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Plant-Based Completely Biodegradable Printed Circuit Boards

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  • CIIRC-JYOTHY INSTITUTE OF TECHNOLOGY

Abstract

Completely biodegradable printed circuit boards (PCBs) have been developed using biocomposites made from natural cellulose fibers extracted from banana stems and wheat gluten, which are normally considered as agricultural wastes or coproducts. PCBs were fabricated using these composites with properties suitable for electronic applications. The biocom-posites are free of chemicals, and an environmentally benign approach was adopted to fabricate the PCBs. Conventional PCBs are critical components in electronics and are currently made using fire resistant plastics (FRPs). FRPs are typically made using glass fibers and epoxy, which are nonbiodegradable when disposed in the environment. Several attempts have been made to develop environmentally friendly PCBs and other electronic components. Although dissolvable electronics and foldable PCBs have been reported, so far, there are no 100% biodegradable PCBs. The dielectric constant for banana fiber/wheat gluten composite varied between 2-36, which is in the range of dielectric materials used for PCB and other electronic components. A significant amount of heat (up to 45 °C) was dissipated through the biocomposite preventing overheating and thus reducing risk of fires. PCBs did not show any deterioration in performance even after exposure to high humidity (90%) or high temperature (100 °C). LED connected to the PCB was able to glow without any interruption. Natural fibers and protein-based PCBs may provide an alternative to the synthetic polymer-based electronic components and help us to reduce the environmental burden due to the disposal of electronic waste (e-waste). Index Terms-Biodegradable, electronic waste (e-waste), natural fibers, printed circuit board (PCB), proteins.
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IEEE TRANSACTIONS ON ELECTRON DEVICES 1
Plant-Based Completely Biodegradable
Printed Circuit Boards
Vijay Kumar Guna, Geethapriya Murugesan, Bhuvaneswari Hulikal Basavarajaiah, Manikandan Ilangovan,
Sharon Olivera, Venkatesh Krishna, and Narendra Reddy
Abstract Completely biodegradable printed circuit boards
(PCBs) have been developed using biocomposites made from
natural cellulose fibers extracted from banana stems and wheat
gluten, which are normally considered as agricultural wastes
or coproducts. PCBs were fabricated using these composites
with properties suitable for electronic applications. The biocom-
posites are free of chemicals, and an environmentally benign
approach was adopted to fabricate the PCBs. Conventional PCBs
are critical components in electronics and are currently made
using fire resistant plastics (FRPs). FRPs are typically made
using glass fibers and epoxy, which are nonbiodegradable when
disposed in the environment. Several attempts have been made
to develop environmentally friendly PCBs and other electronic
components. Although dissolvable electronics and foldable PCBs
have been reported, so far, there are no 100% biodegradable
PCBs. The dielectric constant for banana fiber/wheat gluten
composite varied between 2–36, which is in the range of dielectric
materials used for PCB and other electronic components. A
significant amount of heat (up to 45 °C) was dissipated through
the biocomposite preventing overheating and thus reducing risk
of fires. PCBs did not show any deterioration in performance
even after exposure to high humidity (90%) or high temperature
(100 °C). LED connected to the PCB was able to glow without
any interruption. Natural fibers and protein-based PCBs may
provide an alternative to the synthetic polymer-based electronic
components and help us to reduce the environmental burden due
to the disposal of electronic waste (e-waste).
Index Terms—Biodegradable, electronic waste (e-waste),
natural fibers, printed circuit board (PCB), proteins.
I. INTRODUCTION
PRINTED circuit boards (PCBs) are an integral part of
electronic gadgets. The ever increasing use of electronic
goods and their short-life span result in generation of consider-
able amounts of electronic waste (e-waste). A study by United
Nations University has estimated that the global disposal of
Manuscript received August 23, 2016; revised October 12, 2016; accepted
October 19, 2016. The work of N. Reddy was supported by the Department
of Biotechnology, Ministry of Science and Technology, Government of India,
through the Ramalingaswami Fellowship. The review of this paper was
arranged by Editor H. Klauk. (Corresponding author: Narendra Reddy.)
V. K. Guna is with the Centre for Incubation, Innovation, Research and Con-
sultancy, Jyothy Institute of Technology, Bengaluru 560082, India, and also
with the Regional Resource Centre, Visvesvaraya Technological University,
Belagavi 590018, India.
