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 19^th 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.

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:

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 multilayer 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 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 nonconductors/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:

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) 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 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 silicacellulose 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. 

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.

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, deconstruction 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 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 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 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.

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, E^o = 0.76 V

                               Reduction: Cu²⁺ + 2e- → CuO, cathodic, E^o = 0.34 V

                               Over all reaction: ZnO + Cu²⁺ → Zn²⁺ + CuO, E^o = 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 (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:

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.

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-                                           

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 otherand the bond will therefore be polar. :In this case, the electrons are displaced toward the boron atom, giving the boron atom excess negative character, 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 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 m