How Do You Design a PCB Layout?

by | Feb 9, 2021 | 0 comments

The design process of a PCB will essentially begin by creating a schematic. This schematic is then developed into a PCB layout using CAD software.

In this article, we will be going through the following points:

Overview of PCB design

The first step involved in PCB design is to draw your concept block diagram on a paper. This block diagram is then developed into schematic designs using CAD software. A schematic consists of component symbols and net connections between the symbols. These nets will become traces on the circuit board. 

The next stage of design is the pre-layout stage, in this stage, the BOM from the schematic is validated for long lead time components and obsolete components. During the validation process, the manufacturing part numbers (MPN) and vendor part numbers are verified. During this stage, the stack-up design is also completed. 

The PCB layout stage is next. In this stage, the board parameter settings, board outline, component placement, routing, and production document generation are completed. 

What are the basic steps of PCB design?

The design process comprises various different stages. Each stage has its own defined processes and checklists. To design a successful PCB, it is essential to follow the processes and check through the checklist at every milestone. In this section, we will go through the different steps that are involved in designing a PCB using Altium Designer

Step 1: PCB schematic creation

A schematic diagram is a representation of the elements of a system using abstract and graphic symbols. In this phase, the design is entered into the schematic tool (Altium, Allegro, etc). A schematic shows the components that are used in the design and how they are connected together. If the design uses a hierarchical schematic, where numerous functional schematics are interrelated with each other, the schematic defines the relationships between groups of components in different schematics. An example of the schematic diagram is shown below.

PCB schemtic diagram

An example of a PCB schematic diagram

Schematic generation is the process of creating a logical representation of the electronic circuit. When you generate a schematic, you are connecting a collection of symbols (components) together in a unique way, creating your unique electronic product. 

Below are the steps involved in PCB schematic creation using Altium Designer:

To learn more about schematic symbols and schematic diagrams, read our article What’s the Meaning of a Schematic Diagram?

Schematic symbol generation

Altium Designer symbol generation tool can be accessed by choosing the options Tools → Symbol Wizard command from the main menu. The symbol generation process involves drawing the body of the component, adding pins and pin numbers, defining the reference designators, and assigning a footprint.

Schematic symbol placement

The body of the symbol is created by placing graphical design objects in the schematic library editor workspace. Altium Designer includes a variety of closed symbol shapes including rectangle, pentagon, ellipse, and triangle as shown below.

PCB schematic symbol shapes

PCB schematic symbol shapes

Pin numbering

Pins define the connection points on the component for the incoming and outgoing signals. Pin numbering is made to ensure the connections shown in the schematic, end up connected properly by copper on the PCB. It is the component pins that give the component its electrical properties and define connection points on the component for directing signals in and out. A pin is placed to represent each pin on the actual physical component.

Pin numbers in LED

LED with two pins (1 and 2)

Pins can be placed in a schematic library document using one of the following steps:

  1. Click Place→ Pin in the main menu
  2. Symbol Options dialog will appear. Use this dialog to define the symbol height and width, the length of its pins, and a style for its pins, in relation to the ports on the source sheet and click OK.

Reference designators

Reference designators mainly consist of category, value, manufacturer, manufacturer part number, and supplier. It is recommended that every symbol on your circuit have its own unique designator so that every part is easily identifiable. For example, every resistor should follow a consistent naming sequence of R1, R2, R3, etc.

Assigning a footprint

Footprint gives an idea of the actual size of the component. For example, when we put a component on the sand, it will leave its impression there. This imprint is its actual physical size. Some components come in standard packages and the footprints are easy to find. In some cases, we may have to create the footprint manually. Below are the steps to be followed to create a footprint in Altium Designer.

  1. Create the pads
  2. Key-in component height and area
  3. Provide the silk-screen information
  4. Save the footprint

Connecting the symbols

It is very important for a PCB designer to clearly show how the components are interconnected in the schematic. First, whenever you have two wires that form a junction and share an electrical connection, that intersection needs to have a junction dot. This is standard practice in every schematic design.

