A

Review Paper

Submitted

By

Hardik Gandhi (50166075)

On

3D PRINTING MATERIALS FOR ELECTRICAL

CIRCUITS

In

The partial fulfillment

of

Coursework IE 506

ABSTRACT

One of the greatest challenges for the electrical circuits is formulation and

processing of material, effects of oxidation on inks and various desired

properties required such as surface tension, adhesiveness and density for

ideal inks are discussed in the start to begin with. Here we limit our

discussion only upto conductive materials. In the beginning two different

formulations of particle-free conductive solutions are introduced that are

low in cost, easy to deposit, and possesses good electrical properties.

Further, a metallo-organic decomposition (MOD) ink with its wear and

fracture properties are subsequently introduced in brief [2]. Next, the

paper discusses about Graphene material for ink-jet printing. In addition

to aforementioned materials for 3D printing this paper further extends to

discuss a simple, low cost conductive composite material for 3D printing

in great details. As part of it, “carbomorph” as a composite material and

its physical properties with results are discussed in lucrative way. To

conclude this paper comparison between various materials is discussed in

the end.

Keywords: Aqueous solution ink, conductive inks, Electrical circuits,

Graphene, Piezoelectric effect, Carbomorph, Capacitive Sensors

INTRODUCTION

3D printing (3DP) is a term to describe technology used for the rapid

production of 3D objects directly from digital computer aided design (CAD) files.

The 3D printing process allows 3D objects to be fabricated in a bottom-up, additive

fashion directly from digital designs, with no milling or molding. It can be likened to

clicking on the print button on a computer and sending a digital file, such as a letter,

to a printer sitting on an office desk. The difference is that in a 3D printer the material

or ink is deposited in successive, thin layers on top of each other to build-up a solid

3D object. Various governing factors and individual assessment of each types of inks

is listed below:

Resistance of this conductive material is deciding factor for various industrial

application. One of the general method of fabricating electrical circuits called

photolithography make use of selective masking and etching technologies to create

metal substrate on the surface for upto 100nm levels. But this method of printing is

proven to be time consuming and expensive. In addition to that, photolithography

can’t be used where substrates are manifold in three dimensions[2]. To overcome

these difficulties researchers came up with the idea of inkjet printing, as a subcategory

of direct writing electrical component. There are two distinct categories of inkjet

techniques that have become famous. The first being Drop-on-Demand and second is

Continuous-Inkjet. As part of this review paper we will not go into the depth of these

methods but central idea would be to discuss the challenges of preparing conductive

inks for these methods and their respected properties.

Another method called Instant Inkjet Circuits has ability to print highly

conductive traces and patterns onto flexible substrates such as paper and plastic films

cheaply and quickly [4]. Electrical components such as large-scale sensors, high

frequency antennas, actuators, electrical components for switches, moisture sensors,

and capacitive touch sensors are supported by this technique. One of the biggest

advantages of using this methodology over others is it uses the inks, which don’t

require sintering, and which can be applied cheaply and easily. Further details about

inks, merits and demerits of it are explained in the detail in following chapter. For

electrical component such as transistors, photovoltaic devices, organic light emitting

diodes and displays printed from Inkjet printer uses Graphene, which is the two

dimensional building block for sp2 carbon allotropes and further it can be stacked into

3d graphite, or rolled into 1d nanotubes, or 0d fullerenes. More details on properties

of Graphene and ink requirements are discussed in progressive chapter [3].

Inks used for above all the methods have limited-functionality. For the

expensive business of fabricating electric sensors which finds many applications in

the medical implants uses a new simple, low-cost composite material termed

‘carbomorph’is presented in the end of the following chapter. Material formulation

and testing for carbomorph, its characterization, application on 3DP Flex Sensor, 3DP

Embedded Flex Sensor and 3DP Capacitive Buttons as a latest interest of researchers

are also covered [1].

DISCUSSION

1. CHALLENGES OF CONDUCTIVE INKS FOR INKJET PRINTING

One of the foremost and important property for printing electronic

components is the formulation of the conductive inks with suitable physiochemical

properties. Formulation is critical because it determines the inks’ ability to be printed,

its adherence to a substrate, the resolution of the conductor produced, and the

mechanism for conductivity. Surface tension and viscosity, the most important

rheological properties of the ink, play a crucial role in optimizing the ejected droplet

velocity, size, and stability, as well as the shape of the droplets impinging on the

substrate. The impingement shape establishes trace resolution and thickness, which

ultimately determine the trace mechanical and electronic properties. Inks possessing a

surface tension on the order of 25–70 mN/m and Newtonian viscosity of 1–10 MPa

were shown to be most suitable for Continuous Inkjet printing. But, most

commercially available conductive inks generally possess viscosities above this range

and do not allow them to be inkjet printed.

