Digital Circuit Boards: Mach 1 GHz

6.8 Four-Layer Circuit Board

Multilayer boards are usually built using symmetric layer stacks. The core or middle two layers consists of a copper-clad laminate. The thickness and type of epoxy-glass used in the core, as well as the copper clad thickness must be specified by the designer. In a four-layer board, the outer two conducting layers are copper foil separated from the core by sheets of material known as prepreg. Prepreg is a partially cured layer of woven glass and epoxy. Under heat and pressure, the prepreg bonds the outer copper layers to the core. To get the required thickness and to eliminate the chance of shorts, several layers of prepreg are used. This stacking arrangement is called a layup. A four-layer layup is shown in Figure 6.16.

Figure 6.16 A four-layer board layup.

The outer layers of copper and prepreg add about 0.015 in to the thickness of the board. The remaining board thickness is in the core. Typically, the total board thickness will range from 0.040 to about 0.061 in.

There are several ways to use the four layers of a board to handle logic and power. Layers can be dedicated to logic traces, to ground, to power, to a mix of logic traces and power, or to a mix of logic traces and ground. The choice that is made depends on whether ground/power planes are used for decoupling and if the characteristic impedance needs to be controlled. If traces are limited ...