Connection of wires in PCB schematic

Connection of wires in PCB schematic

Schematic connections: good practice

PCB schematic wiring

PCB schematic wiring: good practice

If you have a pair of intersecting wires that are not electrically connected and are just overlapping, then you will not need a dot. 

Intersecting wires in PCB schematic

Intersecting wires in PCB schematic

The important signals on the board should be marked once the components are connected. This marking includes impedance traces such as 50Ω SE and 100Ω differential pairs. Also, the power traces need to be identified and marked.

Link the footprints to the symbols

The footprints of components are shown on the schematic diagram when you transfer the schematic information to the PCB layout. 

Generate the netlist

Netlist in any PCB designing software contains information of the component name as well as the pad of that component which is connected. Netlist also assigns numbers to the connections in serial order. The Netlist Manager dialog is used to control and manage the netlist of the board. Nets can be edited, added, or deleted as the requirements. The pins (or pads) of the components in the nets can also be edited.

Perform a netlist check

Export Netlist option is used to export the netlist of the PCB to the current document. Once the command is launched, a netlist document with an extension ‘.Net’ is saved in the same folder where the circuit board design document is saved. A net by net verification in the schematic should be carried out (whether all the nets are connected as intended).

Generate BOM

The bill of materials (BOM), is simply a list of required materials for manufacturing a printed circuit board. In Altium Designer, BOM can be generated by choosing the options Report → Bill of Materials from the PCB schematic.

It is always recommended to carry out a general review after each and every step/sub-step to ensure an error-free design.

Step 2: Pre-layout stage

In the pre-layout stage, we initiate the design of stack-up, ensure that the BOM is verified for all parts, and verify that the parts are active and not obsolete.

BOM validation

The BOM, is simply a list of required PCB materials for manufacturing a printed circuit board. The first step in the pre-layout stage is to ensure that all the materials required for your design are available.

During the BOM validation, the following is verified:

  1. Manufacturing part numbers (MPN) are correct 
  2. Vendor part numbers (VPN) are correct
  3. The quantity of the parts is correct
  4. Designators match the schematic
  5. DNI (do not install) components are marked in BOM

Design the stack-up with the help of contract manufacturer (CM)

Designers need to have the details of the stack-up before starting the layout design. Designers always design the stack-up but they are usually assisted by the fab house to get a right stack-up arrangement. They can get the required help from a PCB manufacturer or they can use stack-up tools, like our Stackup Planner

Stackup Planner by Sierra Circuits

The parameters required for the planner include:

  1. The material of the PCB (FR4, I-Speed, Rogers, etc) depends on the frequency requirements and environment (for example, high temperature)
  2. Number of layers – signal layers and power layers
  3. Impedances required like 50Ω single-ended, 90Ω differential, or 100Ω differential
  4. The thickness of the copper (½ / 1 /  2 ounces)

DOWNLOAD OUR CONTROLLED IMPEDANCE DESIGN GUIDE:

Controlled Impedance Design Guide

Step 3: PCB layout stage

The PCB layout stage includes setting up the design tool, board outline, import of netlist, component placement, routing, silkscreen cleanup, DRC check, and generation of documents for production (Gerbers, netlist, etc.). 

Setting up the stack-up

The board layout is started by setting the stack-up and design rules. The stack-up is set up in the tool using the Layer Stackup Manager tool. The stack-up design generated by the stack-up tool from the PCB manufacturer is used as a reference to set up the stack-up in the layout tool.

Steps to design your PCB stack-up:

  1. A single layer stack-up by default is defined when a new board is created.
  2. The currently selected stack is duplicated if you click on the Add Stack button. Once the new stack is added, the name and the properties can be changed in the stack properties tab of the dialog box.
  3. Click on the Add Layer button to add a separate solder mask and over layers.
  4. The order of the stack-up can be changed using the Move Left and Move Right buttons that are present at the bottom right of the stack-up tab.
  5. For flexible PCBs, the flex stack-up should have its Flex option enabled. Flex bending is defined by placing a Bending Line across the flex region (Design » Board Shape menu)

The below image shows the Layer Stackup Manager tool.

