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edited by MARY VIRGINIAORNA,0.S.U College of New Rochelle New Rochelle. NY 10801
The Chemistry of Printed Circuit Substrates: Some of the Latest Developments James H. Freeman Westinghouse Research and Development Center Pinsburgh, PA
Flguie 1. Opening ot the eplrane ring
The first part of this paper traced the development and the physical and chemical requirements for producing perfect and reproducible printed circuit boards.' This second part elaborates on some of the latest developments in this field. Soldering The soldering process itself also has changed as the com~ l e x i t vof the circuits has mown. Orieinallv. the circuit board was dhlled or punched and the c o m ~ n e o i staked s intn place. Then the laminate assembly was floated on a bath of molten solder, and all connections were soldered simultaneously. An alternative method called wave soldering has been developed, which places one or more boards in a rack above the sdder bath. A wave is generated in the bath which causes the solder connections to be wiped by the wave as i t passes. This technique lends itself well to continuous in-line processing. Vapor soldering is a method of mass soldering large numbers of terminals in a circuit board. In this process, a solder rivet is placed in each hole and the components are staked into their corresponding places. The hoard is put into a chamber and the hot vapor of a liquid with boiling point above that of the melting point of the solder is introduced. As the vapor condenses. i t eives UD heat to the samde. which in turn. causes the solder‘;^ mel; and srcure the cbnnection. Proper cure of the laminate is critical to its performance since it is the primary determinant of'the physical/mechanical properties of the lminate. In addition, the curing parameters, time, temperature, catalyst, have a considerabl'ejfect on the output (productivity) of the manufacturing process. For cure of epoxyresins, i t is possible to use various Lewis type acids as catalyst (e.g., BFBcomplexes or phosphonium compounds) or dianhydrides or amino-type compounds (including polyamines). These compounds serve f i t to open the epirane ring of the resin (Fig. 1) which is required for the curing process. The dianhydrides and polyamines compounds are also used in amounts greater than catalytic amounts. These compounds actually become incorporated into the eDoxv chain formine crosslinked svstems (Fie. 2). 0 h e n different catalysts are usid successively LO promote different stages in the reaction. However, to be useful in such respects thecatalyst residues, if they remain in the cured polymer, must not lessen the desired dielectric properties. This means that the residues cannot be ionizable salts, nor can they be moisture-sensitive. Dicy One particular catalyst has proved so superior that it has been almost universally adopted hy the circuit hoard industry. Brlef descriptions of phenomena, topics, facts,etc.. that chemical educators have found to be of Interest In their teaching, will be presented in a "note-type" format throughout THE JOLRNAL.
Figure 2. Crossllnklng epoxy polymers This is cyano guanidine, better known as dicyandiamide or simply as "dicy." NH
II
NH,-C-NH-CEN
Dicy is a dry, stable, granular material and is used alone or in comhination with other catalysts in almost all the high-performance. fire-resistant..eDoxv . " circuit board laminates made today. In addition to its catalyst cure properties and absence of ionic structure, the very high ratio of nitrogen present contributes to improved fire resistance in the laminates. Dicy has several limitations, however. The manufacturing process involves resin impregnation of a moving web of fiberglass cloth, evaporation of any solvents, and reaction to an intermediate or "B" stage. The treated cloth web is cut into sheets and placed between layers of resin-impregnated glass, "prepreg," to produce the desired thickness. The stack is topped with a thin sheet of copper foil (often 1mil in thickness). An elevated temperature cycle, which is provided by a multi-opening hot-compression press, is then required for complete curing. The compound dicyandiamide has very limited solubility in the resin svstem until the curine reaction is well alone. In some systems, therefore, elaborate'prncedural cechniqueiare used to maintain solubility as t h e solvents volatilize. These techniques are criticnl. Should crystalline dicyandiamide fall out of resin solution durinpr "H"stagine and hefore cure, it is apt to be trapped in the prepreg as finite, needle-like cry&, thus resulting in micro voids in the laminate. These voids may appear as one form of so-called "measling," when copper foil is etched in circuit processing. The second-order transition temperature, T,,of the "dicy" cured epoxy is also sometimes not as high as modern service applications demand. Hence, various cocatalysts may be used PTeSentBd at lhe 7th Biennial Conference on Chemical Education, Stillwater. OK. August 9. 1982. ' Freeman. James H., J. CKM. Eom.. 61,875 (1984). Volume 61 Number 11 November 1984
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to overcome this. Often, a T,of 15OoCor higher is sought. The is imporsecond-order, or glass transition temperature (T,), tant from both the physical and the electrical viewpoints. Tg represents the region of the temperature scale a t which molecular motion of the segments of the polymer chain begins t o occur. This temperature corresponds to the change from a hard rigid glassy condition into a rubber-like state of matter. On the macroscopic scale, T, manifests itself in several ways: 1 ) It is as the point above which the electr~calproperties (particularly the dislpation fartor TmhV heEin tc,change rapidly (rise) which can lead 10 a runaway condition and wentunl electriral
breakdown.
