ABSTRACT
The use of printed circuit board is unavoidable within the electrical and electronic
industries. Various types and models exist, all made from synthetic substrates. The
environmental impact of discards of printed circuit boards as well as the need to go
green globally poses challenges to the printed circuit board manufacturing
industry. In the attendant search for wider utility value for agro-waste based
particle boards, this work presents the research of utilizing agro-waste based
particle boards as virgin substrates for the production of printed circuit board
wafers. The agro-waste materials were pre-treated, ground and pressed into boards
using a Novalac resin (Melamine- formaldehyde). After cutting to sample sizes, the
samples were cleaned and electroless deposition was carried out on the boards
using non-precious metal catalyst ( as against the conventional precious metal
catalyst-Palladium). Material strength characterization of the boards was carried
out to determine the durability of samples when in use. Scanning electron
microscopy of the samples showed good deposition and acceptable roughened
topography which compared well with that of a commercial grade sample. A
simple conductivity test was done with an ammeter to prove the transfer of
electrical current at the surface of the substrates. This phase of work concludes that
there can be deposition on natural waste materials and that going ‘green’ in the
area of circuitry is achievable. Optimization of process conditions will create
another niche for the use of conversion products from agro waste discards while
giving the products a value-added status.
TABLE OF CONTENTS
Title Page————————————————————————————–ii
Declaration———————————————————————————–iii
Certification ———————————————————————————-iv
Acknowledgement —————————————————————————v
Dedication ————————————————————————————vi
Table of contents—————————————————————————-vii
List of Tables———————————————————————————xi
List of Figures——————————————————————————-xii
List of plates——————————————————————————–xiv
Abstract ————————————————————————————xvi
CHAPTER ONE
1.0 General Introduction——————————————————————-1
1.1 Natural Polymers (agro wastes)——————————————————6
1.1.1 Corn cob———————————————————————————6
1.1.2 Rice Husk——————————————————————————7
1.1.2.1 Water Imbibition:——————————————————————–8
1.1.2.2 Industrial uses———————————————————————-9
1.1.3 Saw Dust——————————————————————————9
viii
1.2 Background of Study—————————————————————-10
1.3 Scope of Study————————————————————————11
1.4 Importance of Work——————————————————————11
1.5 Literature Review——————————————————————–13
1.5.1 Selection of Borohydride Reducing Agent—————————————16
1.5.1.1 The Borohydride (BH) ion——————————————————–16
1.5.2 The Amine Boranes——————————————————————17
1.5.3 Complexing Agents—————————————————————–18
1.5.4 Stabilizers—————————————————————————-19
1.5.5 Electroless Catalyst Selection—————————————————–20
1.5.6 Non- Noble Metal Application—————————————————-23
1.5.7 Timeline of Printed Circuit Board Manufacture——————————–23
CHAPTER TWO
Materials And Methods——————————————————————–24
2.0 Experimental————————————————————————-24
2.1 Board Materials———————————————————————24
2.1.1 Synthetic—————————————————————————–24
2.1.1.1 Polyvinyl Chloride Sheet———————————————————-24
2.2 Natural ——————————————————————————-24
ix
2.3 Reagents/Chemicals—————————————————————-25
2.4 Equipment—————————————————————————-26
2.5 Analysis ——————————————————————————26
2.6 Process Steps ————————————————————————28
2.6.1 Process Flow Chart —————————————————————–31
2.7 Water Absorption/Imbibition And Thickness Swelling Test —————–32
2.8 Electroless Plating ——————————————————————33
2.8.1 Preparation of Cleaning Solution ————————————————-34
2.8.2 Preparation of Etchants ————————————————————34
2.8.3 Neutralizer —————————————————————————34
2.8.4 Activating Colloid ——————————————————————34
2.8.5 Developer —————————————————————————-35
2.9 Electroless Copper Bath ———————————————————–35
CHAPTER THREE
3.0 Results And Discussion————————————————————-38
3.1 Pictures of Samples produced—————————————————–38
3.2 Peel Test——————————————————————————42
3.3 Results Obtained for Strength of Materials, Water Imbibition And Physical Properties of Samples—————————————————-44
x
3.3.1 Bending Strength ——————————————————————-44
3.3.2 Tensile strength———————————————————————-52
3.3.3 Moisture Absorption/Water Imbibition——————————————63
3.4 Scanning Electron Microscopy—————————————————-71
3.4.1 Surface Morphology of Corn Cob Sample—————————————72
3.4.2 Surface Morphology of Rice Husk Sample————————————–74
3.4.3 Surface Morphology of Saw Dust Sample—————————————76
3.4.4 Surface Morphology of PVC Sample———————————————77
3.4.5 Surface Morphology of Control Sample—————————————–73
3.5 Application of Scanning Electron Microscopy———————————-79
CHAPTER FOUR
4.0 Conclusions and Recommendations ———————————————82
REFERENCES——————————————————————————84
BIBLIOGRAPHY—————————————————————————90
CHAPTER ONE
INTRODUCTION
1.0 GENERAL INTRODUCTION
Printed circuit boards are boards used in the connection of lead lines of various electronic
parts/components. Such important circuitry parts like resistors, capacitors, transistors are
housed and connected using metal-clad non-conducting substrates and the whole network
is known as a printed circuit board(1). These boards are made in three basic structural
classes, (i) with a shield or earth plate; (ii) with a multilayer structure; and (iii) as a thin
film, single layer. They are pathways made of copper or some other conducting material
that is etched or laminated onto a rigid or flexible surface. The “printed” means that the
material is deposited onto the substrate and the discrete wires are not used.
