PCB Schematic: How to Read and Design PCB Schematic

Regardless of the circuit board type, the first step when you want to make one is to develop a schematic. The diagram lays out your idea in a simple layout for easier understanding before proceeding to other complex design and manufacturing phases later. So what is a PCB schematic, and how do you make one? We have looked at all this in detail below, so take a look!

 

PCB Schematics vs. PCB Designs

 

A PCB schematic is a simple 2-dimensional circuit design that shows the functions and connections between components. It is usually the first step in designing a PCB project and uses standardized symbols to define the real circuit connections.

However, the blueprint does not specify the exact location of the components. Its primary purpose is to show the connections.

On the other hand, a PCB design is a 3-dimensional board layout that shows the exact component location. The PCB design process comes right after the PCB schematic is complete. It defines the physical structure of the circuit, including the hole layout.

By focusing on component location, the design process looks more into performance. After the two phases, you can send the design to a PCB manufacturer for fabrication.

Purpose of PCB Schematics

The primary purpose of PCB schematics is to help visualize the electrical connections and overall structure of the circuit. While doing so, this diagram indicates the logical interactions between electronic components, enabling it to act as the circuit’s design blueprint before creating the layout.

An electrical PCB schematic diagram of a doorbell circuit

Schematic diagrams also allow functionality verification using CAD tools like Altium Designer and you can use SPICE simulation tools to simulate the circuit’s behavior to analyze its performance.

If you detect any anomalies, these schematics will help you review the design or even form a troubleshooting reference when debugging or testing.

Once the project passes the design phase, you can use the schematic to document the fine-tuned designs for fabrication and support.

Advantages of PCB Schematics

While visualizing the overall structure of the circuit, PCB schematics simplify the electrical connection overview without layout constraints.

They also allow you to tweak and experiment with different configurations while clarifying the circuit’s functionality.

Thus, you can reuse the design by modifying the existing schematics.

On the cost side, PCB schematics provide a cheap, functional validation tool to help manufacturers create a working prototype.

Remember, these schematics support SPICE modeling tools for simulation, which makes it easy to validate the logical correctness of the connections, expected node voltages, signal integrity, power management, etc.

Standard Specification for PCB Schematics

PCB schematic notations are regulated globally using two international standards, the first being the International Electrotechnical Commission’s IEC60617.

This standard is based on the old BS3939 British standard and provides the graphic symbols for electrotechnical diagrams. Its database has about 1900 symbols, each with a reference number for identification and a title in English and French.

Standardized electronic circuit symbols

The second standard is the ANSI Y32 standard, which initially provided special notations for aircraft but was edited to bring the documents in line with the IEC.

PCB Schematic Symbols and Connectors

The part symbols can vary depending on the country. However, the IEC standards have made them have a similar appearance. Let’s sample the symbols of six common circuit devices.

Resistors

These component symbols can either be zigzag lines with two straight lines on the ends or small rectangles with two connection lines on the short ends.

Capacitors

Capacitors can be polarized or non-polarized. The former’s symbol is two parallel lines with each line having a perpendicular electrical connection and a + sign to indicate the positive side. It can also have a curved line on one side. Non-polarized capacitors have the same symbol but without the + sign.

Different capacitor symbols

Variable capacitors have an additional diagonal arrow above the parallel plates or an arrow pointer on the curved line.

Op-amp

An op-amp symbol is a triangle with two inputs (positive and negative) and one output.

Power and Ground

Ground, analog ground, and digital ground symbols are all triangles pointing downward, with the last two having A and D letters inside the triangle to indicate their type. But earth and chassis grounds are different. The former consists of three parallel lines that shorten as if inside a triangle going downward, while the chassis ground resembles a fork with three bent pins.

Heterogeneous Symbols

Complex devices like microcontrollers, FPGAs, and microprocessors have multiple pins that are represented by multiple parallel lines emerging from the sides of a large rectangle. These lines represent the power, I/O, data, and control connections to the chip.

