An electric circuit is a set of connections between electronic components. In a very abstract form, and electric circuit has a source of electric current, a destination for the current (ground) and electro-mechanical components that draw power from the flow of electricity to do some work. A PCB or Printed Circuit Board is essentially a circuit which can be feasibly and reliably mass produced. Mass production reduces cost and lower cost lead to more consumption leading to virtuous production cycle.
Take note that the primary purpose of a PCB is to finally be mass produced. Several DIY developers design PCB’s for just prototyping and are not necessarily rigorously looking towards mass production. In this guide, I will highlight the entire procedure, in a step by step guide to design production grade PCB. Let’s get started!
Note: In the following tutorial, I use Altium CircuitStudio. However, the principles of PCB design will remain the same no matter which tool you use. The core concepts will remain the same, including the terminologies. This tutorial can easily be adapted into Altium Designer or Autodesk Eagle with remarkable ease.
Step 1. The Circuit
It goes without saying, to design a PCB we need a circuit that we want to design. The circuit you are designing can be simple or complex. In this tutorial I am going to use a fairly intermediate level circuit.
The circuit above is a 555 alarm circuit. From an electronics perspective, it is quite simple and straight forward. It is powered by a 5V source. The NE555 timer will generate a square pulse wave. The speaker will output sound in sync with the waveform when the button is pressed. Simple.
I chose this circuit because it has several elements that a normal circuit designer will use, only in a lesser quantity. Lets look at the components in the circuit. Broadly speaking, we have:
- 555 timer which is an IC (integrated circuit)
- Passive elements like resistors and capacitors.
- Active elements like the PNP and NPN transistors.
- I/O components like the button and the speaker.
We can now start the actual process of PCB designing.
Step 2: PCB Schematic Design
A schematic representation is essentially a drawing of all components and their connections. All PCB design software make use of schematics to then project into the PCB layout design. Thus, the first step in PCB design is to create a good schematic.
The main purpose of a schematic is to be sort of a blueprint, not just for the PCB software but also to any person reading. The components you add to the schematic have pin layout and labels. The components are connected to one another using wires\nets. A well designed schematic is one that is clean and easy to read. The connections need to be clearly visible.
The example above may be a functional schematic but it is not easy to read. The connections are numerous and the wires\nets overlap in several places. In short it is a functional schematic for personal use but not necessarily of any use if you intend to share it. Let’s get started with designing our very own schematic. Before we start off, here are few common keywords:
- Wire\Net : It is the wire connection that establishes a link between pins of the components.
- Component: It is either an IC, a passive unit like resistor\capacitor or an active unit like BJT or MOSFET.
- Source Pins: The VDD and GND are source and drain modules.
Start up your PCB design software and create a new project. Give it any name you want and save it to an easily accessible directory. I am calling my project Alarm_PCB_555. Your project should now appear with no files in it. Right click on your project directory (or go to File… and add) -> Add New Schematic. Save it with a name like Alarm_555_Schematic.
We will place all our components on this schematic view. Every PCB design tool will have its repository of components and the method to access them might be different. However, regardless of the software, the components will be of at least two types.
- THT : Through Hole Type, are the designs which require a hole to be drilled through the PCB. This is usually used in prototyping and single layer PCB.
- SMD: Surface Mount Devices are the ones used in industries and have flat pads. They do not require to be drilled onto the board. This allows different components to be placed on either side of the PCB, each with a different circuit.
In this project we are going to use SMD components. So make sure to select SMD version of the components you bring into the schematic.
Step 2.1: Bringing All Components to the Board
This is our reference circuit again. We first start with importing the IC, followed by the transistors and then the passive components. There is no particular reason for this order other than personal preference but I found this makes arranging the schematic easier.
The SMD packages for passive components come in various package sizes. They are commonly 1206, 0805, 0603, 0201 etc. These are the length and width of the modules. It is a good practice to stick to a particular dimension for the entire project. The smaller package sizes have higher cost and low availability. I mostly use 0603 packages. However, it is best you look online for the package that is most easily accessible to you.
Step 2.2: Bringing in the IC
It is worth noting here that the order of pins in schematic is not the same as that in the actual IC package. That is something we need to worry about in the PCB design stage. There are two ways we can proceed from here. We can import the other components and connect them directly with Nets resulting in a schematic similar to the mess in Fig.3 or we can create small Nets emerging from each Pin and assign those nets a name(Net Label). If any component needs to connect to a pin, then we create a small net from that component and assign it the same Net Label. This way the components are connected without drawing out the actual connections. It also drastically improves the readability of the schematic.
Note: The Net Label can be called anything. In this case I will assign them the Pin Names, but their names may change depending on your specific use case.
Step 2.3: Making Connections
In Fig.10, I have made a connection between the wires of PIN-3 and 1000-ohm resistor using Net Labels. This example is a little small to fully illustrate the benefits of using Net Labels, but I hope you can see how helpful this can be as the size and complexity of a schematic increases.
Note: Make sure that you are using Net Label for naming the wires since most PCB software also come with the option to add text strings.
Step 2.4: Completing the Schematic
I have replicated the entire schematic, with a few practical exceptions.