PCB stackup manager tool

PCB stack-up manager tool

Set the PCB design rules

Design rules are a set of instructions for the PCB layout tool to follow. Each and every aspect of the design is covered in the design rules. PCB design rules can be broadly classified as:

  1. Electrical design rules: Deal with electrical characteristics such as impedance, frequency, etc.
  2. Physical design rules:  Associated with parameters like trace width, via sizes, differential pairs, etc.
  3. Spacing design rules: Deal with spacing between power tracks of high voltage, clearance, or a particular region if we need a 5-mil tracing, etc.

These rules are defined under PCB rules and constraints Editor Dialog box.

Draw the PCB outline

The shape of the PCB is referred to as the board outline and is essentially a closed contour. The board shape can be redefined in different ways:

  1. Manually:  By moving the existing board vertices. This can be done by switching to board planning mode (View → Board Planning Mode) in the design menu.
  2. From selected objects: This is typically done on a mechanical layer if you have the outline of the board imported from an MCAD tool (DWG/DXF file).  Switch to 2D layout mode (View → 2D layout Mode), select the primitives on the mechanical layer (Edit Select All on Layer), then use the Design Board Shape Define from selected objects command.
  3. From a 3D body: Use this option if the blank board has been imported from a CAD tool into a 3D body object (Place → 3D Body). Switch to 3D layout Mode (View → 3D Layout Mode) then use the command in the Design → board shape sub-menu to select the board shape.

Component placement

The first step of component placement begins by breaking down the layout into different sections based on the circuit’s functionality (analog, digital, high speed, high current, power supply, etc)

PCB floor planning based on PCB component categorization

PCB floorplanning based on sections

Cross-select mode

Cross-select mode enables the corresponding selection of the object between PCB layout and schematic. In other words, if you select an object on the layout, the corresponding object on the schematic is also selected. This ensures accurate component placement on your layout. The below image shows the steps to activate cross-select mode in Altium Designer.

PCB layout steps to activate cross select mode

Steps to activate the cross-select mode

The component placement process begins by placing the components which are at the fixed locations as per the design requirements. These components normally include connectors and their associated components. The next step is to place the main components such as CPU, memory, and analog circuits. The third step is to place the auxiliary components to the main components such as crystals, decoupling capacitors, and series resistors.

Follow the below steps to place an object (component) on the PCB layout:

  1.   Select the object to be placed from one of the toolbars or the Place menu.
  2.   Use the mouse to define the location of the placed object in the PCB workspace and its size (where applicable).
  3.   Right-click (or press Esc) to terminate the command and exit placement mode.

Route the board nets

Routing is the process of laying the copper traces between the nodes.  This conductive path is defined by placing tracks, arcs, and vias on the copper layers to establish a connection between the two nodes.  

Interactive routing method is used to route the connection in the PCB designer. This interactive routing command can be accessed by selecting the option Routeinteractive routing to route a single net, RouteInteractive Differential Pair Routing to route a differential pair. The below image shows the steps to access the interactive routing command.

Steps to access interactive routing

Interactive routing

Once the command is launched click on a pad that needs to be routed. The interactive router defines a route path from the selected pad to the cursor location. The size of the track is as per the PCB set design rules.

The sequence of routing is to complete the decoupling capacitors routing and the power vias. Then route the critical traces such as impedance traces and high-speed traces. Then route non-critical traces. It is a good policy to route traces in an orthogonal way.

Once the component routing is completed, the power/ground plane connections are made. The power plane is a layer of copper to which the power supply is connected. The ground plane is a layer of copper to which a ground connection is made.

Perform a design rule check (DRC)

Design rule check (DRC) is a process of checking both the logical and physical integrity of a design. In DRC, checks are made against all the enabled PCB design rules. This feature can also be enabled online so that the checks are performed parallelly as we progress through the design process. This step should be performed on every routed board to confirm that minimum clearance rules have been followed and there are no violations. The below image shows the DRC dialog.