G. Murugesan, M. Ilangovan, S. Olivera, V. Krishna, and N. Reddy are with
the Centre for Incubation, Innovation, Research and Consultancy, Jyothy Insti-
tute of Technology, Bengaluru 560082, India (e-mail: nreddy3@outlook.com).
B. H. Basavarajaiah is with the ACS College of Engineering,
Bengaluru 560074, India.
Color versions of one or more of the figures in this paper are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TED.2016.2619983
e-waste was about 41.8 metric tons in 2014. Although most
countries have laws that regulate disposal of e-waste and
promote take-back systems, only about 6.5 tons of e-waste
was collected for recycling [1]. In most cases, recycling of
e-waste is not done scientifically, and the processes used
to recollect the parts and metals cause substantial harm to
the environment. Electronic goods contain metals such as
mercury, cadmium, and lead and chemicals such as brominated
flame retardants, polychlorinated biphenyls, and hexavalent
chromium, which have been reported to impair mental devel-
opment, and also cause damage to lung, liver, and kidneys
(http://unu.edu/news/news/ewaste-2014-unu-report.html).
Since the use of electronic goods will continue to increase,
attempts are being made to improve the recyclability and
reuse of e-waste. PCBs are particularly difficult to recycle due
to their complex structure and diverse components that they
house [2].
One of the approaches to reduce the environmental bur-
den of e-waste is to make electronic goods biodegradable.
Biodegradable organic electronic materials or green electron-
ics have been developed using proteins, carbohydrates, and
biodegradable synthetic polymers [3]. Such organic electronics
have been applied in organic light emitting diodes displays,
organic FETs (OFETs), conductors and semiconductors, and
in medical devices [3]. Paper-based rechargeable batteries
for energy storage, biointegrated electronics consisting of
antennae and power coils and sensors that can monitor the
functioning of the heart and brain have been developed using
water soluble polyvinyl alcohol [1].
In addition to being biodegradable, electronic goods are also
expected to be biocompatible, particularly for medical applica-
tions. Proteins are considered to be more biocompatible than
carbohydrates and synthetic polymers [4], [5]. Researchers
have attempted to use biopolymers including proteins to
develop electronic goods for food, medical, and other appli-
cations. In one such attempt, a silk-based all protein array of
metamaterial antenna was used to monitor the environment and
predict the quality of food [6]. Similarly, silk fibroin supported
ultrathin electronics (sensors) that could conform to surfaces
but later dissolve and resorb were developed for diagnosing
and treating diseases [7]. The sensor developed was considered
to be biodegradable and edible and hence suitable for food
and medical applications. In another study, organic transistors
that could operate at 4–5 V using source drain current of
up to 0.5 μA were made using natural organic dielectrics
such as glucose on inorganic oxide dielectrics that could
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2IEEE TRANSACTIONS ON ELECTRON DEVICES
self-assemble for food, biomedical, and other applications [8].
Albumin taken from fresh egg was converted into dielectric
material for OFETs [9]. The material developed was found
suitable for flexible OFETs and inverters.
Biopolymers have been used to develop biodegradable
and even water soluble PCBs. Transient PCBs were made
using sodium-carboxymethyl cellulose as the base and mag-
nesium, tungsten, or zinc as electrical connectors [10].
Components were attached onto the board using metal pastes.
The biodegradable transient PCBs had performance similar to
that of the conventional PCBs and could also be made into
rigid structures. Partially biodegradable PCBs were developed
using chicken feathers and e-glass fibers as reinforcement and
soybean oil resins as the matrix [11]. Mechanical and electrical
properties of the biocomposites were found suitable for PCB
application. A prototype circuit was developed on the PCB
and used in a telephone. However, the moisture absorption
and ability to support dc or ac currents was not studied. In
addition, the epoxy and styrene used in the composite make
the PCB partially degradable.
