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Comparisons of dicy-cured materials with non-dicy cured materials, especially the phenolic FR-4 materials developed for lead-free assembly, show that the elimination of dicy can have a positive impact on CAF resistance. This may be due to reduced moisture absorption when dicy is removed, or to the elimination of an electrochemical reaction that may be promoted in the presence of dicy, or to both. Lead-free assembly can stress the resin-to-glass bond, and even cause some level of resin decomposition, which can create microvoids within a PCB. These phenomena can make it easier for pathways for CAF formation to be created. The use of more thermally stable resin systems may be required when CAF resistance is needed in products that will experience lead-free assembly.
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Base material electrical properties are an important consideration in sophisticated printed circuits operating at high frequencies. High data rates, measured in gigabits per second (Gbps), and high clock speeds make the dielectric constant (Dk) and dissipation factor (Df) of base materials very important in high-speed digital circuits. Wireless and RF applications operating at very high frequencies also demand very low Dk and Df values. Moreover, the consistency of these properties over a large frequency range is also important. These properties were defined in Chap. 8 as follows:
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Dielectric constant/permittivity This is the ratio of the capacitance of a capacitor with a given dielectric material to the capacitance of the same capacitor with air as a dielectric. It refers to the ability of a material to store an electric charge. Dissipation factor/loss tangent The property is the ratio of the total power loss in a material to the product of the voltage and current in a capacitor in which the material is a dielectric.
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Importance of Dk and Df These properties are important because they affect signal transmission in the printed circuit. At low frequencies, a signal path in a printed circuit can typically be represented electrically as a capacitance in parallel with a resistance. However, as frequencies increase, at some point signal paths must be considered transmission lines where the electrical and dielectric properties of the base materials have a greater effect on signal transmission. A full discussion of capacitive versus transmission line environments is beyond the scope of this chapter, but the premise is to determine, for the transmission of a signal pulse of a given rise time, the acceptable length of a conductor before a significant voltage difference is realized along its length. Conductors longer than this critical value are then regarded as transmission lines. Because the velocity of signal propagation is inversely proportional to the square root of the permittivity of the dielectric, a lower permittivity value results in faster signal speeds and a longer rise distance. With a larger rise distance, larger conductor lengths are acceptable before a significant voltage drop is experienced. However, if the ratio of conductor length to rise distance is large enough, signal reflections from a mismatched load impedance may be received back at the source after the pulse has reached its maximum plateau value, and pulse additions that occur under these circumstances may lead to false triggering of a device. On the other hand, signal attenuation can result in missed signals. One of the causes of signal attenuation is dielectric loss. As the circuit operates, the dielectric medium absorbs energy from the signal. Attenuation of the signal by the dielectric is directly proportional to the square root of permittivity and directly proportional to the loss tangent. In addition, dielectric
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losses increase as frequencies increase. When a high bandwidth is desired, this effect has a greater impact on the higher-frequency components, and the bandwidth of the propagating pulse decreases and degrades the rise time. Because the permittivity and loss tangent vary with frequency, and other factors to be discussed later in this chapter, the degree to which these properties vary is also an important circuit design consideration. If these properties vary significantly with frequency, designing a circuit with devices that operate at various frequencies becomes that much more complex. In addition, operating within a given bandwidth becomes that much more difficult as different frequency components experience different dielectric properties, which in turn lead to differences in signal propagation and loss. Therefore, base materials with low permittivity values and low loss factors are desired for high-speed, high-frequency printed circuits. In addition, consistency of these properties across frequencies is also required. Besides frequency dependence, since operating environments can also vary, the consistency of these properties across environmental conditions is also important and is discussed in the following sections.
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High-Speed Digital Basics Figure 9.21 is a representation of high-speed digital communication that involves sending bits of information coded in waveforms. The zeros and ones of binary information are coded on the rise time or on both the rise time and fall time. The high voltage represents 1 and the low voltage represents 0. The faster the rise time, the faster the signal. To achieve faster rise times, sinusoidal wave forms are superimposed on one another. The range of frequencies used is called the bandwidth, with the bandwidth given as 0.35/rise time. In short, a faster rise time allows for a greater range of frequencies, or greater bandwidth. Figure 9.22 provides an example of eye pattern analysis. In this analysis, the height of the central eye opening measures noise margin in the received signal.The width of the signal band at the corner of the eye measures the jitter. The thickness of the signal line at the top and bottom of the eye is proportional to noise and distortion in the receiver output. Transitions between the top and bottom of the eye show the rise and fall times of the signal. Figure 9.23 illustrates potential signal integrity differences when different base materials are used. The top chart shows an example of a 10 Gbps signal at the source. Note the pattern in this chart. It changes from 0 in the x-axis (0) to its peak value (1). Now look at the chart in the lower-left corner of Fig. 9.23, representing the use of a standard FR-4 material. Note the change in the pattern, particularly the decreased amplitude. When the signal degrades as illustrated
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