The search for printed circuit boards dates back to the 19th century when telegraph,
telephone and radio inventions were being recognized as practical devices for everyday
use and they all required wiring connections(2). For example, the increasingly complex
radio circuits needed an alternative wiring technology which ought to be simpler than the
existing tedious and error prone wiring technology. As a result, in 1903, Albert Hanson
(3) filed a printed wire patent which was to solve the problem of multi-wire connection
dilemma. His patent clearly described the concept of double-sided through–hole circuitry.
This first circuit pattern touched on so many concepts that are seen to be of modern
origin.
Printed circuit board is synonymous to printed wiring board which is undoubtedly the
most common type of printed circuit. It is a copper-clad dielectric material with
conductors etched on the external or internal layers. It is subdivided into single-sided,
double-sided, and multilayer boards.
2
It performs structural, functional and aesthetic duties in any electronic device, while
ensuring safety and convenience in the handling of point-to-point lead line linkages.
There are five primary types of this board, depending on the desired utility in the
electronic circuitry. These five types are:
1. Motherboard: This is the board that forms the principal circuit board in the
circuitry and it houses the basic components of the system.
2. Expansion board. This is a printed circuit board that plugs into an expansion slot
present alongside the mother board. This board compliments the utility of the
mother board.
3. Daughter board. This is a board that attaches to an expansion board as a
supplementary utility board
4. Network Interface Card (NIC). This is a type of expansion board that is mostly
found in personal computers (PC). It enables the PC to be connected to a local area
network. It is a connector circuit board.
5. Adaptor. This is a type of expansion board that controls the graphics monitor
because it houses the controller chip.
The top side of a printed circuit board is referred to as ‘component side and the bottom
side the ‘solder side’. The components are located on one side of the board and the
conductor pattern on the opposite side necessitating the making of hole (through hole) in
the PCB for the component legs to penetrate the board. Consequently the legs are
soldered to the PCB on the opposite side of where the components are mounted. There
are oftentimes the need for complex PCB designs as a result of product utility and this
prompted the designing and manufacture of PCB boards of various ‘face’ categories(4).
These categories are:
3
1. Single Sided: These are boards that have only the conductor pattern on one side
and the components mounted on the other side. This type of board has serious
limitation with respect to the routing of the wire in the conductor pattern
because the wires cannot cross and have to be routed around each other. This
category of board design is only used in very primitive circuits (5).
2. Double –sided: These are boards with a conductor pattern on both sides of the
board. They have electrical connection between two conductor patterns, this
electrical bridges are called ‘vias’ which are holes in the PCB that are filled
with metal and touches the conductor pattern on both sides. This type of PCB
design is suited for complex circuits.
3. Multi-layer boards: There are boards with one or more conductor patterns inside
them. The multilayer is achieved by laminating several double – sided boards
together with insulating layer in between. The number of layers is known from
the number of separate conductor patterns and is usually even and includes the
two outer layers. The most common ones are the 4 and 8 layers, though some
with as many as 100 layers are obtainable(6). The ‘vias’, which connects the
conductor patterns, becomes a hindrance when only a few of the conductors are
needed in service. Therefore, ‘buried’ and ‘blind’ vias types are used in multi
layer boards. This is feasible because the ‘buried’ and ‘blind’ vias are produced
in such a way that they only penetrate as many layers as are necessary. The
blind vias connects one or more of the inner layers with one of the surface
layers without penetrating the whole board, while ‘buried’ vias only connects
the inner layers.
In multi-layer PCBs, whole layers are almost always dedicated to ground and power and
are classified as signal, power or ground planes (7). In situations where it is necessary to
4
have the different components on a PCB connected to different supply voltages, there is
usually more than one of both power and ground planes.