Biopolymers, such as carbohydrates and proteins, are
biodegradable but require dedicated land, water, and other nat-
ural resources. These natural resources are becoming scare and
their future availability and cost could become a constraint for
commodity applications. Renewable and sustainable biopoly-
mers and materials can be obtained from agricultural wastes
and coproducts particularly from the residues of food crops
for various industrial applications [12]–[14]. For instance,
banana fibers are extracted from the stem of the plant that
is considered as a waste. Similarly, proteins are generated as
coproducts during the processing of cereals for food or feed
applications. Examples of such proteins include soy proteins,
wheat gluten, and corn zein. In addition, considerable amounts
of proteins are also generated when oil seeds are processed
for food or fuel. These agricultural wastes/coproducts have
been used as reinforcement and matrix, respectively, to develop
completely biodegradable composites [13], [14]. In addition to
using renewable and sustainable sources, these composites do
not contain any chemicals and will completely degrade when
disposed in the environment.
In this report, we have used two renewable, sustainable,
and agricultural waste/coproducts (banana fibers and wheat
gluten) as examples of reinforcement and matrix, respec-
tively, to demonstrate the feasibility of developing completely
biodegradable PCBs. PCBs have been fabricated on the bio-
composites, and the performance of the PCB including dielec-
tric properties, electrical performance, and fire resistance has
been studied.
II. MATERIALS AND METHODS
A. Materials
Banana fibers were purchased from Tamil Nadu Agricultural
University, Coimbatore. These fibers are extracted from the
stem of banana plant using mechanical means and without the
use of any chemicals. Wheat gluten with a protein content
of about 80% was purchased from P.D. Navkar chemicals,
Bengaluru. Electrical components required for preparing
the PCB were purchased from local vendors. Spraying
of silver onto the composites was carried out at Siltech
Corporation Inc., Bangalore, India. Chemicals required
for etching were laboratory grade and purchased from
M/S Bangalore Scientific, Bengaluru, India.
B. Preparation of the Composites
Banana fibers were carded to form a mat. Wheat gluten was
sprayed onto the carded mat by hand in a predetermined fiber
and matrix ratio. Three different w/wratios (70/30, 50/50, and
30/70) of fiber to protein were used to prepare the composites.
Water equivalent to the weight of the fiber and matrix used was
sprayed, and the prepreg was allowed to condition for about
30 min. The wet prepreg was compression molded between
two aluminum sheets at 180 °C for 8–10 min at a pressure of
3000 PSI. After compression, the mold was cooled by running
cold water and the composite formed was collected for further
analysis.
C. Mechanical Properties
Tensile and flexural strength, elongation, and modulus of
the composites were measured on a universal tensile tester
(Model UTM-G-312C, Shanta Engineering, Mumbai) accord-
ing to ASTM standards. Samples were conditioned at 21 °C
and 65% humidity for at least 24 h before testing and also at
90% humidity, 21 °C for 24 h. Tensile tests were performed
according to ASTM D638-14 on a universal tensile testing
machine. For the tensile tests, composites were cut into dog
bone shaped samples with overall length of 165 mm, width
of 19 mm at the widest area, and 13 mm at the narrow
segment. About 8–15 samples were tested for each condi-
tion, and the average and standard deviations are reported.
Flexural strength and modulus were determined based on
ASTM D790-15 using samples of 203 mm length and 76 mm
in width. Crosshead speed during the flexural test was
10 mm/min, and at least five samples from three different
composites were tested to determine the flexural properties.
Average and standard deviations of the tensile properties are
reported.
D. Dielectric Properties
To determine the dielectric properties of the composites,
circular test samples of 20 mm diameter with various thick-
nesses were prepared. Samples were prepared by cutting
from the composite specimens using a die. Three readings
were taken from each type of sample and the average values
are reported. During measurement, the test samples were
sandwiched between two parallel gold plates as electrodes for
characterization. The dielectric constant, dielectric loss, tan δ,
and conductivity of these samples were measured at room and
variable temperatures using Alpha–N Analyser (Nova control,
Hundsangen, Germany) in broad frequency range of 0.13 Hz
to 10 MHz.
E. Flame Resistance
Samples measuring 125 mm ×13 mm ×1.6 mm were cut
and placed in a conditioned chamber at 23 °C and 50% humid-
ity for 48 h for the flammability testing. Flame resistance of
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GUNA et al.: PLANT-BASED COMPLETELY BIODEGRADABLE PRINTED CIRCUIT BOARDS 3
the PCB was determined as per UL-94 standard test method.
Briefly, the samples were placed vertically below an igniter.
