Considerations for PCB Circuit Design: Why PCB Labels are Important
Checklists are an important part of life. Airline pilots use them to make sure that they’ve put the wheels down before landing. Shoppers use them to make sure that they don’t forget to buy the eggs. We all use checklists in different ways, and many of us would be lost without them.
Here’s another good use for a checklist; making sure that we don’t forget to include important elements on our PCB design. It’s not likely that we will forget to place and route any of the components and nets. Silkscreen part outlines and reference designators are often created automatically, and design rule checks will help with the rest. There is one area of the design though where it can be easy to forget something important, and that’s the different labeling required on a PCB.
There are two areas of labeling that need to be on your design; identification labels, and markers to aid in PCB manufacturing. I have personally experienced how easy it can be to forget these items when designing the board. The best way to remember is to run through a “design complete” checklist before you send the design off to manufacturing. Let’s look at some of the different labels and markers that you should have in your list.
The first category of labels and markers that must be on your design are the identification labels. If these labels are not included, the board will not be able to be correctly identified and matched up with the bill of materials. Here are some types identification labels that you should consider:
- The PCB name.
- The serial number.
- The model number.
- The fabrication number.
- The assembly number.
- The revision number or letter.
- Any other identifiers such as FCC numbers, etc.
- Clear areas for manufacturing stamps.
These labels are usually created in etch on the top or bottom layers of the board. Some companies may also require you to cover the labels with identical text in silkscreen to make the label more pronounced. You won’t necessarily need all of these labels that I have listed as different companies have different standards for what they want. You must however make sure to include the labels that your company requires, and a checklist will help you to do that.
You will also need to include on your design the markers and labels needed for manufacturing. Without these markers, you will probably get the design kicked back to you from manufacturing. I know this because it has happened to me. Here are some of the commonly used markers and labels that you should know about:
Layer number markers: These are usually small blocks within the board outline containing numbers that are sequentially placed next to each other for each layer of the board. These markers provide a quick check that all board layers are composited in the right order
Global fiducials: These are small metal pads on the top and bottom layers of the board within the board outline. Fiducials are used as reference points for automatic pick and place machines. Targets: These are used for layer registration and are on the outside of the board outline. Although not as important as they once were when PCB layers were aligned manually during manufacturing, these targets still serve as a good final layer-to-layer registration check.
Title blocks: These are blocks of information outside of the board outline. Title blocks contain information such as the company information, board name, fabrication number, and layer numbers. They provide a quick and easy way for the manufacturer to know what they are dealing with.
Some important do’s and don’ts
I’ve got another tip that could be helpful for you. Add these labels and markings at the beginning of the design, or at least be sure to leave space for them when you start. Realizing at the end of the design that there isn’t room for the serial number that you forgot can ruin your whole day. It may not matter to anyone else that you will need to spend time redesigning the board in order to create some space, but it will drive you nuts!
Here’s another tip for you. When you make revisions to a design, make sure that you run through this same checklist. A new revision of the board will change some if not all of the identification numbers on the board. This may seem like a minor detail, but it can have large effects. For example, if the revised design has new functionality but the part numbers haven’t changed, it will be hard to tell the new board apart from the old one. This can cause all kinds of problems in manufacturing and testing, where the board may end up being incorrectly assembled or give false test results. If an incorrectly labeled board were to get all the way out to the field, end users could potentially end up with a circuit board that doesn’t function in the way that they expected it to. Not only could errors like this potentially cause failed products and lots of money to repair and replace, but the finger of blame will eventually find its way back to you.
Having a checklist to make sure that you don’t forget these labels and markings will be a tremendous help to you. As I said, I know from personal experience the pain that you can go through if you forget them. These labels are needed to match your board up with the bill of materials as well as the rest of the PCB documentation. Incorrect or forgotten labels can cause all kinds of problems, and it is up to you to make sure that they are correct.
Labels and other board identification markings are all an important part of the entire BOM documentation process. BOM management tools can help you with the creation and management of PCB documentation, and Altium’s BOM tools can give you the help that you need. Find out more information by talking to an expert at Altium.