Printed circuit board (PCB) substrates are materials that are polymeric, which perform
the function of structural platforms/bases for the mounting of electronic units in the
electronic industry (8). Literarily, from the definition of the two component make-up of
the phrase, “PCB substrates”, are materials of large number of structural units that are
joined by the synergy of linkages, which forms a stratum on which is mounted electronic
units that collectively make-up a system’s circuit. These supports are non
conductors/dielectrics that are dimensionally, thermally and chemically stable when in
use. The choice properties of such materials are:
a. high dielectric strength
b. low dielectric constant,
c. good flexural strength
d. low thermal coefficient of expansion
e. high resistance to humidity and
f. possession of high degree of fire retardancy
The use of polymers (plastics) as substrate in plating process can be traced back to
the plating of celluloid pen parts in 1905, where electroless silver solution was applied to
the surface of the celluloid material after a stannous chloride sensitization of the surface
of the plastic(9). Some of the advantages of using polymers in place of metals in plating
processes are:
5
a. Plastics give extended shelf life because only the surface of a plated plastic is
prone to corrosion whereas all parts of a metal corrode with an eventual failure
in service (10).
b. The plastics require no other production finishing steps such as buffing, before
plating, whereas metals require such steps and this increases the overall cost of
production.
c. When plastics are plated on, they acquire improved tensile strength, elasticity
and flexural strength, with a reduced total coefficient of thermal expansion. The
plastic material also has an enhanced abrasion and weathering resistance.
Some examples of platable plastics are:
i. acrylonitrite butadiene – styrene (ABS) ii. poly (phenylene ether)
iii. nylon
iv. polysulfone
v. polypropylene
vi. polycarbonates
vii. Phenolics
viii. Polycarbonate – ABS alloys
ix. Polyesters
x. Foamed polystyrene
xi. Phenolic-paper
xii. Epoxy-paper
xiii. Polyester-glass
xiv. Polyimide glass,
xv. Poly(vinyl chloride)
xvi. Poly(ethersulfone)
6
xvii. Polyetherimide
xviii. Polyetherketone etc.
1.1 NATURAL POLYMERS (agro wastes)
These materials are wastes from the agricultural sector. Most of them have little or
limited utility values. They all have a common base raw material which is cellulose.
These materials are (a) corn cob, (b) saw dust and (c) rice husk.
1.1.1 CORN COB
A corncob is the central core of a maize (Zea mays ssp. mays L.) ear. The corn
plant’s ear is also considered a “cob” or “pole” but it is not fully a “pole” until the ear is
shucked, or removed from the plant material around the ear. Historically, corn cobs were
used in outhouses in lieu of toilet paper, source of furfural( an aromatic aldehyde used in
a wide variety of industrial processes), as fibre in ruminant fodder, smoking pipes. It
contains not less than 40% phosphorus as P205 (ash). The principal chemical constituents
of corn cobs are cellulose, pentosan and lignin. These are mainly from the wood blast and
cortical layers of the cob. Cellulose and lignin are usually good for board manufacture
while the pentosan content of 20.6 percent shows that the corn cobs could be used in the
manufacture of furfurals and other products(11).
The absence of acid and extractive content shows that the board properties may
not be affected because their presence affects board quality ( 12). It is therefore, assumed
7
tentatively that corn cobs might be suitable raw material for particle board manufacture.
The various production steps involved are outlined using the flow chart.
1.1.2 RICE HUSK
Rice husk/hull is a “biogenic opal,” with approximately 20% opaline silica in
combination with a large amount of the phenylpropanoid structural polymer called lignin.
The silica which is amorphous is bound to water in a very intricate manner. This high
percentage of opaline silica within rice hulls is most unusual in comparison to other plant
materials(13 ). It is proposed(14 ) that during the combustion of rice hulls, the silica ash
may form a “cocoon” that prevents oxygen from reaching the carbon inside thereby
retarding burning. Another viewpoint(15 ) is that, since silica and carbon may be partially
bonded at the molecular level, silicon carbide is formed during high-temperature
combustion, and that the presence of this heat-resisting ceramic impedes the easy
combustion of the rice hull. Still other scientists project that at certain temperatures, the
molecular bond between the silica and carbon in the hull is actually strengthened, thereby
preventing the thorough and uniform burning of the hull. This flame-retarding and, at
ordinary temperatures, self extinguishing character as a result of the peculiar silica
cellulose structure, impede uniform and thorough burning in a combustion process and
also ensures resistance to water penetration and fungal attack.
Attributes:
a) They are highly resistant to moisture penetration and fungal decomposition.
b) They do not transfer heat very well.
c) They do not smell or emit gases.
d) They are not corrosive with respect to aluminum, copper or steel where corrosion is
induced/propagated by either alkaline or acidic environmental conditions.