The igniter was brought in contact with the specimen for 10
s and the time taken for the flame to self-extinguish was
recorded. In addition, any dripping of the specimen onto
the cotton fibers below the sample was also observed. Five
samples were tested for each ratio of reinforcement and matrix,
and the flammability ratings were assigned based on the time
to self-extinguish the flame.
F. Heat Dissipation
Ability of the biocomposite to dissipate heat was determined
using a circuit having 7805 regulator and a 5 V LED connected
as output. The regulator was firmly adhered on to the top
surface of the biocomposite using an adhesive. Transfer of heat
from the base of the regulator to the (other side) bottom of the
composite was measured using thermocouples. A dc supply of
+15 V was applied and the temperature was recorded using a
data logger
G. Developing the Printed Circuit Board
Biocomposite was coated with silver particles (8 μmthick-
ness) to improve the conductivity for copper coating. Coated
composite was immersed in electrolytic copper solution con-
taining 250 g/l of CuSO4.5H2O, 30 ml/L of sulfuric acid, and
0.125 ml/l of hydrochloric acid for 30 min with an applied
current of 1 A. A copper coating of about 20 μm thickness
was deposited on the composite. Coated board was covered
with a negative mask of the desired circuit and immersed
in a ferrous chloride etching solution. After etching, the
required components were mounted to obtain the complete
circuit.
H. Evaluating the Performance of the PCB
Ability of the biodegradable PCB to be used in electronic
applications was tested using a simple voltage regulator circuit.
The circuit comprised of a transformer, a full wave rectifier
circuit with capacitive filters, and a linear voltage regulator
integrated circuit LM7805 and IN4001 diode. A similar circuit
was adopted on a conventional FR-4 PCB for comparison
with the biocomposite PCB. A general purpose glass fiber
reinforced epoxy laminate PCB (FR-4) and the biodegradable
PCB mounted with electronic components were used as a
voltage regulator. An LED was used as load in the voltage
regulator circuit in both conventional and biocomposite PCB.
Deviations in the signal waveforms between the conventional
and biocomposites PCB were determined using a digital
storage oscilloscope (12 GHz, TDS6124C DSO).
I. Effect of Humidity and Temperature
Ability of the biodegradable PCB to withstand high humid-
ity and temperatures was studied. PCB containing the com-
ponents was placed in a humidity chamber at 90% humidity
for 24 to 48 h. After conditioning, the PCB was immediately
tested and the output waveforms were analyzed. Similarly, the
PCB was placed in a hot air oven at 100 °C for 8 h and the
changes in the output (wave forms) were measured.
Fig. 1. (a) Tensile strength and modulus and (b) flexural strength and modulus
of the biocomposites.
III. RESULTS AND DISCUSSION
A. Mechanical Properties of the Biocomposite PCB
Amount of reinforcement and matrix influenced both the
tensile and flexural properties of the biocomposite. Increasing
proportion of banana fibers from 30% to 50% increased the
tensile strength marginally but the flexural strength increases
by nearly 300%. Further increase in the fiber content increased
the tensile strength by about 300% but the flexural strength
decreased by about 300% (Fig. 1). Modulus (Fig. 1) of
the composites showed a different trend than strength. Both
flexural and tensile modulus increased when protein ratio is
increased from 30% to 50%. At 70% of fiber, the tensile
and flexural moduli are lower than at 50/50. Strength and
modulus are related to the properties of the materials and
the binding between the reinforcement and matrix. At low
protein concentrations, there is poor binding due to insufficient
matrix and hence the flexural and tensile properties are lower.
Similarly, at high fiber content, there is a lack of adequate
binder and hence the properties decrease. Among the three
different ratios of banana fiber/ protein studied, an optimum
of 50% of fiber and 50% of matrix provided the highest tensile
properties to the composites (Fig. 1). Commercially available
FR-4 PCBs are reported to have a flexural strength higher than
440 MPa and tensile strength higher than 310 MPa. Highest
flexural and tensile strength obtained for the biocomposites
in this paper was 27 and 45 MPa, respectively. Although
the tensile properties of the biocomposites are considerably
lower than that of the FR-4, it should be noted that the
density of the biocomposites was about 1.03 g/cm3compared
with 1.85 g/cm3for FR-4. The biocomposites may not be
able to match the mechanical properties of FR-4, such high
mechanical properties may not be necessary for most elec-
tronic applications. Increasing the density of the biocomposites
using different fibers or fabrics may provide biocomposites
with tensile properties similar to that of the FR-4 PCB.
