Attività

Another very broad area of high frequency 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 does not 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 in this section is focused on this issue.

Some other aspects of grounding that must be managed include the control of spurious ground and signal return voltages that can degrade performance. These voltages can be due to external signal coupling, common currents, or simply excessive IR drops in ground conductors. Proper conductor routing and sizing, as well as differential signal-handling and ground isolation techniques enable control of such parasitic voltages.

One final area of grounding to be discussed is grounding appropriate for a mixed-signal, analog/digital environment. Indeed, the single issue of quality grounding can influence the entire layout philosophy of a high performance mixed-signal PCB design—as it well should.

Function of OrCAD PCB Editor in the printed circuit board design process

PCB Editor is used to design the PCB by generating a digital description of the board layers for photoplotters and CNC machines, which are used to manufacture the boards. Separate layers are used for routing copper traces on the top, bottom, and all inner layers; drill hole sizes and locations; soldermasks; silk screens; solder paste; part placement; and board dimensions. These layers are not all portrayed identically in PCB Editor. Some of the layers are shown from a positive perspective, meaning what you see with the software is what is placed onto the board, while other layers are shown from a negative perspective, meaning what you see with the software is what is removed from the board. The layers represented in the positive view are the board outline, routed copper, silk screens, solder paste, and assembly information. The layers represented in the negative view are drill holes and soldermasks. Copper plane layers are handled in a special way, as described next.

Fig. 1.17 shows routed layers (top and bottom and an inner, for example) that PCB Editor shows in the positive perspective. The background is black and the traces and pads on each layer are a different color to make it easier to keep track of visually. The drill holes are not shown because, as mentioned already, the drilling process is a distinct step performed at a specific time during the manufacturing process.

PCBs usually contain epoxy resin, fiberglass, copper, nickel, iron, aluminum and a certain amount of precious metals such as gold and silver; those materials and metals along with electronic parts are attached to the board by a solder containing lead and tin. The main material composition of PCBs was determined and is shown in Table 13.1. From the table, the composition of metals, ceramic and plastics could reach 40%, 30% and 30%, respectively. Further, the concentrations of precious metals in waste PCBs are richer than in natural ores, which makes their recycling important from both economic and environmental perspectives. Table 13.2 shows the average content and value ratio of different metals in PCBs. One can see that Au, Cu, Pd and Ag account for nearly all of the economic material value in waste PCBs. Therefore, PCB recycling focuses on recovering these metals above all else.