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PCB Design Guide

The PCB Design Guide is an invaluable resource for those learning pcb design.  Designing a pcb isn’t something you’ll do in a couple of hours, its a highly technical learned skill that will take years to master.  The PCB Design Guide will provide the basics of pcb design as well as more advanced topics.    The guide is a work in progress, so feel free to contact us to suggest topics or contribute.

PCB Design Process

First and foremost its important understand the pcb design process.  This design process/workflow will give you a birds eye view of each step necessary in taking your circuit design through assembly and final test.  This section is not to explain the pcb fabrication process. We’ll cover that at the end of the guide.

The PCB Design Process & Workflow

Now that you’ve got a good understanding what it will take to design your printed circuit board let’s get started. We’ll assume that since you made it to this site you’ve probably completed your circuit design or that you plan to prototype your circuit on a pcb. That leads us to the second step of pcb design.

Choosing PCB Design Software

With your circuit design complete, its time to capture your design electronically.  To do that you’ll need a pcb design software package.  We’ve got an entire section of this site dedicated to pcb design software. There are a couple open source packages, some free options (sponsored by pcb manufacturers) as well as several very expensive options the larger corporations will use to design 16 layer, very high density pcbs.

Here are a couple articles that will help you choose the best pcb design software for your project.

Open Source PCB Design Software
Sponsored PCB Design Software
High-End PCB Design Software

In general there is a learning curve associated with many of these packages. They are all alike in several ways, but they all have their nuances that can take several weeks to master and become proficient in. We hope to provide tutorials on the more popular pcb software packages, but until then you’re on your own. Should you find the time to document your experiences please pass them along and we’ll publish them here for others to reference.

PCB Design Guide – Schematic Capture

If you’ve completed your circuit design and know which pcb design software you want to use you’re ready for the third step – schematic capture.

Schematic Capture Basics

Schematic capture is the crux of a solid circuit board design. Whether you’ve inherited a pcb design, trying to design your own printed circuit board, or just trying to understand how to read a schematic, this section is for you. Here you’ll find information on many different aspects of schematic capture like the components of a schematic, what a good schematic looks like, and how to read and work with schematics.

In the electronic age we live schematic capture software has become readily available. Today you can download many different free pieces of schematic capture design software. Most of these tools are user friendly and allow hobbyists and seasoned engineers to begin capturing their schematics in minutes. However in order to become proficient there are many standards practices that should be followed. Following these standards will greatly reduce your design and debug time after you’ve had your circuit board manufactured.

What is Schematic Capture?
Schematic Capture Standard Techniques

Component Creation

Your pcb design software likely comes with several of the standard components already loaded in a library. You’ll want to use those components when available, but its inevitable that you’ll need to create your own custom library of components. You’ll need the manufactures datasheet for the component and begin the process of adding the necessarypins to the part. You’ll then be able to instantiate these components where needed to form your schematic.

Wiring up Components

To make all the components interconnected you’ll need to place wires between pins you want connected. This is a pretty straightforward process and will probably become second nature in no time. It should be mention to avoid 4-way ties at all costs. They only cause confusion.

For power and ground you can use universal symbols to make connections. This greatly reduces the amount of wires travelling across your schematic. In the netlist all pins found to be connected to the symbol will be connected together. It’s highly likely that you’ll want symbols for each of the planes in the printed circuit board. In other words, you’ll want a symbol for digital ground, analog ground, power ground, +5VDC, +12VDC, etc.

If you’ve got a large schematic you’re going to need to send wires across several sheets of schematics. There are generally “ports” to accomplish this task. These ports and can be inputs or outputs and will connect signals across several pages. Nice pcb design software packages will also automatically tell you which page the signal can be found. This greatly reduces the amount of time it takes to examine a schematic during debug.

Design Rule Check

Any respectable pcb design software with schematic capture will have a schematic level design rule check. You should run this periodically for a couple reasons. You don’t want to get to the end of the schematic capture process to find that you’ve been doing something wrong and then have to do it all over again. It will also help you get familiar with the process and fix any minor issues along the way. The design rule check generally looks for any reference designators that are duplicated, inputs that are connected to inputs, etc. This tool can’t tell you if you hooked up your schematic correctly so double check your work!

Netlist

Once you’ve got a clean well organized schematic, its time to make a netlist.  The netlist is what the pcb design software uses to figured out what components it should instantiate and how those components are connected.  Generally speaking you can capture a schematic in Orcad Capture and then layout a board in Mentor PADS.  Orcad has become the industry standard but is having a hard time holding onto the pcb layout portion of their design suite.

Advanced Schematic Capture Articles

With electrons moving faster and faster across the traces, there are several circuit design techniques that must be followed in order to ensure your pcb works properly after fabrication and assembly. Here are a couple excellent resources to help ensure your design words as you expect.

Considering EMI / EMC During Schematic Capture

PCB Design Guide – PCB Footprints

Step 4 in the pcb design guide is PCB footprints.

As you’re creating your circuit design / schematic be sure to keep a directory of all your component datasheets. These data sheets will minimize your time searching for manufacturer recommended land patterns. Many pcb design software packages come with a library of pcb footprints a designer can use, however they are limited in the number of packages as components are constantly changing and shrinking in size. It’s likely you’ll end up having to create several new footprints during your design.