- The Battery has been replaced with a micro — USB B receptacle. This allows the PCB to be powered from any source and not be limited to just cells. It increases the usability of the device while also making the package as small as possible. The USB module has 5 accessible pins, however we will need only the V_Bus and GND pins. We connect them to the Source Pins and then connect the source pins to other components.
- The Speaker Connection has been replaced with Connection Pads (TP). Connection pads are metallic exposure points which are usually 1mm in diameter. Wires can be soldered onto them with remarkable ease, thus allowing users to connect to a variety of modules and not being restricted to just speakers. It also makes it easier to replace faulty components.
- TP stands for Test Point. This is one of the most essential parts of a PCB designing which beginners leave out. Test points are exposed connections which are used to test connection and signals during debugging. This is done using either a multi-meter or an oscilloscope.
Step 2.5: Adding Test Point
TP1 and TP2 are the pads to which the Speaker\Output device is soldered onto. We add test points at any junction we need to test the flow of current. It is usually placed near components which may experience varying power. Test points are also added near power sources to ensure power is flowing correctly. Additionally, test points help to check for faulty connections as well.
Step 3: PCB Layout Designing
Now that our schematic ready, we can move into the process of laying out the PCB. So go to Files menu and create a new PCB layout. I am going to name my PCB as Alarm_555_PCB.
You should now have a Black background as your PCB. Now we need to import the schematic. To do this, go to your schematic and right-click to get schematic actions and select, “Update PCB with Schematic”.
Note: This option will vary from one software to another. The process flow however, will be the same.
The imported components will be placed outside the board. Now, we can either resize our board and then place our components or vice-versa. I prefer to layout my components before resizing the board. However, before that there is one more step. We need to define the rules of PCB layout.
Step 3.1: Set PCB Design Rules
PCB Design rules normally set the width of the trace and size\diameter of holes in the PCB. They are usually set to default value but not every manufacturer caters to all sizes. Therefore, you need to determine the basic sizes after looking through the guidelines page of the PCB manufacturer from which you plan to order your PCB from.
Make sure you are using Metric units instead of the Imperial units. Some important ones are:
- Clearance: I set it to 0.25 mm for all units.
- Trace Width: I set this as well to 0.25 mm
- Routing Via: Via’s are a connection from the top layer of the board to the bottom layer. This is so that the same circuit can be continued in the bottom layer if the top layer is blocked. The diameter is set to 0.5 mm and the hold width is set to 0.2 mm.
Step 3.2: Laying out the Components
Starting with the IC, we bring it to the bottom edge, since we want to keep the size of the PCB as small as possible. It is a convention to bring in all IC’s first before bringing in the components they connected to.
Note: If you scroll in, you can see the connections that exist in the form of wires. These will be removed as soon as you create an actual connection using PCB trace. Try to place the components with connections as close to one another as possible.
If you look closely, the C3, R5, U1 are all overlay texts which are used for identifying the component and its orientation. Their location and position can be rotated or moved. It is good practice to keep the overlay on the top of the component. Also, make sure to not overlap the white text on any component. It helps when the PCB is being assembled manually for testing and prototyping.
Step 3.2: Routing The PCB
Routing is the process of making connections on the PCB. Most PCB software come with an auto-router. Just click on the auto-route option and it will create the traces using a predefined algorithm. This works amazingly well when you have a simple PCB design but as the complexity rises, its accuracy and efficiency dips drastically. Most professional users prefer to route their design manually. Although it takes more time, it ensures quality. Even in the manual process, the Design Rules that you specified will be adhered to with no issues at all.
Some times you may come across overlapping connections as in this case. You can go around in a long path, but it is not good practice. So instead we will add a via. A via is used to make connection between two layers. Here we will cross the overlap in the bottom layer of the PCB and use another via to come back to the top layer for making the connection.
The Green Trace is the trace in the lower layer.
Step 3.3: Resizing the PCB
The default size of the board in my case is 10 cm * 10 cm. However our board does not need to be so big. So we can go ahead to resizing it appropriately.
Step 4: The Last Mile
There is one last step before our PCB can be production ready and that is adding a copper pour to the board. In the present form, copper will only remain in the blue\green traces and all copper from the board will be etched away. This naturally will take a lot more etching solution and as a result be costly. To solve this, we fill the entire layer with copper and connect it to the ground pin. The other traces will be isolated in the etching process. Thus, instead of removing 95% of the copper, we will remove only 3% copper.
Copper Polygon Pour has another advantage. Since it is connected to the ground plane, it will help the PCB dissipate heat in a better way and reduce EMI interference. We will do this on both the top and the bottom layer of the PCB.
Your PCB Design is now complete. You can now export the Gerber files along with the assembly and bill of materials to any PCB manufacturer and you are good to go. Most PCB software also generate automatic CAD files which can be exported and used in software like Fusion360 and AutoCAD.
In the tutorial above I have highlighted the basic processes in professional PCB designing using a very simple example. These methods can easily be extrapolated to a more complex project as the underlying principles will remain the same.
Hope it was helpful. The following few images are from a more complex project I worked on in a college project, just as reference to illustrate the potential of the steps mentioned above.
PCB CAD Software