To satisfy the comprehensive requirements of inkjet printing of electrical

circuits, the conductive inks must contain the appropriate precursors, vehicles, and

other components. Various adhesion promoters are generally included in the

composition, depending on the nature of the precursors. In the case of inks for

conductors, the contents must be adjusted to provide the required resolution, adhesion,

and electronic conductivity for the printed lines. Conductive inks are typically based

on noble metals because of the chemical inertness in ambient atmosphere and good

electrical conductivity. There are several categories for precursors of conductive inks

that include nanoparticles and metallo-organic compounds. One of such precursor of

conductive ink is Aqueous solution ink based on Silver Nitrate is discussed below.

Nano-scale particles or metallo-organic decomposition (MOD) precursors are

two general resources for synthesizing conductive inks whose rheological properties

permit inkjet printing. They work well under small-scale experimental conditions, but

these materials are expensive and sometimes not available commercially. In this paper

I would like to focus on the precursor developed by the researchers. These precursors

are commercially available and substantially less expensive than metallo-organic

compounds. Due to its low cost, silver nitrate was chosen as the primary inorganic

compound in an aqueous ink composition. Silver nitrate is used in almost all

processes for producing silver compounds and has a wide range of applications that

vary from painting, xerography, chemical electroplating, electric batteries, and

medical catalysts. Silver nitrate melts at 212 C and decomposes to silver at

temperatures between 440 C to 500 C. It also has solubility of 219 g in 100 g water

at room temperatures, which is significant when very dense ink is needed for high-

resolution electric circuits. Considering its broad resources and physical properties,

silver nitrate makes an excellent candidate for conductive inks. But, after being

printed on a glass slide and cured at 500 C for 15 min, a structure was obtained that

was neither aesthetical nor functional. So, additives are deemed necessary to change

the characteristics of this Ink Based on Silver Nitrate.

After numerous tests, the researchers found Aluminum nitrate nonahydrate

(molecular formula: Al NO 9H O) to be the most effective additive for improving

the conductive silver nitrate ink. Fig. 1 shows a scanning electron microscopy (SEM)

structure of the conductive trace that was produced after curing.

Figure1Microstructureofconductivetraceafternovelconductiveinkwascured

From the above figure they conclude that the silver has a purity of 95% and

produces good electric conductance. The high purity of silver demonstrates that the

oxidation is insignificant during the 485 C curing in an ambient atmosphere. In

addition, the electric conductance was found to remain unchanged when the material

was cured in a protective atmosphere (964%H) that is known to minimize oxidation.

Silver film from different cells is continuous, which assures silver conductivity

between cells. This property is crucial for continuous conductive traces in a macro-

scale because the slightest amount of discontinuity will make the trace nonconductive

as a whole. The durable aluminum oxide ridges surround the silver cells to protect the

soft silver film from wear and also allow it to strongly adhere to the glass substrate.

This special functional composite structure explains the traces’ beneficial

characteristics.

Once the preliminary ink solution was obtained, researchers carried out a

factorial design with MINITAB to determine the critical processing parameters for

further optimizing the conductive inks. Mechanical adherence of the ink was not

considered because it was found to be a very stable property and closely correlated

with the quality of the electric resistance and thus only they screened only three

factors:

• Ink density: high (1.68 g/mL) and low (1.4 g/mL);

• Substrate temperature: high 75 C and low 20 C;

• Curing time and temperature: High Temperature at short interval and low

temperature at large interval

It is clear from Fig. 5 that all three factors—ink density, plate temperature, and

curing parameters—were important variables. By further analysis, curing temperature

and ink density were found to be relatively more important. Based on the factorial

analysis, dense inks, low substrate temperatures, as well as low curing temperatures

were found to provide the best results for reducing electric sheet resistance.

Figure2FactorialDesignandMainEffects

The response surface method, shown in Fig 3., was used to find the optimum

composition for the new ink. There were two additional factors studied: the amount of

silver nitrate (ranging from 0.4 1.5 g), and the amount of additive (ranging from 0.05

0.4 g) material in 1 mL water. Taking into consideration the solution stability and

cost of the ink, 1.0 g silver nitrate plus 0.2 g of additives was determined to be the

best combination studied and worthy of further investigation [2].

