edited by MARY VIRGINIA ORNA,0.S.U Callege of New Rochelle New Rochelle, NY 10801
thumbnail zketchez Chemistry of Printed Circuit Substrates: Physical and Chemical Requirements James H. Freeman1 Westinghouse Research and Development Centel Pittsburgh. PA We are all witnesses to a modern revolution which is occurring in all forms of communication and device control systems throughout our society. These range all the way from remote-controlled tow and TV r- m e devices such= ParMan' t o sophisticated weapons control systems and complex, computer-baaed, information processing networks. They include diverse items such as push-button telephones, handheld calculators, communication satellites, navigation devices, missile guidance systems, robots, and computer-controlled manufacturing lines. lnterconnectlons All of these diverse applications have one feature in common: the information signal, however sensed and processed, must be transmitted and interconnected to a multitude of other points where it is received and some action results. Much of today's information processing is handled by very sophisticated silicon chips. These integrated circuits are interconnected to one another and to an output system before a desired action occurs. The interconnection is by means of a set of electrical conductors between points and could, of course, he attained by wires. One such examole is a twical .. teleohone exchange network. Such systems tend to be physically large and cumhersome, relying heavilv on individual human effort to construct, assemble, test, maintain, repair, etc. Space and weight limitations are often critical, especially in airborne vehicles. The printed wiring board (PWB), also called printed circuit board (PCB), was developed to solve these problems. Additionally, these devices improve the systems' reliahility by means of simplified wiring patterns, redundant circuits, rigid soldered multiple connections and rapid total circuit test capabilities. PWB's also permit rapid assembly and repair hy simple plug-in of entire circuit subsystems. In microcircuitry, PWB's serve to protect delicate conductor patterns by holding them rigidly at a fixed position. Most importantly, they permit the rapid production of many identical systems hy means of photographic printing and chemical etching of the circuit patterns.
and dried leaving a very accurate and precise wiring pattern that is ready for component assembly. These steps lend themselves readily to an in-line, continuous operation where high volume production is desired. I t is possible to provide metal foil on hoth sides of the laminate, to print and etch hoth sides in appropriate circuit patterns, and to produce, therehy, a two-sided circuit board. Alternatively, three or more laminates can he stacked to provide a multi-layer circuit. Interconnections between the circuits on oooosite or drillina.. sides are handled hv. ounchina . holes at appropriate points, staking components into the holes, and filling the interlaver holes with solder (through-hole soldering; The prohlehs of appropriate registration of top and bottom patterns and maintenance of dimensional requirements throughout processing (including printing, etching, rinsing, drying, drilling, soldering) grow in importance as the line patterns of circuitry become finer and more closely packed (in line density per unit area). The Substrate The substrate for all of this is a resin bonded laminate. All that is required of this suhstrate is that i t he perfect in all respects and that it be producible by the thousands! The definition of perfection varies (expands) as the intended electronic ap&&ns become more and more complex. Hefore 1960, the circuit laminate was composed of cellulose paper sheets impregnated with a phenolic resin. Phenol plus formaldehyde yielded a methylol-substituted, water soluble intermediate stagr (resole~bhichwas highly compatible with the paper substrate giving good impregnation and thorough (see wpt out .~~~ , - Pie. ~ " 1~ .). On aoolication of heat and oressure. the final multi-ply laminate became a well-bonded structure which adhered to the copper foil surface sheet and stood up well against the etchants. This product could be plasticized to some degree by means of diluents in the resin, by ring-chain extenders, or by large alkyl suhstituents on the phenolic ring. I t could he readily punched and cleanly sheared, especially if warm, without fraying or delamination a t holes. The ounchine aualitv and electrical characteristics (aood dielectiic constiin; andlow dishipa~ionfactor) Ird to NEMA (National Electrical Manufacturers' Association) grade classifications as XI'. XXP. XXXP where X indicated the degree of electrical performance and P indicated pmchalnlity.