8
In their raw and unprocessed state, rice hulls constitute a Class A or Class I insulation
material( 16). It is a by-product with very low protein and available carbohydrates, but
contains very high crude fiber, crude ash and silica. Of all cereal byproducts, the rice hull
has the lowest percentage of total digestible nutrients (less than10%) ( 17). Surprisingly,
rice hulls require no flame or smolder retardants. Nature has freely given to this
agricultural waste product all of the combustion properties needed to pass the Critical
Radiant Flux Test (ASTM C739/E970-89), the Smoldering Combustion Test
(ASTMC739, Section 14), and the Surface Burning Characteristics Test (ASTM E84).
Recent testing( 18) done by R&D Services indicates an average Critical Radiant Flux (CRF) of 0.29W/cm2, a smouldering combustion weight loss between 0.03% and 0.07%,
a Flame Spread
Index (FSI) of 10 and a Smoke Development Index (SDI) of 50.
1.1.2.2 Water Imbibition:
All organic materials will absorb or release moisture until they come into equilibrium
with the relative humidity of the surrounding air. The high concentration of opaline silica
on the outer surface of the rice hull impedes the atmospheric transfer of moisture into the
hull. Also, 2.1% to 6.0% of the rice hull consists of a bio polyester called cutin, which, in
combination with a wax produced by the rice plant, forms a highly impermeable barrier.
Nature employs several very effective strategies to protect the kernel of rice from the
water and high humidity generally associated with the cultivation and growth of this
plant. Consequently, studies done(19 ) on rice hulls at 25°C indicate that the equilibrium
moisture content of rice hulls at 50% relative humidity is at or below 10%, while at 90%
relative humidity, the equilibrium moisture content of rice hulls remains at or below 15%.
A Moisture Vapor Sorption Test (ASTM C739, Section12) conducted by R&D Services(
20) indicates a gain in weight of only 3.23%. This is well below the moisture content
needed to sustain the growth of fungi and mould.
9
1.1.2.3 Industrial uses-
These include:-
Mesoporous molecular sieves, which are applied as catalysts for various chemical
reactions, as a support for drug delivery system and as adsorbent in waste water
treatment, Pet food fibre, Building material, Pillow stuffing, Fertilizer, SiC production,
Fuel, Brewing to increase the lautering ability of a mash, Juice extraction to improve
extraction efficiency of apple pressing and as rice husk ash aggregates and fillers for
concrete and board production, economical substitute for micro silica / silica fumes,
absorbents for oils and chemicals, soil ameliorants, as a source of silicon, as insulation
powder in steel mills, as repellents in the form of “vinegar-tar”, as a release agent in the
ceramics industry, as an insulation material for homes and refrigerants
1.1.3 SAW DUST
Wood waste is wood that no longer has value at its current location, it may be a waste
product of a process, it may be from shipping/receiving, it may be from construction or
de-construction, etc. It is produced from manufacturing, forestry, construction, de
construction sectors, municipalities and utilities. Sources include pallets/skids, crates,
wire reels, scrap wood, sawdust, shavings, milling residue, processed wood, cut offs,
trees, branches, brush, stumps and bark. Sawdust is composed of fine particles of wood. It
is produced from cutting with a saw, hence its name. It has a variety of practical uses,
including serving as mulch, fuel, manufacture of particle board. Until the advent of
refrigeration, it was often used in ice houses to keep ice frozen during the summer(21 ).
Historically, it has been treated as a by-product of manufacturing industries with inherent
hazard, especially in terms of its flammability( 22). It is also sometimes used to soak up
spills, allowing the spill to be easily swept clean. Perhaps the most interesting application
10
of sawdust is in pykrete, a slow-melting, much stronger ice composed of sawdust and
frozen water.
The environmental impact of saw dust comes from the large amount of sawdust and
wood waste that is generated which is dumped without being fully utilized. Over a period
of time the wood waste is burnt or used for heating and when not removed from dump
area decomposes and emits methane, a greenhouse gas that is about 21 times more
harmful to the environment than carbon dioxide(23 ).
1.2 BACKGROUND OF STUDY:
This project stems from the fact that Nigeria today is matching towards a technological
independence of which the actualization of skill acquisition in the area of electronic
components, starting from the basic circuitry manufacture is one of it. Consequently the
production and acquisition of the skill of manufacturing this bare board will ensure the
non-dependence of our industries on imported bare boards which invariably cuts down
the overall production of electrical and electronic components/parts.
The utilization of waste cellulosic agro-materials will further make the cost of producing
the bare boards significantly cheaper while providing another means of converting the
wastes to utility items supporting the laudable ‘waste to wealth’ initiative of the country.
It will also create employment for the teaming youths.