Figure3ResponseSurfaceforElectricalResistivity

2. Sintering-free Conductive Ink

Recent advances in materials science have resulted in a variety of

commercially available conductive inks and paints, which do not require sintering and

which can be applied cheaply and easily. Bare Paint and related products are already

popular among hobbyists and artists, but unfortunately the properties of these paints

mean that they are not suitable for on-demand rapid prototyping. For example, their

high sheet resistance (e.g. 55 Ω/�1) makes it virtually impossible to use narrow traces

or printed antennas. Moreover, because of their high viscosity, screen-printing must

be employed instead of inkjet printing in order to print precise patterns. Unfortunately,

screen-printing requires a time-consuming and expensive process of making a screen

and stencil, and requires large amounts of ink, which are ultimately wasted.

Conductive ink pens are popular with electronics professionals. These are

designed for the manual rework of electrical circuits to fix breaks in electrical

conductivity. However, the main draw- back of this technology is the relatively large

size of the silver particles, which are suspended in the ink in order to make it

conductive. This makes it hard to create patterns less than 1 mm wide. It is also

impossible to deposit the thin, consistent layer of ink required for flexible circuitry, so

the traces are very brittle [4].

3. Graphene as an Inkjet Printed Material

Graphene is at the center of an ever-expanding research area. Near-ballistic

transport and high mobility make it an ideal material for Nano-electronics, especially

for high- frequency applications. Furthermore, its optical and mechanical properties

are ideal for micro- and Nano-mechanical systems, thin-film transistors, transparent

and conductive composites and electrodes, and photonics. Graphene was isolated by

micromechanical exfoliation of graphite. This technique still gives the best samples in

terms of purity, defects, mobility, and optoelectronic properties. However, large- scale

production approaches are needed for widespread application. These encompass

growth by chemical vapor deposition, sublimation of Si atoms by heat treatment of

silicon carbide, segregation from metal substrates, and liquid phase exfoliation (LPE).

Among these, LPE is ideally suited to produce printable inks. LPE was first achieved

through sonication of graphite oxide.

A key property of inks viable for printing is their ability to generate droplets.

Ink viscosity, surface tension, density, and nozzle diameter influence the spreading of

the resulting liquid drops. These can be arranged into dimensionless figures of merit

such as the Reynolds, Weber, and Ohne-sorge numbers. During printing, the primary

drop may be followed by secondary (satellite) droplets. This needs to be avoided in

drop-on-demand printing. When inks contain dispersed molecules or nanoparticles,

the latter should be smaller than the nozzle diameter, to prevent clogging. Research

suggests that, as a sufficient condition, that they should be at least 1/50 of the nozzle

diameter, in order to exclude any printing instability, such as clustering of the

particles at the nozzle edge, which may cause deviation of the drop trajectory, or

agglomerates, eventually blocking the nozzle.

Stable dispersions of Graphene require the Gibbs free energy of mixing,

ΔGmix, to be zero or negative, where ΔGmix = ΔHmix _ KΔSmix, K being the

temperature, ΔHmix the enthalpy of mixing, and ΔSmix the entropy change in the

mixing process. For Graphene and nanotubes, ΔSmix is small. Therefore, for

dispersion and stabilization of Graphene in solvents, ΔHmix needs to be very small.

This can be achieved by choosing a solvent whose surface energy is very close to that

of Graphene [3].

4. Composite material “carbomorph” for 3d printing of electronic

sensors

Carbon Black (CB) is an amorphous form of carbon, produced from the

incomplete combustion of heavy petroleum products such as FCC tar, coal tar,

ethylene cracking tar and a small amount from vegetable oil. As such it is a readily

available and inexpensive. Amorphous CB has been previously shown to be a good

filler material in conductive polymer composites. [11]. CB is preferable for this

application over a material such as copper because finely divided copper is prone to

oxidation and becoming non-conductive. A transition from insulating to non-

insulating behavior for composites with conductive filler is generally observed when

the volume concentration of filler reaches a threshold of about 25%. [12]. To provide

a printable thermoplastic matrix for the composite, we chose a readily available

modeling plastic, polymorph, a commercial formulation of polycaprolactone (PCL).

PCL is biodegradable polyester with a low melting point of around 60degC and a

glass transition temperature of about 260uC. The low temperature processing

conditions of the polymorph offers significant advantages for formulating the final

composite to work in the 3D printer as it did not require high temperature or

expensive extrusion equipment. This new composite material is termed as

‘carbomorph’.

The electrical conductivity of the of the carbomorph depends on the physical

mixture of the conductor with the insulator in high enough proportions that electrons

can either percolate through a network of carbon black. The filler ratio had to be high

enough to deliver a useable electrical conductivity but low enough so as to enable the

material to exit the heated extrusion nozzle of the 3D printer.

