What You Can Takeaway
How important it is to work with detailed and correct PCB component footprint models
What the different features and capabilities are in your component model editor to help with the creation and alteration of PCB footprints.
How you can put the resources of your CAD tools vendor to work for you in creating the most robust footprint models.
An assortment of PCB components sitting on their footprints in a 3D layout
For the first plastic model kit that I ever built, my dad helped me by patiently explaining each step along the way. One of the things that he insisted on was that I take the time to trim off the excess plastic from each piece to smooth it out. On later model projects, when he wasn’t around to check up on me, I would forgo this step and jam all the pieces together in a rush to finish. You guessed it, the pieces didn’t fit together well and the models ended up looking terrible.
As PCB designers it is always tempting to want to quickly jump into the layout. The challenge of the layout can be invigorating, and placing and routing components can be a lot of fun. But if we don’t spend the time up front to make sure that our PCB footprint models are absolutely correct, we could end up with a real disaster on our hands. Fortunately, our PCB design CAD tools have helpful utilities for this, and we can get through the part building process using a component model editor.
The Importance of Working with Detailed PCB Footprints
When design tolerances were looser and there were a lot more thru-hole components being used, there used to be a lot more slop in how component footprints were created for PCB design. It wasn’t unusual for similar dual inline parts (DIPs) on a board to be created with the same component outlines no matter what their actual sizes were. Although this was not the recommended way to do things, it usually worked and a lot of boards were designed this way. That is no longer the case, as today’s high-density surface mount designs demand that you claw and scrap for every millimeter of space that you can get.
In addition to making sure that you don’t cheat yourself out of any available design space on the board with parts that are too large, you also need to make sure that your parts aren’t too small as well. Component footprints need to be built exactly as specified starting with the correct pad size and spacing to support the design rules of the board. This may include thermal pads to combat excessive heat issues, and specific pad shapes and spacings to help guard against noise and support high-speed design or RF requirements. At the same time, you still need to build your component footprints so that their pad layout and clearances allow for the best routing for the board technology that you are designing. But wait, there’s more.
You also have to build your component footprints so that the final board design can be efficiently manufactured without errors. Pad sizes have to support the soldering process that that board will be assembled with, while at the same time not violating design for manufacturing (DFM) clearance rules. The footprint pattern must also include the correct body size so as to work together with automated and hand assembly processes. Also, the correct component height must be used in order to avoid clearance violations in the “Y” axis. 3D step models should also be included with the footprint for advanced PCB design tools to leverage. Not only do these allow for the layout to be checked and verified in 3D, but they will also be a large part of the overall ECAD and MCAD co-design process as shown here.
As you can see, it is more important now than ever before to invest a lot of effort into creating the most accurate component footprints for your PCB library. Fortunately, your PCB design CAD tools are set up to help you with this in many different ways.
Using the Padstack Editor in Cadence Allegro to create a surface mount pin
Using a Component Model Editor to Create Your PCB Footprints
There are many different elements in a PCB design CAD library such as schematic symbols and drawing pieces, and of course the component footprints. When building a footprint in your component model editor, it can sometimes be helpful to start with an existing footprint as a template, while other cases will require being built new from the ground up. In either case, the first thing that a component will need is a padstack.
You probably already have many padstacks in your PCB library, but as we’ve already discussed, many new parts will have specific padstack requirements necessitating a new shape. Here is where you need to work closely with the land pattern requirements of your new footprint to get the correct dimensions. PCB design CAD tools like Cadence Allegro offer you the use of a Padstack Editor as shown above, which gives you the ability to choose the pad type and then build it according to the specific parameters that you need.
Next comes building the component footprint itself. Although there are different automated tools within a design system that can help in footprint creation, there will always be land patterns that will require manual creation or editing. Component model editors like the one shown below give you a lot of control over placing pins, drawing outlines and adding reference designators. You can see how this model editor in Allegro gives the users a lot of control over exactly where and how to place the pins.
Manually placing the pins for a 1206 component footprint
Wizards and Magic: Design Automation Tools and Online Services that Can Help
Fortunately though, you don’t have to do all of the work building footprints yourself. As we mentioned above, most PCB design CAD tools will provide you with an assortment of different functions to help with the creation of component footprint land patterns. These features can range anywhere from common templates to repeatable pin placement commands for high pin count parts like ball grid arrays (BGAs). Another very useful automated feature for the creation of component footprints is a part generator like the package symbol wizards shown in the picture below. A wizard like this one will give you the ability to specify the details and dimensions of your component footprint, and then it will build the part for you. Once created you will still have the option of going in and editing the part to make alterations if necessary.
Another extremely helpful feature in the software toolbox of some printed circuit board design CAD systems is the ability to use online PCB library services. These tools, like Cadence’s Unified Part Search, allow designers to search through an online library service for the part they need while still in schematic capture. To help the designer find the part they are looking for, there are search and filter features similar to what you would find on any kind of web browser. By selecting the desired part and symbol, the associated data including specification sheets and component footprint models are downloaded into your design environment. Since these parts are created by the component manufacturer and fine-tuned for the CAD system that is being used, the designer will end up with the perfect footprint for their PCB layout.
Specifying the dimensions of a surface mount discrete using the Package Symbol Wizard in Allegro
How to Get the Most Out of Your PCB Design Software
To get the full use out of your component model editor, you need to arm yourself with as much information as possible on the best ways to use it. One good spot to find information like this is in the web pages of your CAD software provider. There you will find online video demonstrations, tutorials, and a host of white papers and manuals presented by the CAD software vendor. Cadence for instance has multiple online content, videos, and other services available by going to their resource page here at this link.
The next step is to jump in and start using your tools to create the footprints that you need. Don’t be content to merely create the bare essentials, but take the time to create a component footprint that will serve everyone’s needs. Depending on what your company’s requirements are, this may mean incorporating different component shapes for assembly drawings, or specific font sizes and line widths for silkscreen. In addition, you should also take the time to make your footprint exactly according to specified dimensions. You, your design department, and your manufacturer will be very glad that you did.
You need the best PCB design system in order to have the tools and features needed to create, edit, and download perfect PCB footprints. Allegro PCB Designer has these capabilities, and the images you’ve seen here are all thanks to the multiple software features that Allegro is equipped with.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.
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