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Printed circuit boards (PCBs) are by far the most common method of assembling modern
electronic circuits. Comprised of a sandwich of one or more insulating layers and one or
more copper layers which contain the signal traces and the powers and grounds, the
design of the layout of printed circuit boards can be as demanding as the design of the
electrical circuit.
Most modern systems consist of multilayer boards of anywhere up to eight layers (or
sometimes even more). Traditionally, components were mounted on the top layer in holes
which extended through all layers. These are referred as through hole components. More
recently, with the near universal adoption of surface mount components, you commonly
find components mounted on both the top and the bottom layers.
The design of the printed circuit board can be as important as the circuit design to the
overall performance of the final system. We shall discuss in this chapter the partitioning
of the circuitry, the problem of interconnecting traces, parasitic components, grounding
schemes, and decoupling. All of these are important in the success of a total design.
PCB effects that are harmful to precision circuit performance include leakage resistances,
IR voltage drops in trace foils, vias, and ground planes, the influence of stray capacitance,
and dielectric absorption (DA). In addition, the tendency of PCBs to absorb atmospheric
moisture (hygroscopicity) means that changes in humidity often cause the contributions
of some parasitic effects to vary from day to day.
In general, PCB effects can be divided into two broad categories—those that most
noticeably affect the static or dc operation of the circuit, and those that most noticeably
affect dynamic or ac circuit operation, especially at high frequencies.
Another very broad area of PCB design is the topic of grounding. Grounding is a problem
area in itself for all analog and mixed signal designs, and it can be said that simply
implementing a PCB based circuit doesn’t change the fact that proper techniques are
required. Fortunately, certain principles of quality grounding, namely the use of ground
planes, are intrinsic to the PCB environment. This factor is one of the more significant
advantages to PCB based analog designs, and appreciable discussion of this section is
focused on this issue.
Poor design & material & heat up issues
