Tutorial 1: PCB Basics

(Selected materials from SparkFun.com)

Overview

One of the key concepts in electronics is the printed circuit board or PCB. It’s so fundamental that people often forget to explain what a PCB is. This tutorial will breakdown what makes up a PCB and some of the common terms used in the PCB world.

PCB is an acronym for printed circuit board. It is a board that has lines and pads that connect various points together. A PCB allows signals and power to be routed between physical devices. Solder is the metal that makes the electrical connections between the surface of the PCB and the electronic components. Being metal, solder also serves as a strong mechanical adhesive.

Composition

A PCB is sort of like a layer cake or lasagna- there are alternating layers of different materials which are laminated together with heat and adhesive such that the result is a single object.



Let’s start in the middle and work our way out.

FR4:
The base material, or substrate, is usually fiberglass. Historically, the most common designator for this fiberglass is “FR4”. This solid core gives the PCB its rigidity and thickness. There are also flexible PCBs built on flexible high-temperature plastic (Kapton or the equivalent). You will find many different thickness PCBs; the most common thickness for SparkFun products is 1.6mm (0.063").

Cheaper PCBs will be made with other materials such as epoxies or phenolics which lack the durability of FR4 but are much less expensive. You will know you are working with this type of PCB when you solder to it - they have a very distictive bad smell. These types of substrates are also typically found in low-end consumer electronics. Phenolics have a low thermal decomposition temperature which causes them to delaminate, smoke and char when the soldering iron is held too long on the board.

Copper:
The next layer is a thin copper foil, which is laminated to the board with heat and adhesive. On common, double sided PCBs, copper is applied to both sides of the substrate. In lower cost electronic gadgets the PCB may have copper on only one side. When we refer to a double sided or 2-layer board we are referring to the number of copper layers (2). This can be as few as 1 layer or as many as 16 layers or more.

The copper thickness can vary and is specified by weight, in ounces per square foot. The vast majority of PCBs have 1 ounce of copper per square foot but some PCBs that handle very high power may use 2 or 3 ounce copper. Each ounce per square translates to about 35 micrometers or 1.4 thousandths of an inch of thickness of copper.

Soldermask
Soldermask The layer on top of the copper foil is called the soldermask layer. This layer gives the PCB its green (or, at SparkFun, red) color. It is overlaid onto the copper layer to insulate the copper traces from accidental contact with other metal, solder, or conductive bits. This layer helps the user to solder to the correct places and prevent solder jumpers. In the example below, the green solder mask is applied to the majority of the PCB, covering up the small traces but leaving the silver rings and SMD pads exposed so they can be soldered to.
Soldermask is most commonly green in color but nearly any color is possible.

Silkscreen
The white silkscreen layer is applied on top of the soldermask layer. The silkscreen adds letters, numbers, and symbols to the PCB that allow for easier assembly and indicators for humans to better understand the board. We often use silkscreen labels to indicate what the function of each pin or LED.


Silkscreen is most commonly white but any ink color can be used. Black, gray, red, and even yellow silkscreen colors are widely available; it is, however, uncommon to see more than one color on a single board

More about the terminology in the PCB industry can be found in the PDF printed from the SparkFun's website.

Introduction to Eagle PCB:

EAGLE is one of many PCB CAD softwares out there.
For non-profit academic use, EAGLE is free, but there are some limitations:
Exploring the Control Panel

The first time you open up EAGLE, you should be presented with the Control Panel view. The Control Panel is the “homebase” for Eagle, it links together all of the other modules in the software.


You can explore the six separate trees in the control panel, which highlight separate functions of the software:

Libraries – Libraries store parts, which are a combination of schematic symbol and PCB footprint. Libraries usually contain a group of related parts, e.g. the atmel.lbr stores a good amount of Atmel AVR devices, while the 74xx-us.lbr library has just about every TTL 74xx series IC there is.

Design Rules (DRU) – Design rules are a set of rules your board design must meet before you can send it off to the fab house. In this tree you’ll find DRU files, which are a a pre-defined set of rules.

User Language Programs (ULPs) – ULPs are scripts written in EAGLE’s User Language. They can be used to automate processes like generating bill of materials (bom.ulp), or importing a graphic (import-bmp.ulp).

Scripts (SCR) – Script files can be used to customize the EAGLE user interface. In one click you can set the color scheme and assign key bindings.

CAM Jobs (CAM) – CAM jobs can be opened up by the CAM processor to aid in the creation of gerber files.

Projects – This is where each of your projects are organized into a single project folder. Projects will include schematic, board design, and possibly gerber files.


If you select a file in a tree, information about it will appear in the right-hand portion of the window. This is a great way to explore libraries, project designs (EAGLE comes with some fun examples), or to get a good overview of what a script’s purpose is..

Using the SparkFun Libraries

Included with EAGLE is an impressive list of part libraries, which you can explore in the Control Panel view. There are hundreds of libraries in here, some devoted to specific parts like resistors, or NPN transistors, others are devoted to specific manufacturers. This is an amazing resource! But it can also be a bit overwhelming. Even if you just want to add a simple through-hole electrolytic capacitor, there are dozens of libraries and parts to sort through to find the right thing. Instead of using the hundreds of default libraries, you can use the SparkFun EAGLE Libraries, which are filtered down to only include the parts that we’ve used in designs ourselves. And they’re constantly updated with new parts we’ve discovered. Here’s how you can install and use the SparkFun libraries instead of (or in addition to) the default ones:

Step 1: Download the SparkFun Libraries

The most recent version of the libraries can always be found in the GitHub repository. For help using GitHub, check out our Using GitHub tutorial. Basically, all you’ll need to do from the main repository page is click “Download ZIP”.