Design rule check dialog

Design rule check dialog

Complete PCB fab/assembly notes

PCB fab notes consist of the numerous design-related information as follows:

  1. Class of the PCB (class 1, class 2 and class 3)  
  2. Number of layers
  3. Overall board thickness
  4. IPC standards to be followed
  5. Color of solder mask
  6. Color of silkscreen
  7. Layer-wise impedance details
  8. Cut-out details
  9. Stack-up details
  10. Drill-hole details (drill chart)
  11. Version number and date

Filling all the specific information on the fab notes is very important as it documents all the vital information of the PCB design for future reference.

If the board is being designed for a customer it is recommended to get approval from the customer after the above step. An example of fab notes is shown below.

PCB fab notes

PCB fab notes

Drill chart

The drill chart lists the number and size of the holes for each drill to be used on the board. It is recommended to insert the drill chart in the fab notes. An example of a drill chart is shown below.

PCB drill chart

PCB drill chart

To know more about the PCB drilling process read our article PCB drilling explained: The do’s and the dont’s

Step 4: Generate the production files

Below are the files that need to be generated:

  1. Generate Gerbers and other production files:

    1. TOP – Top copper layer (extension: board.gtl) Indicates the copper traces on the top layer of the PCB.
    2. SMT – Solder mask top layer (extension: board.gts). A solder mask is used for protection against oxidation and to prevent solder bridges formed during the soldering process.
    3. SPT – Solder paste top layer (extension: board.gtp). Solder paste is used to connect surface mount components to the pads on the PCB. The paste is applied by jet printing, stencil printing, or syringe.
    4. SST – Silkscreen top (extension: board.gto). The silkscreen is a layer of ink used to identify components, marks, logos, and so on.
    5. BOT – Bottom copper layer (extension: board.gbl). Indicates the copper traces on the bottom side of the PCB.
    6. SMB – Solder mask bottom layer (extension: board.gbs)
    7. SPB – Solder paste bottom (extension: board.gbp)
    8. SSB – Silk screen bottom (extension: board.gbo)
    9. Inner layers signal and power /GND (extension: board.g1)
  2. NC drill file: Shows the orientation of drill holes on PCBs (extension: board.txt)
  3. Pick and place file
  4. IPC 356 netlist file
  5. ODB++ file (Open Database). Exchanges the information between design and manufacturing steps.
  6. PDF of schematic and layout
  7. PDF of assembly drawings 

Perform a DFM check

Design for manufacturability (DFM) is a set of design guidelines that verify the manufacturability of the design. DFM analysis identifies the PCB layout issues that can create manufacturing problems during assembly and fabrication. DFM issues are related to the geometry, and most of the time, go undetectable during the DFM checks.

Better DFM by Sierra Circuits

Sierra Circuits’ Better DFM tool helps you to check the design for manufacturability. It runs on your PCB design files (Gerber file format) and pops up detailed information about the design rule issues in your files. For example, if you think your design has a minimum trace of 6 mils, and you run Better DFM, it will highlight any areas where the traces are just 5 mils.

To know more about DFM read our article on 6 DFM issues designers should check before PCB manufacturing.

Popular PCB design software

Designers and engineers use electronic design automation (EDA) software, also referred to as electronic computer-aided design (ECAD) software, to create a blueprint of the desired circuit board. This requires an in-depth knowledge of architecture and a good understanding of the associated libraries. The produced design pattern must be in compliance with the IEEE electromagnetic compatibility (EMC) standards. This design is brought into life (later put into production) by a PCB manufacturer.

The prime factors to be considered while choosing a PCB design software are:

  • User interface (UI)
  • Features
  • Large components libraries

The automation software programs establish a convenient approach in PCB designing that can be easily transformed into a physical board. See the list of PCB design software, and learn more about Allegro, Altium Designer, Eagle, OrCAD, etc.

We have now covered the basic steps involved in designing a PCB layout. Let us know in the comment section if there are any specific topics that you would like to read more about.

 

DOWNLOAD OUR DFM HANDBOOK:

DFM Handbook

 

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