This series of articles discusses the different steps of PCB development from the basics of creating a design schematic with specific requirements, to finalizing a board and preparing it for fabrication. The articles are written in the context of the National Instruments circuit design tools NI Multisim and NI Ultiboard.
Part one of this article, PCB Design Basics, outlines the major terminology in PCB design. In this part, we discuss schematic drawing and footprint selection as well as transferring the design to layout. This article outlines an example design flow.
Schematic drawing and footprint selection
The first step in successfully creating a PCB is the proper creation of an appropriate schematic drawing of the circuit. All of the components selected in the schematic must have correctly assigned packages associated with them so that the generated part list and netlist representing required connections will be correct. Figure 1 is a Multisim SPICE simulation schematic that helps illustrate the basic steps from schematic to final PCB generation:
Figure 1: A Multisim SPICE simulation schematic.
Notice here that power and ground symbols are considered virtual parts that connect into a single net, and these symbols are treated specially in the schematic environment. Since these are net connections only and do not connect to a footprint for layout purposes, it is appropriate to have them appear in a black color.
Once satisfied with the schematic (and functionality or performance through the use of simulation), it is time to prepare the schematic for transfer to the PCB layout environment.
Preparation Steps before Transferring
In the example schematic, both an analog and a digital ground are used. For layout purposes, it is critical to specify whether the analog and digital grounds should be considered one ground signal (i.e. connected together) or be kept separated for the purpose of routing two separate ground connections.
This example requires that the two grounds be connected together (or C2 would not have the same common return path as the rest of the circuit). The digital ground shows a net name of GND, and the analog ground has a net name of 0. When the grounds are connected, the netlist transferred to the PCB environment will use the net name ‘GND’ for both analog and digital ground net names.
Set the export settings (mils, mm, etc.) that will be used for clearance and trace width units-- mils (0.001 inch) are the most common units in North America. From within either the schematic or the PCB layout environments, trace and clearance constraints can be predefined to specific values. For example, based on the expected peak currents in a circuit, the default power or ground traces can be set to 25 mils.
Setting the number of layers
If the circuit is relatively simple and electromagnetic compatibility is not an issue, a cost-effective single-layer PCB design may be able to be used. If power or ground planes are required or the circuit is slightly more complex, two, four, or more layers can be selected. When planning for the board design, consider cost impacts as layers are added to the board. For this circuit example, four layers will be selected so that the design can use a power and ground plane.
Transferring the design for PCB layout
Once the final preparation steps are complete, the design is ready to be transferred to the PCB layout environment. A window is typically displayed showing all of the components and nets that are being transferred (see Figure 2).
Figure 2: Screen capture of transferred components and nets.
This is particularly important when updating a design layout with new changes in the schematic at a further stage of the design.
Board Outline Selection
As shown in Figure 3, a generic board outline of a rectangular shape is automatically generated in the tool. However, a custom board outline shape needs to be created to match package dimensions or other specifications.
Figure 3: Automatically generated generic board outline.
Usability features such as selection filters are essential for fast selection of different attributes of a design. In our example, a selection filter is used to change the board outline without interacting with the remainder of the design (see Figure 4).
Figure 4: A selection filter helps change the board outline.
The default rectangular board outline can be repositioned or resized, or one of the following methods can be used to customize the shape of the board outline. With any of these techniques, the existing board outline will first need to be selected and deleted and a new shaped created.
1. Using Board Wizard tools.
2. Import a board outline shape via DXF file.
3. Place a polygon on the Board Outline layer to define a custom or complex shape.
Part 3 of this series will cover component placement tips and strategies.