(I) Use of Carbomorph as a 3DP Flex Sensor

Resistivity tests proved that the printed carbomorph material exhibited

piezoresistive behavior. The piezoresistive effect describes the changing resistivity of

a semiconducting material due to applied mechanical stress. Piezoresistivity is a

common sensing principle for micro-machined sensors. In order to test the sensing

properties of the printed material and incorporate the electrical connection method

into the devices, a CAD design was made of a standalone monolithic printed device

composed of a carbomorph track (composed of a single printed filament) with two

printed sockets at the end for connection to ‘banana plugs’ (Fig. 4) below.

Figure43DPFlexSensor

By printing the sockets for connection in the same single build process the

need for connecting to the tracks using conductive paint or glue for instance is

removed, making sensor implementation and use much easier.

(II) Use of carbomorph as a 3DP Embedded Flex Sensor

Under this topic researcher showed an ‘exo-glove’ to detect resistance changes

upon movement of the finger (Fig. 5). The design incorporated loops for attachment

of a securing a strap and printed jaws for gripping to fingers. The whole device was

printed in a single, un-paused print run with the flat back portion of the ‘glove’

printed first (Fig. 5i). Contacts were made to the tracks in this case using silver loaded

epoxy resin in order to maintain an ohmic contact during testing. Upon the authors

putting on the glove and flexing a finger (Fig. iii & iv) the resistance of the tracks was

seen to change.

Figure53DPEmbeddedprintedglove

This effect could be repeated for each of the fingers of the glove. A difference

in initial resistance of the individual fingers was observed due to the differing length

of the carbomorph tracks within each finger. Fingers 1 and 3 (the index and ring

fingers respectively) are of similar length, while finger 2 (the middle finger) is much

longer and hence exhibits a higher resistance and finger 4 (the little finger) is shorter

and hence exhibits a lower resistance. With the manufactured sensors, the response

was still detectable with a simple potential divider and basic electronics and did not

require amplification.

(II) Use of carbomorph as a 3DP Capacitive Buttons

In addition to its piezoresistive properties, the carbomorph could be used to

print capacitive devices. Capacitive sensing as a Human Interface Device (HID)

technology, for example to replace the computer mouse, is growing increasingly

popular. An example 3D printed capacitive interface was designed to interface to a

computer as described in figure 3a. Again, the device was designed to accept

commonly available ‘banana plugs’ into a 3D printed socket. To use the printed

device, a user touches the printed conductive pad, the capacitance of the pad increases,

which is then sensed by the board and used to trigger an operation [1].

Figure6CarbomorphusedasHID

RESULTS AND CONCLUSION

This review paper was aimed to cumulate and compare various levels of conductive

materials used in Electrical or composite circuit. To conclude this paper I envision

that in future researchers would definitely find materials or inks better compared to

aluminum nitrate to overcome oxidation of it when applied on substrates, which can

be used as multiple ranges of temperature varying from low to very high. Further, till

the time Graphene has provided us lots of excitement as a 3D Printing material. But,

further research on Graphene would definitely lead to improve its mobility; thus

improving its viability for flexible electronics. In the end, Capacitive sensing can

further be used in many different types of sensors, including those to detect and

measure proximity position or displacement humidity fluid level and acceleration to

biomedical and medical industry. In sum, research can be done in the direction to

cancel out various materials for various applications to one unique and standardize

material for 3D Printing, which have noble adherence, surface tension, viscosity,

Glass Temperature and above all high-speed and industrial low cost such that the use

of 3D Printing as a next advanced manufacturing technology can be commercialized.

REFERENCES AND NOTES

1. A Simple, Low-Cost Conductive Composite Material for 3D

Printing of Electronic Sensors by Simon J. Leigh, Robert J.Bradley,

Christopher P.Purssell, Duncan R.Billson and David A. Hutchins at

School of Engineering, University of Warwick, U.K.

2. Formulation and Processing of Novel Conductive Solution Inks in

Continuous Inkjet Printing of 3D Electric Circuits by Junfeng Mei,

Michael R. Lowell and Martin H. Mickle, Life Fellow, IEEE

3. Inkjet-Printed grapheme Electronics published in ACS NANO

from Department of Engineering, University of Cambridge,

Cambridge, U.K.

4. Instant Inkjet Circuits: Lab-based inkjet Printing to Support Rapid

Prototyping of UbiComp Devices by Yoshihiro Kawahara, Steve

hodges, Benjamin S. Cook, Cheng Zhang, and Gregory D.Abowd