6.7 Two-Layer Boards
In conventional construction, a two-layer circuit board does not have a ground or power plane to form transmission lines. This means that a signal trace that interconnects components must have a nearby ground or power trace to form a controlled transmission line. For a logic and ground trace running in parallel on one surface, the characteristic impedance will usually be above 70 ohm. This results because the edge capacitance per unit length is a small value. There are several ways by which the capacitance can be increased to reduce the characteristic impedance, such as if the logic trace is placed between two ground/power traces, if the traces are thicker, or if the trace spacing is reduced.
The trace geometry we will consider for a two-layer board is a logic trace between two ground or power traces. The largest part of the field energy is concentrated in the space between the inner trace edges. We will consider trace widths and spacings of 10 mil.
A possible construction for a fast double-sided board is to run traces left to right on the top surface and top to bottom on the reverse surface. The ground and power traces serve to connect power to the components and to form transmission line for each signal. It is good practice to use vias to interconnect ground and power traces on the two surfaces at regular intervals. These connections in effect form a pseudo ground/power plane for the board. Decoupling capacitors should be placed near the power and ground ...