEA electroplating formed a porous structure while the copper growing was taking place with a speed rate a few orders of magnitude higher than the growth speed in the conventional copper electroplating. The presented results in this dissertation verify the feasibility of using the HEA electroplating as a viable method for making copper-based circuit layouts directly on fabrics.

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