2) It also is the point of rapid escalation in dimensional instability
of the material as measured by coefficience of thermal expansion. 3) It is seen as distortion under load. Dimensional changes can result in warping of the substrate and also lead to problems in maintenance of orooer . . reeistw .. . when manv fine-line. closelv spaced circuits are employed. Hence, 'I,#should be uell ahuve the intended service temperature of the vircu~tsyrtem as well as above the soldering temperature Kevlar For some military applications, the dimensional stability (Xand Y directions) is critical because of high packing density of circuit paths. If a multilayer concept is contemplated, then dimensional stability in the Z (interlaminar) direction also becomes important. In the Z direction, expansion is governed solely by the characteristics of the resin since there are no fibers to serve as a restraint. Demands in the thermal/dimensional area have begun to exceed the limits of epoxy resin and glass fiber structures, and a new candidate material is becoming more and more popular. Kevlar, an aramid (aromatic amide) polymer fiber produced by the E. I. duPont Company, was originally developed for its extremelv " hieh .. modulus (stiffness) DroDertv. . . . However. it also possesses an extremely low coefficient o f t hermal expansion (in the fiber direction) and woven Krvlar fahrics are substituted for glass fahrics when X, Y dimensional stability is critical in printed rircuit laminates. In combination with the aramid fibers, an ammatic imide polymer may he substituted for the epoxy resin as the hinder, esperially when Z direction dimensional stability is important. (For example, to reduce the possihility of fracture of delicate solder connections by thermal expansion. Maleimide Copolymers . . Aromatir imide polymers were first developed because of their hieh thermal-oxidative stabilitv and remain the hest in this reg&d among commercially avaiiahle polymers. However, these ~olsimidesare difficult to cure, except in very thin sections (mils) as films or coatings. Olefin-containing compounds from the maleic anhydride family have thus been
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employed as end stoppers for short polyimide chains. The resulting olefin-ended, aromatic polyimides are handled in solution or melted for impregnation onto cloth, and are cured without by-product volatiles by reaction of the double bond using appropriate catalysts. This polymer sacrificed some of the excellent long-term thermal-oxidative stability of the parent imide structure. However, circuit board service temperatures are limited anyway by thermal limits of their semiconductor components. The maleimide conolvmers retain a high T e which adants them readily for soiderhg operations andValsoimparts to thkm a low thermal expansion coefficient. This is particularly valuable in the Z direction which is dominated by resin only, rather than by the fiber reinforcement. The principal maleimide polymer in use is Kerimid, a product made in France by Rhone-Poulenc and marketed by Rhodia. Inc.. in New York City. Flexible Circulls For some ap~licationsof circuit interconnects, the planar structure of ihe printed wiring board is inappropriate. In certain designs, a very thin circuit laminate is needed especially wherebending, folding, or other contorting of the shape to fit a confined space is required. For this purpose, copper foil has been laminated to a polymeric film by means of an adhesive. After a typical print and etch procedure, the resultant flexible circuits are confirmed into the required space. The Polaroid SX-70 camera makes use of such flexible film circuit laminates. For lower temperature applications such as the "Princess" telephone, an aromatic polyester film such as Mylar may be used. For more demanding applications, an aromatic polyimide film, duPont's Kapton, is chosen. Again, the inherent long-term thermal stability is less important than the shortterm solder temoerature resistance and the dimensional stability which it imparts. One final particular application of flexible circuitry is that in which the circuits consist simply of long runs of close-spaced parallel conductors terminating in a plug or edge board connection. There are referred t o as flat or tape cables and are useful as jumpers or interconnects on such complex systems as computers and information processing networks. The IBM 360 makes use of such a jumper harness. Ever-Expanding Horizon Chemistry is playing an ever-expanding role in high technology. New polymer systems continue to be developed and tested to expand the usefulness of present products and to overcome the limitations. Tangent 6 expresses me traction of current which contributes to power loss (evidencedas bat) in a material acting as a capacita placed in an akmating cwmt fiald. ll derives Iran molecular motiom induced by presence of dipole groups.