The lower tensile properties and susceptibility of the bio-
composites to moisture could be advantageous to recover the
components. When exposed to 95% relative humidity at 21 °C
for 24 h, the tensile and flexural strength of the composites
decreased by about 45% and 55%, respectively. Similarly,
the tensile and flexural modulus decreased by 55% and 45%,
respectively. At high humidity, composites had become con-
siderably soft and the components could be extracted easily.
B. Flame Resistance
Irrespective of the amount of protein and fibers, the bio-
composites exhibited good flame resistance and passed the
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4IEEE TRANSACTIONS ON ELECTRON DEVICES
TAB L E I
COMPARISON OF THE FLAME RES ISTANCE OF THE BIOCOMPOSITE MADE
USING THREE DIFFERENT PROPORTIONS OF PROTEINS AND FIBERS
Fig. 2. Biocomposite was able to self-extinguish and passed VL-1
requirements for PCBs.
UL-94 V-1 classification requirements (Table I). As seen from
Fig. 2, flames were able to propagate through the composite
but were self-extinguished. The burned sample (1C) was
intact without any dripping, which is desirable to prevent
damage to other components. Typical FR-4 PCBs have a
higher flame resistance rating of V-0. Biopolymeric compos-
ites made from chicken feathers, soy-based resin, and E-glass
fibers and containing halogen-free melamine polyphosphate
and diethylphosphinic salt also had a rating of V-O [11]. The
biocomposites developed in this paper can also be treated
with flame resistant chemicals to achieve higher level of
flame retardancy. However, the fibers and proteins in the
biocomposites can self-extinguish and turn into ash rather
than melt and cause damage to the adjacent components. This
would reduce the risk of fire and component damage when the
biocomposites PCBs are used in electronic goods.
C. Dielectric Properties
The 50/50 fiber/protein composite had dielectric values
ranging from 2.38 to 50.38 when the frequency was varied
from 0.13 to 107 MHz. The dielectric constant (Fig. 3) also
varied with temperature with higher temperature decreasing
the constant, particularly at higher frequencies. There was
considerable decrease in the dielectric constant when the
temperature was increased from 25 °C to 60 °C but subse-
quent increase in temperature up to 120 °C showed marginal
decrease. In fact, the dielectric constants at 60 °C and 100 °C
overlap with each other. Higher temperatures remove moisture
Fig. 3. Dielectric constant (relative permittivity) was dependent of the
frequency and temperature during measurement. The curves for 60 °C and
100 °C overlap with each other.
Fig. 4. Changes in the conductivity of the biocomposite with increasing
frequency at two temperatures.
leading to reduction in the dielectric values. When moisture
content is reduced, the number of available free electrons and
the mobility of the electron decrease leading to lower con-
stants. Lower dielectric values are desirable since in integrated
circuits, a decrease in the dielectric constant increases the
operating speeds, particularly at higher temperatures. Previous
studies on dielectric properties of banana fiber and epoxy com-
posites have suggested that inclusion of the fibers increased
the orientation polarization and also moisture content leading
to higher dielectric constants [15], [16]. Higher temperatures
also remove moisture and cause a reduction in the dielectric
values. Conductivity (Fig. 4) of the composites increased
continually with increasing frequency due to the interfacial
polarization between the matrix and reinforcing fibers. The
interfacial polarization depends on the orientation of the fibers,
atomic, and electronic polarizations in the material [17].
Higher temperatures marginally increased the conductivity,
especially at higher frequencies mainly due to the easier and
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GUNA et al.: PLANT-BASED COMPLETELY BIODEGRADABLE PRINTED CIRCUIT BOARDS 5
Fig. 5. Up to 45 °C heat was dissipated by the biocomposites, which reduces
the risk of overheating and fire hazard. Curve A: temperature of the regulator.
Curve B: temperature measured on the other side of the biocomposite.
Fig. 6. Electronic device fabricated on the biocomposites had similar
waveform compared with a standard FR-4 (C1 and C3 are input and output
wave forms for FR 4, respectively, and C2 and C4 are input and output
waveforms for biocomposites-based PCB).
better mobility of the molecules in the composites. FR-4 PCB
typically has dielectric constant of 4.3 at frequencies between
500 MHz to 1 GHz. Dielectric constant of the biocomposites
varied with thickness and frequency. Increasing the density of
the biocomposites to match that of the FR-4 could provide
dielectric constants of about 4 similar to that of FR-4 PCBs.