The understanding and utilization of the cheap non precious metal catalyst reagents will
not only reduce the cost of manufacturing but also encourage the search for safer, cheaper
and more environmentally friendly alternatives to the palladium and/organic catalysts that
are presently in use, thereby keeping us abreast with the western technological approach
to this technology. The primary target of this study is to discover alternative raw
materials for the production of printed circuit board, different from the petrochemical
11
based resources (synthetic polymers) considering the fact that our petrochemical industry
is not actively operational. The successful completion of the study will open a new
frontier in the actualization of the concept of ‘green’ electronics which will provide a
sustainability and efficient cycling status to this sector. It will also enhance the required
techno-socio-economic impact of utilizing renewable resources while bringing down the
cost of producing these board with attendant low cost electrical/electronic products. The
dependence on foreign expertise and/product importation will be reduced or completely
eradicated.
1.3 SCOPE OF STUDY:
This project boarders around the preparation of particle boards from agro wastes and the
determination of relevant properties that will ensure the possible utilization of the
material wafers for printed circuit board manufacturing. A preliminary electroless copper
deposition will be carried out to ascertain the feasibility of depositing copper on the
substrates. A further work at a higher level will perfect the plating process and upscale to
the industrial manufacturing stage and mass production.
1.4 IMPORTANCE OF WORK:
The aim of the electronic manufacturing industry has long been to achieve a reliable
circuit design with repeatable electrical characteristics, good mechanical properties and
acceptable aesthetics(24). Until the 1950s, electronic circuits and systems were
assembled by using individual wires to connect each of the components. The components
were then mounted on what were known as long strips and sockets.
In response to the desire by the consumer for repeatable performance, smaller sizes
and lower costs, it became very necessary for the development of assembly schemes that
would allow for greater manufacturing efficiency. The printed circuit board method
12
proved very successful in providing the contact between components, laminates of an
insulating material are best suited for these work.
This study which is aimed at locally producing a non-conducting substrate for use
in the manufacture of printed circuit board, is very important to the scientific world
considering the areas of interest in the search for raw materials (synthetic (PVC) and
natural (sawdust, rice husk and corn cob)). The use of the agro-wastes, if successful will
open up a cheap source of raw material supply, apart from the fact that the overall cost of
producing those substrates will be reduced due to the elimination of chemical roughening
step of the sheets (etching). The overall time and energy for processing the board will be
minimized from the skipping of the etching step. On the other hand, the utility value of
those ‘ascribed’ wastes will be greatly enhanced and the negative environmental impact
will be totally eliminated. The study will also explore the possible recyclability of the
boards in any event where damage occurs to the circuit. There is also the possibility of
having an electronic item with close to hundred percent local content raw material input
which are also environmentally friendly and inexpensively sourced.
The study will also present us with the understanding of the possibility of cladding on
unprocessed (not like paper sheets) cellulosic material knowing that all three natural
materials chosen for this study (saw dust, rice husk and corn cob), are all cellulose based.
The degree of permanence achieved with the metal deposition will be verified by the
simple peel test.
13
1.5 LITERATURE REVIEW:
The term electroless plating was originally adopted by Brenner and Riddell to describe a
method of plating metallic substrates with nickel or cobalt alloys without the benefit of an
external source of electric current( 25). Over the years, the term has been broadened to
encompass any process that continuously deposits metal from an aqueous medium. In
general, electroless plating is characterized by the selective reduction of metal ions only
at the surface of a catalytic substrate immersed into an aqueous solution of said metal
ions, with continued deposition on the substrate through the catalytic action of the deposit
itself. Since the deposit catalyzes the reduction reaction, the term autocatalytic is also
used to describe the plating process.
The chemical deposition of a metal from an aqueous solution of a salt of said metal has
an electrochemical mechanism, both oxidation and reduction (redox), reactions involving
the transfer of electrons between reacting chemical species.
The oxidation of a substance is characterized by the loss of electrons, while reduction is
distinguished by a gain of electrons. Further, oxidation describes an anodic process,
whereas reduction indicates a cathodic action. The simplest form of chemical plating is
known as metal displacement reaction. For example, when zinc metal is immersed in a
copper sulfate solution, the zinc metal atoms (less noble) dissolve and are spontaneously
replaced by copper atoms from the solution. The two reactions can be represented as
follows: Oxidation: ZnO → Zn2+ + 2e-, anodic, Eo = 0.76 V Reduction: Cu2+ + 2e- → CuO, cathodic, Eo = 0.34 V Over all reaction: ZnO + Cu2+ → Zn2+ + CuO, Eo = 1.1 V
As soon as the displacement reaction begins, the surface of the zinc substrate becomes a
mosaic of anodic (zinc) and cathodic (copper) sites. The displacement process continues
until almost the entire substrate is covered with copper. At this point, oxidation
14
(dissolution) of the zinc anode virtually stops and copper deposition ceases. Chemical
plating by displacement yields deposits limited to only a few microns in thickness, usually 1 to 3µm. Hence, chemical plating via the displacement process has few
applications. In order to continuously build thick deposits by chemical means without
consuming the substrate, it is essential that a sustainable oxidation reaction be employed
as an alternative to the dissolution of the substrate. The deposition reaction must occur
initially and exclusively on the substrate and subsequently continue to deposit on the
initial deposit. The redox potential for this chemical process is usually more positive than
that for a metal being deposited by immersion. The chemical deposition of nickel metal
by hypophosphite meets both the oxidation and redox potential criteria without changing
the mass of the substrate:
.