Transistors

Transistors are three-terminal semiconductors (base, emitter, and collector) with two PN junctions.

Their symbol is a circle with the PN junctions inside and the base terminal in the middle. The defining factor is an arrow that points to the emitter terminal in NPN transistors and to the middle plate from the emitter in PNP transistors.

Different transistor symbols

How To Read Circuit Board Schematics

When reading and interpreting a PCB schematic, you need to know these four basics.

Reference Mark

These designators indicate the name of each physical component in the circuit. They include a letter code to indicate the component type followed by a number to make it unique.

Component Symbol

The component symbol is the logical representation of the parts you’ll solder to the circuit, and they are standardized images.

A schematic diagram

Network

A schematic network refers to the electrical connections in the circuit and they appear as lines in the drawing that interconnect the component symbols.

Output

The outputs of the schematic creation process include the bill of materials and the netlist, which helps in the circuit wiring and component placement.

PCB Schematic Design Software

There are several PCB schematic design software in the market, but these are the top five.

Altium Designer

This software has advanced automation tools that make it easy to design large, complex boards.

PCB design using CAD software of a desktop PC

KiCad EDA

If you’re on a budget, KiCad is the best option. It is an open-source software suite that offers an integrated environment for creating PCB layouts and capturing schematics.

EasyEDA

For those who prefer using SaaS, this web-based platform allows you to develop schematics, simulate the circuit’s electrical functions, and publicly or privately share the final electrotechnical diagram with your colleagues.

Autodesk Fusion 360

If you’re looking for all-in-one PCB design, computer-aided design, computer-aided manufacturing, computer-aided engineering, and 3D modeling tools, try Fusion 360.

DipTrace

DipTrace is built specifically for electronic schematic capturing and PCB layout creation with a 3D view and shape-based autoroute. The software has four tools, the last two being a pattern editor and a component editor.

Guidelines to Design a PCB Schematic Diagram

 

These guidelines will help you design a successful schematic design.

 

Page Size Selection

 

Most design tools have different page sizes, but A4 is the most common. As a designer, you should pick a page size that suits the size of your circuit design.

 

Page Naming Convention

 

Separate the logic blocks of the schematic using the pages, then name each page using letters or numbers. After doing so, you can place the pages in a naming convention order, such as:

• 1_Block Diagram
• 2_Power Supply
• 3_MCU Interface
• 4_Memory Interface
• 5_Revision History

Or replace the numbers with letters.

 

Grid Setting

 

A grid system gives some reference, enabling the designer to refer to the parts correctly when connecting them. To help probe the nets in the analysis stage, you must place the circuit components and connections on the grid.

 

Electronic components

 

Page Title Block

 

A page title block sits in the footer of the schematic page, and while not a must, it is good practice to fill in all the details. These include the following:

• Update date
• Page size
• Document number
• Revision
• Name and function of the circuit
• Company disclaimer

 

Notes/Comments

 

Like programming, notes and comments help explain the schematic pages to the person building the design and other designers. As a general rule, layout designers make notes on separate pages for complex circuit designs but comment on the same page in simple schematics.

 

Revision History

 

A revision history tracks all changes made to the design pages, providing helpful information, such as the following:

• Description and date of the changes
• Author and reviewer name
• Review comments

The history typically goes to the first or final page of the schematic.

 

Schematic Document Table of Contents

 

Since the schematic document has different topics, it is easier to get to a specific page using a table of contents. The table comes in handy if you have to get to a particular module in a large and complex design. However, you can skip this guideline if the schematic is simple.

 

Block Diagram

 

A block diagram shows the signal flow across different modules in the design. It simplifies the reviewer's work because it is easier to understand the schematic during the review process.

 

Hierarchical Schematic Design

 

While a block diagram makes the design easy to understand, it might not be enough for complex circuits. Therefore, you need to use a hierarchical schematic to show the signal flow from one module to another.

