4Implementation process

The PCBs are the base for any electronic circuit construction. A PCB used to place and mount the electronic components included in the device and it provides the means of electrical connection between these components.

The necessary connections between parts with a wire in PCBs are called nets.

Software tools are used for the PCB design, including schematic capture, component footprint editor, PCB layout design and auto router. Moreover, most of these tools are including analog/digital simulation capabilities and signal integrity analysis tools. Some commercial software tools are available in the market from several companies expert in the development of software packages on this topic, as Cadence, Altium, Mentor Graphics, Zuken or Cadsoft. On the other hand, many free tools can be found for easy development of simple designs, as Kicad or Eagle.

The design of a PCB includes some steps that must be carefully full filled in order to achieve an optimized circuit. These steps are detailed in table 2.

Table 2 – PCB design. Main steps.
| Main Characteristics
| Schematic component symbols are created in schematic libraries. Read data sheet to identify the pins. Draw the symbol of the component. Draw the package. Adding Component Parameters.
Draw the schematic | Place the part symbols. Draw the nets and buses. Place, move and edit the electronic components.
Create the board | Transfer the schematic diagram into a drawing of the PCB. Board size and trace width. Placement of the electronic components and labeling. Definition of ground planes. Trace horizontal and vertical routing including specific angles.
Place the parts and route the signals | Components should be placed according to their connections to other components in the board. Thermal considerations, mechanical requirements, as well as signal integrity and rout ability must be taken into account. Some landpatterns (or footprints) of devices are configured on the board. All electronic components which have connections to each other should be placed in the same vicinity if possible. Each placed part consists of pins which are terminals that need to be connected in order to complete the design. Auto-Rout functions can be used in this step in most popular software design tools. These functions are useful, but it is difficult to get a good design by using just the auto-route tools.
Edit the board and check | Check the board designed to identify any errors. Several check functions are avalaible in all software design tools.
Create PCB manufacturing data | Creation of the files needed by PCB manufacturers in order to finish the fabrication of the electronic circuit.

Some considerations must be taken into account in the implementation process of a PCB. The following points summarize some of these relevant issues:

- Ground planes: The current returns must consist of large area ground planes for low impedance to high frequency currents in order to avoid parasitic resistances and inductances, especially at high frequencies. Ground planes provide a low-impedance ground connection. Eliminate the loop area return by using both separate ground plane and power plane. If you need to split the ground plane and run traces across the split, you should first connect the planes. Use solid ground planes next to power planes. In PCBs including analog and digital circuits and signals, it is always recommended to use separate supplies for digital and analog circuitry.
- Power supply routing, clearances: Power planes are designed using the same rules as ground planes. In mixed circuit PCBs, separate power supplies for analog and digital circuits, even if the voltages are the same. Moreover, the digital power plane does not overlap an analog power plane.
- The power supply pins should be decoupled directly to the ground plane by using ceramic capacitors located as close as possible to the integrated circuit power pins.
- High frequency signals and impedance control: High frequency circuits and signals are going to be emitting electromagnetic radiation as they travel from source to sink. In general, frequencies higher than 1GHz are considered as high frequencies.

These signals can interfere with each other or a nearby component. To avoid conflicts, try to reduce parallel signals on the board at a minimum to avoid any signal coupling or crosstalk. In addition, maintain as much distance as possible between signal traces, and even consider routing signals on a separate layer if they are particularly noisy.

The sampling clock generation and high frequency circuitries should be treated like analog circuitry and also must be grounded and heavily-decoupled to the analog ground plane. It is also important to isolate these components from noisy digital circuits.

Take into account that ground planes will minimize RF emissions.

The impedance is an AC characteristic, meaning that it is related to frequency. The impedance is the combination of the capacitance and inductance of a circuit when operated at high frequency. Though also measured in Ohms, it is somewhat different than resistance which is a DC characteristic.

The PCB traces do not behave like simple connections at high frequencies, controlled impedance helps us ensure that signals are not degraded as they route around a PCB. In fact, controlled impedance is the matching of substrate material properties with trace dimensions and locations to ensure the impedance of a trace's signal is within a certain percentage of a specific value. Controlled impedance boards provide repeatable high frequency performance. Impedance control requires of a planning of the required stack-up and the calculation of electro-magnetic effects surrounding the traces.

- EMC recommendations: Some precautions such as minimizing crosstalk, proper grounding and namely proper layer stack-up will significantly reduce Electro Magnetic Interference (EMI) problems. An ideal stack-up will be a ground plane under each other plane.

In electronics, crosstalk is any phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel.

- Routing tecniques : When routing signals on different layers, be sure to route them orthogonally to each other. That is, on one signal layer your traces will be routed horizontally, and on the other vertically, 45 degrees, etc.

The two basic routing choices are maze routing and X-Y routing. The X-Y routing involves at least two routing layers with wires travelling in only one direction on each layer. On the other hand, the Maze routing allows the wiring of complete nets on a single layer, eliminating the need for layer changing vias.

The Manhattan length is the shortest path that a wire can have when it must be connected using only segments that are confined to either the X axis or Y axis.

Nowadays, the PCB designers have at their disposal a good number of well developed routing tools to effectively route the board. These tools use to include autorouter options. However, these options should not be considered as the only routing option.

It is important to route the nets for critical components, as connectors or FPGA components, by using either manual or auto routing techniques, because these components require routing beneath/between multiple surface mount pins.

In specific applications, it is important to conduct online measurement and the characterization of temperature during printed circuit board (PCB) routing process as well as the optimization of router design based on the investigation of routing temperature [6].