15
Fig.1 Thickness vs. time-comparison between electroless and immersion deposition. Fundamental aspects of electroless plating. G. Mallory, Plating, 58,319 (1971).
In 1844, Wurtz (26) observed that nickel cations were reduced by hypophosphite anions.
However, Wurtz only obtained a black powder. The first bright metallic deposits of
nickel-phosphorus alloys were obtained in 1911 by Breteau (27 ). In 1916, Roux ( 28)
was issued the first patent on an electroless nickel plating bath. However, these baths
decomposed spontaneously and formed deposits on any surface that was in contact with
the solution, even the walls of the container. Other investigators studied the process, but
their interest was in the chemical reaction and not the plating process. In 1946, Brenner
and Riddell ( 29) published a paper that described the proper conditions for obtaining
electroless deposition as defined above.
16
1.5.1 Selection of Borohydride reducing agent
Reducing Agents Containing Boron
Years from the discovery of “electroless” nickel plating by Brenner and Riddell,
hundreds of papers describing the process and the resulting deposits have been published
( 30). Although other electroless systems depositing metals such as palladium, gold, and
copper are covered, the vast majority of these publications (papers and patents) are
concerned with nickel and cobalt-phosphorus alloys and the plating solutions that
produce them. Attempts to develop alternative reducing agents led several workers to
investigate the boron-containing reducing agents, in particular, the borohydrides and
amine boranes. Subsequently, several patents were issued covering electroless plating
processes and the resulting deposits.The deposits obtained from electroless systems using
boron-containing reducing agents are Metal-boron alloys. Depending on the solution
operating conditions, the composition of the deposit can vary in the range of 90 to 99.9
percent metal, with varying amounts of reaction products. In some cases, a metallic
stabilizer will be incorporated in the deposit during the plating reaction.
1.5.1.1The Borohydride (BH) ion
The borohydride reducing agent may consist of any water soluble borohydride
compound. Sodium borohydride is generally preferred because of its availability.
Substituted borohydrides in which not more than three of the hydrogen atoms of the
borohydride ion have been replaced can also be used; sodium trimethoxyborohydride
(NaB(OCH,)SH) is an example of this type of compound. The borohydride ion is a
powerful reducing agent. The redox potential of BHI is calculated to E, = 1.24 V. In basic
solutions, the decomposition of the BH, a unit yields eight electrons for the reduction
reaction: BH + 8OH- → B(OH) + 4H2O + 8e-
17
It has been found experimentally that one mole of borohydride reduces approximately
one mole of metal. The reduction to boron is approached differently in each case.
Case l(31 )
Here the assumption is that only three hydride ions are oxidized to protons and that the
fourth hydride is oxidized to a hydrogen atom, which leads to the formation of a molecule
of hydrogen gas:
Case 2 (32 )
In this instance, it is assumed that all hydride ions are oxidized to protons.
Case 3 ( 33)
Boron reduction is, as assumed , the catalytic decomposition of borohydride to elemental
boron that takes place independently of metal reduction. Gorbunova, lvanov and Moissev
(34) raised an objection to the three above hypotheses. They argue that, based on data
relating to the reduction reactions by hypophosphite, it is doubtful that the hydrogen
atoms, formed during the oxidation of the hydride ions of BH are intermediate products
that can take part in either metal or boron reduction: BH + 4H2O → B(OH) + 4H + 4H+ + 4e
1.5.2 The Amine Boranes
In the BH molecule, the boron octet is incomplete, that is, boron has a low-lying orbital
that it does not use in bonding, owing to a shortage of electrons. As a consequence of the
incomplete octet, BH can behave as an electron acceptor (Lewis acid). Thus, electron pair
donors (Lewis bases), such as amines form 1:1 complexes with BH, and thereby satisfy
the incomplete octet of boron. The amine boranes are covalent compounds whereas borohydrides such as NaBH are completely ionic, that is, Na BH = Na+ + BH─. Although
the amine boranes do not ionize, one of the atoms has a greater affinity for the electrons
than the other and the bond will therefore be polar. : In this case, the electrons are
displaced toward the boron atom, giving the boron atom excess negative character,
18
whereas the nitrogen atom displays excess positive charge. The electrical polarity of a
molecule, expressed as its dipole moment, plays an important role in the reactions of
covalent compounds. The commercial use of amine boranes in electroless metal plating
has, in general, been limited to dimethylamine borane (DMAB), (CHI) NHBH.
