Project Abstract

<p> The use of printed circuit board is unavoidable within the electrical and electronic<br>industries. Various types and models exist, all made from synthetic substrates. The<br>environmental impact of discards of printed circuit boards as well as the need to go<br>green globally poses challenges to the printed circuit board manufacturing<br>industry. In the attendant search for wider utility value for agro-waste based<br>particle boards, this work presents the research of utilizing agro-waste based<br>particle boards as virgin substrates for the production of printed circuit board<br>wafers. The agro-waste materials were pre-treated, ground and pressed into boards<br>using a Novalac resin (Melamine- formaldehyde). After cutting to sample sizes, the<br>samples were cleaned and electroless deposition was carried out on the boards<br>using non-precious metal catalyst ( as against the conventional precious metal<br>catalyst-Palladium). Material strength characterization of the boards was carried<br>out to determine the durability of samples when in use. Scanning electron<br>microscopy of the samples showed good deposition and acceptable roughened<br>topography which compared well with that of a commercial grade sample. A<br>simple conductivity test was done with an ammeter to prove the transfer of<br>electrical current at the surface of the substrates. This phase of work concludes that<br>there can be deposition on natural waste materials and that going ‘green’ in the<br>area of circuitry is achievable. Optimization of process conditions will create<br>another niche for the use of conversion products from agro waste discards while<br>giving the products a value-added status. <br></p>

Project Overview

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