5 Design
5.5 Placement and routing

Component placement is the next step in the process. This is one of the most important and critic step in the design of a PCB. It is said that the design of a PCB is 90% placement and 10% routing. What it really means is that the placement of your components will end up deciding how much time your routing will take.

It is said that PCB design is both an art and a science. There is a big set of technical know-how and measurements to consider when dealing with trace widths, layer stack-ups, schematics, PCB ground plane design, etc. The artistic side of PCB design has to do with component placement. The truth is that there is no right way to place components, and this freedom is ultimately what makes the PCB layout process so creative. If you give a schematic to 100 different engineers, you will likely get 100 different layouts back. This is why the PCB layout process is viewed as an artistic process [12].

One of the first things to do will be to adjust the board area to the maximum allowed size of the PCB. The number of layers will depend on power levels and complexity.

Here there are some general design guidelines you should know and take into account when designing a manufactural, functional and reliable PCB. Obviously, there are more, and the PCB design guidelines list below should not be thought of as a complete list [13].

When placing components make sure the snap to grid option is turned on. Start placement of components in the following order: connectors, power circuits, sensitive and precision circuits, critical circuit components (MCU, DSP, FPGA, memory, and clock devices) and then the rest. In addition, components should be placed according to their connections to other components.

Similar components must be oriented in the same direction as this will help with an efficient and error-free soldering process.

Avoid placing components on the solder side of a board that would rest behind plated through-hole components.

Place all surface mount components on the same side of the board, and all through-hole components on the top side of the board to minimize the number of assembly steps.

Place decoupling capacitors close as much as possible to VCC pin on active components.

Components absorbing greater than 10 mW or conducting more than 10 mA should be considered powerful enough for additional thermal and electrical considerations. Sensitive signals should be shielded from noise sources with planes and be kept impedance-controlled.

Power management components should use ground planes or power planes for heat flow. Make high-current connections according to the acceptable voltage drop for the connection. Layer transitions for high current paths should be made with two to four vias at each layer transition. Place multiple vias at layer transitions to increase reliability, reduce resistive and inductive losses, and improve thermal conductivity.

Always create a ground plane. This can be a large copper area on a single layer board or even an entire layer dedicated as a ground plane on multilayer boards. And once your ground plane has been added, it is simply a matter of connecting all of your components that need to go to ground with vias.

Real-world copper traces have resistance. This means that a trace has a voltage drop, power dissipation, and a temperature rise when current flows through it. The width of the lines should be sized according to the estimated current that flows through them. Therefore, power lines should be wider because all the current is supplied by these wires. Commonly, copper traces on PCBs have 35 micron thickness. Thus:

Table 1 – Trace width and maximum current
Trace width | Maximum current value
4 mm | 10A
2 mm | 5A
1.5 mm | 4 A
1 mm | 3 A
0.5 mm | 2A
0.2 mm | 0.5 A

Minimize trace lengths when placing components and avoid 90 degrees angles. Use two 45 degrees bends instead. Reasons for that are that in the board manufacturing process, the outside corner can be etched a little narrower thus changing the trace impedance. The use of 45 degrees angles shortens the electrical path between components. In addition, high speed logic signals can get reflected off the back of the angle, causing interference.

Keep digital and analog grounds separate because voltage and current spikes from digital circuits can generate interference (noise) in the analog circuits, affecting their performance.

These rules are in no particular order, can generally be applied to any PCB design project, and should prove as a useful guide for novice designers.

As a result of the placement and routing processes, we finally get the PCB layout which is the drawing where the electronic components with their footprint appear in the position they will occupy in the final PCB and the interconnection traces between pins.

Fig. 5. PCB Layout: 2D (top) and 3D (bottom).