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rapid rate of change. A high Tg is important for PCBs that are very thick to guard against barrel cracking or pad fractures during the soldering operation. Coefficient of thermal expansion TCE Surface-mount assembly process subjects the printed wiring assembly to more numerous temperature shocks than typical through-hole processes. At the same time, the increase in lead density has caused the designer to use more and more layers, making the board more susceptible to problems concerned with the base material s coefficient of thermal expansion TCE. This can be a particular problem with regard to the z-axis expansion of the material, as this induces stresses in the copper-plated hole, and becomes a reliability concern. Figure 13.12 shows typical z-axis expansion for a variety of printed circuit base laminate materials.
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FIGURE 13.12 Typical z-axis expansion via thermal mechanical analysis. (Courtesy of Nelco International Corp.)
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Relative dielectric constant er This characteristic measures the effect that a dielectric has on the capacitance between a trace and the surrounding structures. This capacitance affects impedance as well as the velocity at which signals travel along a signal line. (See Figs. 13.3 and 13.4) Higher er produces lower impedance, higher capacitance, and lower signal velocity. Loss tangent, tan (f), or dissipation factor A measure of the tendency of an insulating material to absorb some of the ac energy from electromagnetic fields passing through it. Low values are important for RF applications, but relatively unimportant for logic applications. Electrical strength or dielectric breakdown voltage DBV The voltage per unit thickness of an insulator at which an arc may develop through the insulator. Water absorption factor WA The amount of water an insulating material may absorb when subjected to high relative humidity, expressed as a percent of total weight. Absorbed water increases relative dielectric constant as well as reduces DBV.
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13.5.1 Reinforcement Materials The principal reinforcement for PCB substrate materials is cloth woven from glass fibers.A variation of this glass is cloth made from quartz fibers. The resulting material has a slightly lower dielectric constant than ordinary glass, but at a substantial cost premium and a more difficult drilling cycle. Kevlar is an alternate woven reinforcement that results in a lower-weight material system with a lower dielectric constant, also at a higher cost and higher difficulty in processing.
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TABLE 13.3 Properties of Some Common PCB Materials Systems* Tg Std. FR-4 epoxy Multifunctional epoxy Tetrafunctional epoxy BT/epoxy Cyanate ester Polyimide Teflon 125C 145C 150C 185C 245C 285C N.A. er 4.1 4.1 4.1 4.1 3.8 4.1 2.2 tan (f) 0.02 0.022 0.022 0.013 0.005 0.015 0.0002 DBV, V/mil 1100 1050 1050 1350 800 1200 450 WA, % .14 .13 .13 .20 .70 .43 0.01
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* All with E-glass reinforcement, except Teflon.
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The original reinforcement material for PCBs was paper or cardboard in some form. Paper impregnated with a resin system is still in use in consumer applications where lowest possible cost is necessary and where performance is not an issue. 13.5.2 Polyimide Resin Systems Polyimide resin-based laminates are the workhorse of electronics that must withstand high temperatures in operation or in assembly or repair. Common applications include down-thehole well-drilling equipment, avionics, missiles, supercomputers, and PCBs with very high layer count.The principle advantage of polyimide is its ability to withstand high temperatures. It has approximately the same dielectric constant as epoxy resin systems. It is more difficult to work with in fabrication, is more costly than FR-4 systems, and absorbs more moisture. 13.5.3 Epoxy-Based Resin Systems Epoxy resin-based laminates are the workhorse of virtually all consumer and commercial electronic products. There are several variations of this pervasive laminate family, each developed to answer a special need.Among these are standard FR-4, multifunctional epoxy, difunctional epoxy, tetrafunctional epoxy, and BT or bismaleimide triazine blends. Each of these was developed to answer the need for a resin with a successively higher Tg or glass transition temperature. Multifunctional epoxy is the most commonly used form. 13.5.4 Cyanate Ester-Based Resin Systems This resin system is a recent entrant into the high-performance resin system category. It is said to offer processing characteristics superior to those of the FR-4 blends while offering a higher Tg. 13.5.5 Ceramics A wide variety of ceramic or alumina substrate materials have been developed for use in hybrids and multichip modules. These materials are the subject of specialized manufacturing processes beyond the scope of this handbook. The reader needing information on this group of materials is advised to contact a major manufacturer of ceramic materials. 13.5.6 Exotic Laminates Kevlar, Kapton,Teflon, and RO 2800 are materials developed for specialty applications.The first two, in the form of thin films, are commonly used as substrates for flexible circuits.The latter two
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