D. Heat Dissipation
Biocomposite was able to dissipate considerable amount of
the heat generated, a property highly desirable for electronic
applications. After reaching steady state, the temperature at
the base of the regulator was consistently at about 130 °C for
the entire test period of 2 h. Maximum temperature recorded
on the thermocouple at the other side of the composite and
directly below the regulator was 87 °C. A heat loss of about
43 °C (Fig. 5) had occurred suggesting that the biocomposite
Fig. 7. Components mounted on the biodegradable PCB were able to support
the LED suggesting their suitability for electronic applications.
Fig. 8. There was no deviation in the performance of the electronic device
on the biocomposites even after exposure to 90% humidity for 48 h or 100 °C
for 8 h. D1 and D3 are input and output waveforms for FR 4, respectively,
and D2 and D4 are input and output waveforms for biocomposites-based PCB
after exposure to 90% humidity for 48 h.
was able to dissipate considerable amounts of heat mainly due
to its porous structure. Ability to dissipate heat would prevent
over heating of the components and may also reduce the risk
of fire. Increasing thickness of the composite or changing the
proportion of proteins and fiber could lead to further increase
in heat dissipation.
E. Comparison of the Wave Forms
Fig. 6 shows the input sinewave from the transformer used
in the voltage regulator circuit in conventional PCB (FR4)
and biocomposite PCB, respectively. The dc regulated out-
put voltage from conventional PCB (FR4) and biocomposite
shows that the input waveform and the regulated output from
both conventional PCB and biocomposite PCB are identical.
Ideally, the regulated output voltage should be 5 V from
LM7805 integrated circuit with a tolerance of +/4%.
Biocomposite PCB produced a voltage of 5.26 V compared
with 5.30 V in the FR4 PCB. A slight clamping in input
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6IEEE TRANSACTIONS ON ELECTRON DEVICES
waveform was observed, since the PCBs were powered from
an inverter line which regenerated ac from dc. An LED
connected through the circuit in the PCB was able to glow
without any interruption (Fig. 7). Further evaluation of the
PCB using an ac circuit will validate the applicability of the
PCB for all electronics.
F. Performance at Extreme Conditions
There was no deviation in the output of the biocomposite
PCB after exposure to high humidity or temperature (Fig. 8).
A steady output of 5 V was obtained from both the fire
resistant plastic-4 and the biocomposite (Fig. 8). Sinusoidal
wave without any clipping was produced indicating that there
was no deterioration in the performance of the biocompos-
ite PCB. Similar results were also obtained after exposure to
high temperature. Load (LED) connected to the circuit was
able to glow without any interruption.
IV. CONCLUSION
A completely biodegradable PCB was developed using
a natural protein and natural cellulose fiber. No chemicals
were used during the preparation of the PCB making it
completely environmentally friendly. In addition, the bio-
composite softens when in contact with hot water or high
humidity, which facilitates easy removal of the components.
The biocomposite satisfies the V-1 specifications for flame
retardancy and dielectric requirements for a PCB. No loss
in performance of the biocomposite PCB was observed even
after exposure to 90% humidity for 48 h or 100 °C for 8 h.
We have demonstrated the feasibility of using agricultural
waste/coproducts to develop biodegradable PCBs. This paper
paves the way for exploring the innumerable options avail-
able to develop agricultural waste-based biocomposites for
PCBs and other electronic applications. Although matching the
mechanical properties of an FR-4 PCB by the biocomposite
may be a challenge, PCBs with lower mechanical properties
may be sufficient for most applications. We demonstrate a
simple, environmentally friendly, cost-effective, and efficient
method to fabricate completely biodegradable biocomposites.
The relatively short span of modern electronic gadgets and
increasing affordability and consequent dumping of e-waste is
a major environmental concern. Biodegradable PCBs could be
a stepping stone in adopting biocomposites and biopolymers,
specifically those derived from agricultural byproducts and
coproduct for electronic goods.
ACKNOWLEDGMENT
The authors would like to thank the Center for Incubation,
Innovation, Research and Consultancy for their support to
complete this research.
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