 

Component Referencing

 

Component referencing is usually in the form of a table that shows the electronic components in use and their reference designators.

These designators get assigned per the IEEE standard, and the recommended procedure is to name the components with their standard reference designators. On top of that, you should use capital letters to label the schematic symbols.

 

Symbol Generation

 

A schematic diagram consists of different electrical components (active & passive components) and connectors. Make sure you use the symbols in the standard library to represent these parts in the schematic. However, if you don't get the required ones, it is acceptable to create new symbols.

 

Electronic circuit symbols

 

Net Connections

 

When two wires form a junction on the same electrical connection, each should have its junction dot.

It is best to embrace the net labeling conventions when drawing integrated circuit symbols and specific pins for easy readability. Also, adhere to the net labeling guidelines, off-page connections, and signal flow representation.

 

Component Placements

 

The placement of components is also a crucial part of the design process, and you should do it correctly to improve readability to the layout engineer.

 

Crystal Placement

 

Components linked to the crystal should always be nearby because the signals could be the high-frequency type.

 

DRC Check

 

Computer-Aided Design software offers a Design Rule Check (DRC) to check the physical and logical integrity of the design. You can do the checks live as you design online against all enabled design regulations.

 

Netlist Verification

 

Netlist files either have .txt or .mnl extensions, and you can generate them after the schematic design is complete and ready for importation in the layout.

MNL files are machine-readable, while TXT files show all the nets and connections between the component pins. Ensure you verify the traps manually to prevent design errors.

 

Bill of Material (BOM)

 

CAD software has a BOM creation feature. You can only generate a complete, sufficient BOM if the designer provides all the inputs in the tools when creating/importing library components.

Some of the BOM inputs include:

• Manufacturing Part Number
• Vendor Part Number
• Vendor name
• Package name

The general rule of thumb is to provide all the required information in the symbol creation stage.

 

Converting a Schematic into a PCB Layout

 

Designers usually use CAD software to convert a schematic into a PCB design consisting of ratlines and component footprints. Ratlines refer to electrical connections that will be physical later. To do the conversion:

1. Open the schematic project in the control panel of Autodesk Eagle.
2. Select the BRD/SCH icon to begin the conversion process. It will generate the board layout using ratlines and components as guides to create copper traces, vias, and pours.
3. If you get a warning saying that the BRD file does not exist, click "Yes" to create it from your schematic.

That's it. You will get a window that displays the following:

• A white square outline showing the board's physical dimensions
• All components resting outside the square outline
• Multiple lines/air wires connecting the components

How to Convert PCB to Schematic Diagram?

 

Here are some of the ways to reverse the process explained above.

 

How to Draw Circuit Diagrams According to Real Products?

 

Engineers often encounter problems locating the drawings required to repair legacy electronic products. Therefore, drawing the circuit diagram is necessary to analyze and improve the product. The process involves these steps:

1. Identify and use the components with multiple pins and large volumes as references for drawing the electronic circuits.
2. When printing the circuit board, label the components and pay attention to the serial number regularity. Label them according to the rules to ensure you don't make mistakes when drawing.
3. Number the components to improve proofreading and analysis, preventing difficulties later. Make sure you mark all the vital parts to avoid missing them when drawing the PCB file.
4. Distinguish the printed circuit board wires correctly with their layout positions, functions, and rules. These include the power, signal, and ground wires, plus others.
5. Use transparent tracing paper to draw the sketch and mark it using colorful pens. Before sketching, try to find and use an existing diagram on paper as a reference to create some standard in your drawings, which will be helpful to beginners involved in your project.

The process is not complicated, but it might be for beginners due to a lack of experience.

