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High-Speed Circuit Board Signal Integrity
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Characteristics and Construction of
Printed Wiring Boards
Introduction
This is a book about high-speed signaling on printed wiring boards (PWBs). The
physical construction of PWBs determines the conductor’s resistance (discussed in
Chapter 2), its self capacitance (covered in Chapter 3) and inductance (Chapter 4),
and the coupling to neighboring conductors (Chapters 5 and 9). At the high frequencies
of interest in this book, these electrical primitives appear on a PWB as distributed
rather than lumped elements, giving rise to transmission line behavior.
It is thus necessary for the high-speed circuit designer to have an understanding
of how PWBs are constructed and a sense of the trade-offs fabricators must make
when manufacturing high-density, high-layer count PWBs. This chapter summarizes
those characteristics impacting the electrical characteristics of PWBs and introduces
some of the terminology used in the PWB design industry.
Unit System
The PWB industry nearly universally uses an inched-based measuring system rather
than the metric system. Trace width and length and dielectric thickness are thus
specified in decimal fractions of an inch, as are most component dimensions. However,
many micopackage dimensions (most notably the pin or ball pitch) are specified
with metric millimeters, and the trace thickness is specified in ounces (relating
to the amount of copper plating, as described in Section 1.4). The Appendix tabulates
some common conversion factors, but here it’s noted that 1m = 39.37008 in
and 1 mil = 0.001 in. Therefore, 1 mil = 0.002539 ∼ 0.00254 cm = 0.02539 ∼
0.0254 mm.
PWB Construction
The typical multilayer PWB is formed as a stack of alternating layers of prepreg mats
and laminate sheets. The general idea is shown in Figure 1.1.
The prepreg mats are a weave of glass fiber yarns preimpregnated (hence
prepreg) with a resin that is intentionally allowed to only partially cure. The sheets
come in many stock sizes and yarn styles (classified by the number and diameter of
the glass threads, the weave, and the percentage of resin impregnation) and serve to
strengthen the resin. The typical resin content of the mats is in the 45% to 65%
range.
Copper foil is attached to one or both sides of fully cured prepreg sheets to form
the laminate sheets (also called cores). Similar to the prepreg mats, cores come in
standard stock sizes and thicknesses, from which the fabricator must choose to construct
a PWB. It‘s common for outer layers (such as layers L1 and L6 in Figure 1.1)
to be formed on prepreg [1], but some manufacturers prefer to form the outer layers
on cores.
Reinforcements
Fibers (usually from a form of glass) are used to strengthen the resins, but adding
them changes the electrical and mechanical characteristics of the composite structures
roughly in proportion to the amount of fiber to resin (the glass-to-resin ratio).
As shown in Table 1.2, the glass fibers have a higher εr but superior loss tangent values
than the resins.