Best Footprint Pad Layout Guidelines for Your PCB

Tempo Automation

May 29, 2018 , in Blog

Living near the coast can be wonderful. A mesmerizing sunrise over the horizon of a sparkling ocean is hard to match. However, for a part of every year, coast dwellers deal with the looming threat of hazardous weather. One year, over a decade ago, a severe hurricane decided to visit the East Coast and leave thousands of residents in the dark for a couple of weeks. The lack of power forced us to rely on crude survival skills for cooking and other activities. This experience reminded me that it’s always good to know how to perform needed tasks with the most basic tools because you never know when you may have to personally perform seemingly simple tasks without sophisticated tools or technology.

The sophistication and capabilities available in PCB software packages have expanded to the point where you can usually find the exact component library needed for your component. Yet, just as a rare hurricane can cause you to resort to primitive cooking methods, an unavailable component library can force you to create the footprint pad layout yourself. As incorrect footprint layouts can slow down or even halt the manufacturing of your board, it is necessary to follow footprint pad layout guidelines to ensure accuracy and coordination with other board elements. Before defining the best footprint pad layout guidelines to follow, let’s first take a look at what footprint pad layout guidelines are; why they’re important, especially for your design for assembly (DFA); when you need to use them; and how to do so.

The Four W’s of Footprint Pad Layout Guidelines

Your PCB layout may contain several different types of pads. Generically, a pad is an electrical connection point that may join two component pins, a component pin and a trace, a trace and a via or even a trace with an external or off-board device. The function of most pads is to carry current, which may be used for signal transfer or, in the case of thermal pads, for high power components to transfer heat. Pads vary in size and shape but are usually square, rectangular, round or elliptical. Footprint pads are the connection points for components. Depending on your component’s package type, the pads may be arranged around the perimeter of the component or underneath it, as for ball grid arrays (BGAs). The number of pads per component may range from two to thousands. Regardless of the number of pads per component, there are specific guidelines for how the footprint pad layout should be arranged on your board.

What are footprint pad layout guidelines?

Footprint pad guidelines are rules or suggestions that stipulate how pads should be spaced from other pads, other surface elements, drill holes and the board edge. They also define how pads for a single component should be arranged.

Why are footprint pad layout guidelines important?

Components are the most important elements of the board, and pads are the points of connection. If the pad layout fails to accurately identify your component locations or isn’t arranged such that the component can be securely attached, the PCB cannot be assembled. If the pads don’t line up properly with other surface elements like traces and drill holes, your board will not function properly or at all.

When are footprint pad layout guidelines needed?

Just like your PCB layout, your component footprints must also always adhere to certain restrictions. Therefore, every board design should institute footprint pad layout guidelines.

How can I implement footprint pad layout guidelines?

Among PCB designers, creating component libraries probably inspires the least amount of enthusiasm during design. Building symbols and footprints can be time-consuming and trying; therefore, it’s usually worth the effort to search for the needed pad layout in accessible library databases. These libraries are available from PCB design software companies, such as Altium, Autodesk (Eagle) and Cadence (OrCAD); from component manufacturers, such as Texas Instruments; or other sites, such as element14 or SnapEDA. You can also generate a component library, for example from the datasheet by Pinpoint Parts. To employ these options, the CAD format for the component must be compatible with your PCB design program; otherwise, you will have to build the footprint from the landing pattern provided in the component’s datasheet or use a concierge service, where a third party builds the footprint for you. Building the footprint yourself introduces the possibility of transcription errors and there may be a delay associated with using a concierge service.

Best Footprint Pad Layout Guidelines

Whether you can find a library for your part, have one built for you or must create one yourself, you need to ensure that the footprint pad layout adheres to the following guidelines:

  • Make sure that the component pad layout is symmetric. This will prevent tombstoning, which is the detachment of one side of the component due to unbalanced stress.
  • Make sure that opposite pads for a single component have the same size and shape. This helps prevent tombstoning and component shifting.
  • Make sure that the solder mask dam between footprint pads and all other surface elements (i.e. other component footprint pads, traces and drill holes) is adequate. This prevents solder bridging, which occurs when solder joints intended to be separate are too close together. It also allows for the application of solder mask, which protects the board from the environment.
  • Make sure that clearance between footprint pads and the board edge is sufficient. This helps with depanelization, which is the separation of the processed panel into individual boards.

As component placement and mounting are performed during assembly, it is also important to implement these DFA tips to ensure components are properly aligned and secure:

  • Include pin 1 indicators. This ensures that the component pins are connected to the right pads.
  • Include polarity indicators for polarized components, such as diodes and capacitors. This ensures the component functions correctly. Having the polarity reversed could be catastrophic to circuit operation. For example, a diode that should be in conduction mode would act like an open circuit.
  • Include component outlines. These are helpful in assuring the component is placed correctly and pad connections areas are the same.

Following these guidelines helps ensure that your components are placed correctly on the board; do not interfere with the function of other components or elements; and facilitate the timely manufacturing of your board.

However, to optimize the manufacturing of your PCB, including implementing the best footprint pad layout guidelines, you should incorporate comprehensive design for manufacturing (DFM) based on the processes and equipment tolerances of your contract manufacturer.

Tempo‘s Custom PCB Manufacturing Service

  • Performs entire turnkey process in as fast as 3 days.
  • Emphasizes DFM to eliminate time-consuming back-and-forth design corrections.
  • Performs BOM verifications to reduce possibility of component obsolescence issues.
  • Sources components from the most reputable suppliers in the industry to reduce procurement time.
  • Performs multiple automated inspections during assembly to ensure PCB quality for prototyping.
  • Provides support throughout the PCB manufacturing process, beginning with design.
  • Smooth transition from prototyping to production.

Using the right component library to ensure footprint pad layout accuracy is critical to the PCB manufacturing process of your board. If incorrect, the board may not be manufacturable or function properly. Therefore, instituting the best footprint pad layout guidelines is essential, even if you have to build the library yourself. And to help you survive the storm of having to create your own component library, Tempo Automation furnishes detailed footprint pad layout specifications and tolerances as part of the information provided for your DFM. We also enable you to easily view and download DRC files. If you’re an Altium user, you can simply add these files to your PCB design software.

If you’re ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on footprint pad layouts or how to incorporate the best footprint pad layout guidelines into your design, contact us.

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