The version I am using can be found here.



Step 2: Updating the Directories Window Back to the EAGLE Control Panel window now. Go to the “Options” menu and then select “Directories”. This is a list of computer directories where EAGLE looks when it populates all six objects in the tree view…including libraries.


Note: Mac and Linux users should place a colon (:) between directories instead of the semicolon.

In the “Libraries” box is where we’ll add a link to the directory where the SparkFun EAGLE libraries are stored. There are a few options here. If you’d like to keep the default libraries and add the SparkFun library, add a semicolon (;) after “$EAGLEDIR\lbr”, and paste the SparkFun EAGLE Libraries directory location after that.

Step 3: “Using” Libraries Now, when you go back and look at the “Libraries” tree, there should be two folders included, one of which should be our SparkFun Eagle Libraries. The last step is to tell EAGLE that, for now at least, we don’t want to use the default libraries. To do this, right click on the “lbr” folder, and select “Use none”.

Now, when you go back and look at the “Libraries” tree, there should be two folders included, one of which should be our SparkFun Eagle Libraries. The last step is to tell EAGLE that, for now at least, we don’t want to use the default libraries. To do this, right click on the “lbr” folder, and select “Use none”.



Then, right-click on the “SparkFun-Eagle-Libraries-master” folder, and select “Use all”. Then check the libraries in each of the two folders. Next to them should be either a grey or green dot. A green dot next to a library means it’s in use, a grey dot means it’s not. Your libraries tree should look a little something like this:



Opening a Project and Explore

EAGLE is packaged with a handful of nifty example PCB designs. Open one up by expanding the “Projects” tree. From there, under the “examples” folder open up the “arduino” project by double-clicking the red folder (or right-clicking and selecting “Open project”). Note that, in this view, project folders are red and regular folders are the standard yellow.



Opening the project should cause two more EAGLE windows to spawn: the board and schematic editors. These are the yin and the yang of EAGLE. They should be used together to create the finished product that is a functional PCB design.


The schematic editor (on the left above) is a collection of red circuit symbols which are interconnected with green nets (or wires). A project’s schematic is like the comments in a program’s code. It helps tell the story of what the board design actually does, but it doesn’t have much influence on the end product. Parts in a schematic aren’t precisely measured, they’re laid out and connected in a way that’s easy to read, to help you and others understand what’s going on with the board design.

The board editor is where the real magic happens. Here colorful layers overlap and intersect to create a precisely measured PCB design. Two copper layers – red on top, blue on the bottom – are strategically routed to make sure different signals don’t intersect and short out. Yellow circles (on this design, but they’re more often green) called “vias” pass a signal from one side to the other. Bigger vias allow for through-hole parts to be inserted and soldered to the board. Other, currently hidden, layers expose copper so components can be soldered to it.

Keep Both Windows Open!

Both of these windows work hand-in-hand. Any changes made to the schematic are automatically reflected in the board editor. Whenever you’re modifying a design it’s important to keep both windows open at all times. If, for instance, you closed the board window of a design, but continued to modify a schematic. The changes you made to the schematic wouldn’t be reflected in the board design. This is bad. The schematic and board design should always be consistent. It’s really painful to backtrack any changes in an effort to reattain consistency. Always keep both windows open! There are a few ways to tell if you don’t have consistency between windows. First, there’s a “dot” in the lower-right hand corner of both windows. If the dot is green, everything is groovy. If the dot is magenta, a window’s probably closed that shouldn’t be. Second, and more obvious, if you close either of the two windows a big, huge warning should pop up in the other:



If you see that warning STOP doing anything, and get the other window back open. The easy way to get either a board or schematic window back open is by clicking the “Switch to board/schematic” icon


Navigating the View

This is a subject that’s usually glazed over, but it’s important to know how to navigate around both of these windows. To move around within an editor window, a mouse with a scroll wheel comes in very handy. You can zoom in and out by rotating the wheel forward and backward. Pressing the wheel down, and moving the mouse allows you to drag the screen around.

Configuring the UI

EAGLE’s user interface is highly customizable. Anything from the background color, to layer colors, to key bindings can be modified to fit your preference. Better tailoring your interface can make designing a PCB much easier. On this page we’ll talk about how we at SparkFun prefer to customize our UI. None of these steps are required. Customize your UI as you see fit. These are just the settings that we’ve grown accustomed to. Setting the Background Color The first adjustment we always make to the UI is the background color of the board editor. The standard white background doesn’t always meld very well with the array of colored layers required for board design. Instead, we usually opt for a black background. To change the background color, go up to the “Options” menu and select “User interface”. Inside the “Layout” box you can set the background to black, white, or a specific color.





There are other options in this box to be explored, but you may want to hold off on adjusting most until you have more experience with the software.

Adjusting the Grid Another UI improvement we like to make in the board editor is turning the grid on. Dimensions and sizes are so important to the design of your PCB, having some visible reminders of size can be very helpful.