Experimental data indicates that the hydrogen gas evolved during the plating reaction
originates, in the main, from the reducing agent; this fact is not supported by the
electrochemical mechanism. Electrochemically speaking, an electroless deposition
reaction can be considered the combined result of two independent electrode reactions:
The cathodic partial reactions (e.g., the reduction of metal ions).
The anodic partial reactions (e.g., the oxidation of the reductant).
The electrons required for the reduction of the metal ions are supplied by the reducing
agent. Mixed potential theory interprets many electrochemical processes in terms of the
electromechanical parameters of the partial electrode reactions. Paunovic (35) was the
first to identify electroless metal deposition in terms of mixed potential theory. He
suggested that electroless deposition mechanisms could be predicted from the
polarization curves of the partial anodic and cathodic processes. In simple terms, mixed
potential theory leads to the assumption that electroless metal plating can be considered
as the superposition of anodic and cathodic reaction at the mixed (deposition) potential,
EM. Accordingly, the rates of the anodic reactions are independent of the cathodic
reactions occurring simultaneously at the catalytic surface, and the rates of the separate
partial reactions depend only on the electrode potential, the mixed potential. If these
assumptions are correct, it should be possible, experimentally, to separate the anodic and
cathodic reactions at different electrodes.
1.5.3 Complexing Agents
The additives referred to as complexing agents in electroless plating solutions are, with
two exceptions, organic acids or their salts. The two exceptions are the inorganic
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pyrophosphate anion, which is used exclusively in alkaline EN solutions, and the
ammonium ion, which is usually added to the plating bath for pH control or maintenance.
There are three principal functions that complexing agents perform in the EN plating
bath:
1) They exert a buffering action that prevents the pH of the solution from changing.
2) They prevent the precipitation of nickel salts, e.g., basic salts or phosphites.
3) They reduce the concentration of free metal ions.
In addition to these functions, complexing agents also affect the deposition reaction and
hence the resultant metal deposit. Metal ions in aqueous solution interact with and are
bound to a specific number of water molecules. The water molecule is oriented so that the
negative end of the dipole, oxygen, is directed toward the positive metal ion. The number
of water molecules that can attach to the metal ion is called the coordination number.
When water molecules coordinated to the metal ion are replaced by other ions or
molecules, the resulting compound is called a metal complex and the combining, or
donor, group is called a complexing agent or ligand.
Complexing agents, being electron donors, also have a considerable affinity for hydrogen
ions. Complexing agents can be considered metal buffers in a manner analogous to the
function of hydrogen ion buffers. When a complexing agent is added to a solution of free
metal ions, M”, equilibrium is established.
1.5.4 Stabilizers
An electroless metal plating solution can be operated under normal operating conditions
over extended periods without adding stabilizers. However, it may decompose
spontaneously at any time. Bath decomposition is usually preceded by an increase in the
volume of hydrogen gas evolved and the appearance of a finely-divided precipitate
throughout the bulk of the solution. Fortunately, chemical agents called stabilizers are
available to prevent the homogeneous reaction that triggers the subsequent random
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decomposition of the entire plating bath. To use stabilizers effectively, the chemist must,
a) identify those problems that can be solved by the use of stabilizers.
b) The compatibility of the stabilizer with the process being used, to avoid any adverse
loss in catalytic activity due to a synergistic action with any other additive present in the
bath. C) Compatibility of two or more stabilizers if desired in the same bath at the same
time (it is important to be sure that the action of one does not inhibit or lessen the
effectiveness of the other stabilizers).
d) Stabilizer selection should be on the basis that they only affect the plating process in a
manner that the resultant deposit will be able to meet any required performance criteria.
1.5.5 Electroless Catalyst selection
Several works have been carried out by many authors to provide a catalyst as a
pretreatment for giving electroless plating on a non-metal or a metal having no (or little)
catalytic activity(36 & 37). There has been employed in most cases a method which
comprises the steps of giving sensitivity by use of a tin-containing solution and then
giving catalytic activity by use of a palladium-containing solution (38&41). In addition, a
method of treating an object by use of one solution containing both palladium and tin has
also been widely used (42 & 45). However only palladium metal has been substantially
used industrially as a catalyst for electroless plating (46).
The palladium and tin catalysts have such problems as:
i. A production cost increases as a result of an increase in the price of palladium.
ii. In a production process of, for example, a printed board, palladium adsorbed on the
surface of a resin in providing a catalyst for electroless copper plating remains as a
smut even after etching of a copper plated film, and subsequent electroless nickel
plating is inconveniently deposited not only on a circuit pattern portion but also on
the resin.
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Research is being targeted at replacing palladium with inexpensive metal. Through:-
i. a method using a colloidal solution of a hydroxide or oxide of a metal such as
nickel or copper have already been made known(47).