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A Photocopy
The circuitry is produced by making a large scale photocopy of a desired wiring pattern. This is then reduced phototo the scale desired and orinted onto a thin foil -maohicallv . surface, usually about 1-mil thick copper, which is mounted on some sort of suhstrate, usually film or a reinforced laminate. The film mask is removed and the unwanted copper is then removed by chemical etching using, for example, ferric chloride or ammonium persulfate solution. The hoard is rinsed Presented at the 7th Biennial Conference on Chemical Education, Stillwater. Oklahoma, August 9. 1982. Present Address: 3436 Woodland Drive, Murrysville, PA 15668
Figure 1. Phenol-formaldehydepolymerization Volume 61 Number 10 October 1984
875
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OGEBPA
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DGEBPA
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Higher MW Polymer
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Figure 3. Chain edension of epoxy polymer.
Figure 2. Diglycidyl ether of blsphenol-A.
This product was relatively economical to produce and is still used today in low-cost consumer devices such as radios, toys, games, etc., and is in wide use especially in Japan. As technology progressed, the user demands on the system became more severe. Military devices and, later, information network requirements demanded improved technical performance. To the circuit-hoard manufacturer, this translated t o reouirements for retainine the traditional chemical ietchantj resistance while providyng for thiiner lines with tighter line spacine. meater solderahilitv. and increased dimensional stabiiity. c & e a few mils per inch position error becomes cumulative across the width of a board (which may be up to 20 or more inches wide), it is possible for a line pattern based on .5-15 mil lines and 10-15 mil spacing between lines to go completely out of registration if thermal cycling in service causes X-Y dimension exuausion or contraction. To attain these highe; periormance characteristics, the printed cirrui~laminate industrv shifted almost romnletelv use of woven glass fiber reinforcements and epoxy resin binders. The euoxv resins are a familv. of ~olvmers derived . . from the reaction of epichlorohydrin and an aromatic diol, often the comwund 4.4'-dih~droxvdiphenvlnronane.more commonly known as hisphenol-A (See Fig. 2): The parent compound DGEBPA (diglycidyl ether of hisphenol A) can be caused to react with additional bispheuol A or a substituted derivative to cause chain extension to a higher molecular weight polymer. Extent of this reaction, hence the molecular weight and resin viscosity, is controllable by the reactant proportions used, length of reaction time, and effectiveness of the catalyst chosen (see Fig. 3).If a substituted BPA is used (e.g., tetrachlorobisphenol A), alternate properties may be provided. Use of heavily halogenated monomers gives improved fire resistance to the polymeric product. Fire
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Journal of Chemical Education
resistance can also be enhanced by addition of certain filler compounds such as hydrated aluminum oxide, AI(0H)a (i.e., A1203.3HzO) or antimony oxide, Sb205.The amount of such fillers which can be used is. however. limited hv their effect on the resin viscosity (they may interfere with good impregnation). and bv their possible effect on the dielectric .~ r o.~ e r t i e s of the resultant laminate. The shift to use of glass fiber reinforcement also caused a shift from punching to use of numerically-controlled multiple drills for establishing the many thru holes needed for interlayer connections and component staking. The punchlshear capability of the laminate became aprohlem not only of drill capability, hut also of drill wear. The latter is very important when multiple gang drills are drilling thousands of holes daily. Drill wear is a somewhat suhjective factor related to the user's installation. I t includes drill speeds, sharpness, maintenance, replacement schedule, etc. The property is determined mainly by experience in user's equipment. There is no universally accepted standard test, though a number have been pronn.na y"---.
Manufacturers control the production of their circuits by monitoring the degree or extent of cure as afunction of time, temperature, crosslink density, hardness, and plasticization, if any. A compromise is necessary since too hard a hoard will spa11 or chip resulting in ragged holes or in blooming-a surface delamination immediately surrounding the hole. Such faults lead to difficulties in solderiue or maintenance of solder joints. Too soft a board will result yn resin smear. To Follow
Some of the more recent developments in printed circuitry will be presented in the second part of this article which will appear in the next issue of the JOURNAL.