Footprint Naming Convention

If you can’t find the component footprint in your software’s library you’ll need to create your own. If its a small project footprint naming may not seem like a big deal, but if you’ve got hundreds of components or you plan on reusing any of your footprints at a later date, being able to find those newly created footprints is critical. An excellent resource for pcb footprint naming convention is IPC-7351. The latest version of the standard naming convention is found below for your reference.

IPC 7351A PCB Footprint Naming Convention

Anatomy of a Footprint

Before we go to much further you need to have a good understanding what is included in a footprint. Most pcb software bundles the same information in the footprint file, so we’ll discuss this information generally in hopes you can correlate it back to your specific software.

Anatomy of a PCB Footprint

PCB Footprint Resources

In order to accurately design your footprints you really need access to the IPC-7351 standard. However, IPC has teamed with PCB Matrix and is distributing a free Land Pattern Viewer. This tool is essential for design land patterns and will save you hundreds of hours. This tool will provide the dimensions for pad dimensions, pin spacings, solder masks, etc. Therefore its still a manual process, but the outcome is a land pattern guaranteed to meet manufacturing standards.

PCB Matrix Land Pattern Viewer

You can spend hundreds of hours working on pcb footprints. Hopefully a couple of these tools, resources, and articles will help you get started in the right direction and drastically reduce that time.

Since Step 5 is heavily realted to the footprints we’ll go ahead and cover that here.

PCB Board Outline

The board outline is basically just another footprint, just bigger. It has a outline like a component, mounting holes, keepin and keepout zones, serial numbers, etc. The board outline should be roughed out early in the design phase as an estimate will need to be made for board density. A standard practice is to place the origin of the board in the bottom left hand corner.

PCB Design Guide – PCB Layout

We’ll cover steps 6 – 9 on this page of the pcb design guide. These 4 steps together make up the entire subject of pcb layout.

There are a few topics that need to be discussed before proceeding with pcb layout.

Using Grids for PCB Design
PCB Design Units – Imerial vs Metric?

Board Setup and PCB Design Rules

Step 6 of the pcb design guide is getting the board setup and ready for placement.  There are several board setup options and pcb design rules to consider when designing a pcb.

Multi-Layer Board Design

At this stage in the game you’ve probably got an idea of what shape your board, but you might not have considered the inside of the board yet.  A pcb is composed of layers, each facilitating routing of signals or power.  Depending on your design you may need several layers to even make your board routable, however simple designs can be completed on a single or double layer printed circuit board.

Understanding PCB Layers
Ground / Power Planes

Board and Layer Thickness

If you have any high current circuits like power supplies, motor drivers, or high-power LEDs you’ll need to consider the copper thickness needed to keep the current from burning up the traces. You’ll also need to consider the FR4 thickness and how thick the overall stackup will be after production.

Choose PCB Board Copper Thickness
Overall Circuit Board Thickness

PCB Design Rules

There are many options and  design rules that may be setup to assist you during the placement and routing phase of your design. Rules such as minimum trace width and clearance between traces are two of the most common. During placement or routing if you violate a design rule you’ll generally receive a warning so the issue can be corrected.

It should be mentioned that design rules are heavily influenced by manufacturers capabilities. You should check with your manufacturer’s capabilities to ensure your board is producible.

Place Components

Component placement is thought to be the single most important aspect of pcb design in general. With good placement the routing becomes extremely easy. However there is no single way to place components. Here are a few general guidlines to assist in placing components effectively.

Manual Routing

Traces that should be manually routed are clocks and transmission lines. These traces generally need to be shorter and have routing priority over other traces.

PCB Structures: Vias, Pads, Lands, Dimensions, Traces, and Planes
Laying Tracks
Understanding Electrical Clearances

Using Autorouter

The autorouter will save you hours of time. Learn this tool well.

PCB Design Guide – Gerber File Output

The last step in the pcb design guide is outputting the gerber files. No matter which pcb design software you chose it will output some form of Gerber data. It essentially takes all the information about your board and breaks it up by layer. Each layer turns into a single file. The data in the file can then be used to plot the information in order to construct the circuit board. These files will guide a pcb manufacturer exactly how to build your circuit board. This is the industry standard way to have a pcb board quoted so learning how to properly generate the gerber files is essential.

Gerber Files Standard

The newest standard for gerber files is called RS-274X. Below is the specification for this standard and is worth having access to for reference.

Gerber RS-274X Format Specification

Standard Gerber Files

.GBL – Gerber Bottom Layer
.GTL – Gerber Top Layer
.GBS – Gerber Bottom Solder Resist
.GTS – Gerber Top Solder Resist
.GBO – Gerber Bottom Overlay
.GTO – Gerber Top Overlay
.GBP – Gerber Bottom Paste
.GTP – Gerber Top Paste
.GKO – Gerber Keep-Out Layer
.GM1 – Gerber Mechanical 1
.GM2 – Gerber Mechanical 2
.GPT – Gerber Top Pad Master
.GPB – Gerber Bottom Pad Master

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