Methods such as using a metal colloidal solution, the following methods are
disclosed. Japanese Patent Application Laid-Open No. 6861/1994 discloses a silver
colloidal solution having excellent storage stability and its preparation method (48)
Japanese Patent Application Laid-Open No. 195667/1998 discloses a catalyst solution
containing at least one of palladium, platinum, gold, silver and copper salts, an inorganic
acid and a water-soluble unsaturated organic compound (49). Japanese Patent
Application Laid-Open No. 209878/1999 discloses use of a tertiary amine polymer or
quaternary ammonium polymer as a colloid stabilizer in preparing a colloidal solution by
reducing ruthenium, rhodium, nickel, palladium, platinum, silver and gold with a boron
hydride compound, an amine borane compound, formalin, hydrazine and a
hypophosphite (50). Japanese Patent Application Laid-Open No. 241170/1999 discloses a
solution containing an iron, nickel or cobalt compound as well as a silver salt, an anion
compound and a reducing agent (51). Japanese Patent Application Laid-Open No.
167647/2001 discloses use of a hydroxy acid salt having at least three—COOH and—OH
groups in total, the number of—COOH groups being equal to or larger than the number
of—OH groups, particularly use of a citrate, as a dispersant (52). Japanese Patent
Application Laid-Open No. 32092/2001 discloses use of a noble metal salt of methane
sulfonic acid as a noble metal colloid (53). However, from the viewpoint of practical
performance, a metal colloid which may possibly be industrialized is limited to a silver
colloid.
Considering method 2 using a hydroxide colloidal solution, methods such as Iwai et al
(54) have reported the results of carrying out electroless copper plating by immersing
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objects to be plated in hydroxide colloidal solutions prepared by addition of alkali to
solutions of NiSO4, NiCl2, CuSO4 andCuCl2 and then immersing the immersed objects in
a KBH4 solution so as to reduce the colloids and provide catalytic activity. They also
studied colloids of lead, cobalt, cadmium, zinc, manganese and aluminum, in addition to
nickel and copper.
In recent years, new attempts have been made, such as Japanese Patent Application Laid
Open No. 209878/1999 (55) disclosing a method of stabilizing a metal hydroxide colloid
where the preferred reducing agent for reducing the colloid, is a mixture of one or more
components selected from the group consisting of a boron hydride compound, an amine
borane compound, formalin, hydrazine and a hypophosphite. Japanese Patent Application
Laid-Open No. 82878/2000 (56), disclosing the use of the above method for production
of a buildup multilayer printed wiring board and introduced potassium borohydride as an
example of a reducing agent. Tsuru et al.(57) reported a study to improve adhesion by
reducing a metal hydroxide colloid adsorbed to the surface of an object to be plated in the
same manner as described above by use of a sodium borohydride solution and then
reducing the resulting colloid by use of a hypophosphorous acid solution. They also
reported that when carbon and zinc are deposited by vacuum deposition after adsorption
of the metal hydroxide colloid and the resulting colloid is immersed in acid, the colloid is
reduced at the time of dissolution of zinc, whereby a catalyst for electroless plating can
be provided. Yanagimoto et al (58) obtained a thin copper film by coating a solution
having superfine copper oxide particles dispersed in ethanol on an AlN board by spin
coating, firing the coated board at 600 to 1,000°C., reducing the copper oxide in a
hydrogen atmosphere and then carrying out electroless copper plating.
Despite these scores of studies, methods using metals other than palladium are not yet
used industrially because they provide lower catalytic activity than the method using
palladium and a method for preparing a stable solution is not yet established.
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1.5.6 NON NOBLE METAL APPLICATION:
The term “hydrous oxide”, encompasses the insoluble oxides, insoluble hydroxides,
insoluble oxides – hydroxides or insoluble mixtures of oxides and hydroxides of metals
preferably selected from the group consisting of cobalt, nickel, copper, and mixtures
thereof. It is also recognized that metallic colloids (e.g., copper and nickel and alloys) due
to their pyrophoric nature when in contact with air and water are really metallic nuclei
with an outer surface which is oxidized. Due to the catalytic phenomenon on hand it is
the surface properties which are of greatest interest. Hence, it should be recognized that
in the use of pyrophoric metallic particles for catalytic solutions, they may be considered
hydrous oxide colloids, and such use falls within the context of this work.
1.5.7 Timeline Of Printed Circuit Board Manufacture
Paul Esler of Germany invented printed wiring board in the year 1943(59). The invention
consisted the use of rectangular sections of thin copper supported on a dielectric substrate
as a replacement for discrete wiring using round insulated copper wires, and using
printing technology to produce rectangular copper sections.
The printed wiring boards are identified using several criteria. One criterion is the line
width. Another is the type of substrate used, and the other is the method of
manufacturing. The PWB has evolved through all these interrelated categories to arrive at
the present state of on-going value additions and customization.
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