 

Printed circuit board

 

Protel+PCB Convert Into Schematic

 

You can also use Protel PCB to convert a PCB into a schematic by using these steps:

1. Begin by opening the PCB diagram, then click the "File-Export" menu to export the Protel network table. It should have "Serial.Net" as its abbreviated file name.
2. Start the Omninet program, then pick "Protel" as the file input type. In Input File 1, browse to the location of the netlist file and select it (Serial.Net). For the output, pick EDIF as the file type, specify the name and path of the output file, then click the running icon. An output window should pop up. Click "Accept Data," then click "OK" after finishing, and "Done" to close the window. Exit Omninet.

 

Start E-Studio software and open the EDIF file created in step 2.

 

1. Right-click on the Serial EDF file, then generate the schematics. A window should pop up, and click "OK" on it.
2. Regardless of the complexity of the circuit, the drawings have no hierarchy, so expect the single-plane graph to be highly detailed. Click on "File">"Save As," then pick the output format "OrCAD." Save as "Design V9.10" in the drop-down menu, then click "Save" and "OK" in the pop-up window that shows up.
3. Next, convert the OrCAD file into a Protel schematic, and Protel 2004 gives a better conversion effect. Launch the software, then open the DSN file (pick OrCAD Capture Design as the file type).
4. Once you click "Open," you will get an error message, but the file conversion was a success. Click "Ok" to close the error pop-up window.
5. Double click the "1.SchDoc" file, then select save it as a .sch file (schematic binary 4.0). Conversion is complete, and you can open the file in Protel 99Se.

However, it is crucial to note that the conversion is primarily helpful for PCBs with few components due to the lack of a hierarchy. Also, there will be no unconnected pins in the schematic, and the footprint information will disappear, so you must fill it again.

 

Convert Eagle Circuit Diagram to Altium Designer PCB Format

 

You can download the Eagle file on their website using open-source hardware from manufacturers like Arduino. Next, convert the file into a format that Altium Designer software can open

Use the following steps:

1. Download the ULP file
2. Place the file in the ULP folder in the Eagle installation directory
3. Click the ULP command in the command toolbar
4. In the pop-up dialog box directory, pick "export-Protel PCB.ulp," then open.
5. Select a suitable path to save the converted PCB file
6. Click "Save," then "Ok" in the dialog pop-up box
7. Open the saved PCB file in Altium Designer by double-clicking it.

 

PCB Schematic Checklist

 

A schematic checklist helps to avoid any design error and should include the following:

• Verify the pin numbering and labels of each component as per the datasheet
• A polarity check on all polarized components
• Check if there are any overlapping PINs and labels
• Verify the BCE transistor terminals with the footprint package, datasheet, and schematic symbol
• Validate the location, reference designators, and value of the components
• Check the schematic symbol descriptions (Manufacturing Part Number, vendor part number, etc.)
• Look for off-page connectors.
• Check for inter-sheet reference.
• Do decoupling capacitor checks for the integrated circuits and ground pin separation depending on the signal type (ground, digital, analog)
• Quantity and part number BOM checks

 

How To Verify The Correctness of PCB Schematics

It is important to verify and validate the PCB schematic correctness before proceeding to create the layout and you can do this in four ways.

Visual Inspection

Performing a visual inspection can bring to light errors like missing connections or networks between component terminals, unterminated ports or wires, unrealistic parameter values, and unlabeled components (no reference marks).

CAD Electrical Rules Check

Most CAD software has electrical rules-checking tools built in that can help you identify unconnected pins, missing ground or power connections, invalid part combinations, and conflicting I/O electrical connections.

Circuit Simulation

You can use the open-source SPICE simulation tool to check the expected voltage at circuit nodes, connection or net correctness, and if factors like signal integrity are within the required limits.

Virtual circuit simulation using SPICE tools

Peer Review

It would also help if you get a fresh set of eyes to inspect your schematic because this person can identify issues you’ve overlooked or hidden discrepancies.

Summary

 

There you have it! Even though some people use PCB schematic and PCB design interchangeably, the two are very different, as explained above. Therefore, if you have a PCB project, start by sketching the schematic, make the design, and contact us with the final board layout to begin fabrication.