Do you remember when you were a kid, it seemed like you were forbidden from going anywhere you really wanted to? Either a grownup was reprimanding you for the mere thought of entering a restricted area, or there was a huge sign with capital letters proclaiming “NO CHILDREN ALLOWED!” At the time, these obstructions may have only served to give more allure to the prospect of growing up. But no matter how arbitrary rules to keep children out of certain places may seem, there are usually good reasons for enforcing them.
PCB keepout examples
Ironic though it may be, after reaching adulthood, many people discover that there are indeed situations where preventing entry is for the best. PCB layouts are prime examples of this. Obviously, every component has its own footprint pad layout that informs your contract manufacturer (CM) where it should be placed. However, there are components that require additional spacing from other board elements to accommodate trace fanout for ICs like microprocessors, minimize EMI for antenna transmission and/or reception or other issues. Let’s explore these and other PCB keepout types and then see how best to stay out of them.
PCB Keepout Types
In the simplest terms, a PCB keepout can be defined as follows:
A PCB keepout is an area on a circuit board set aside by the designer where no external components, copper traces or other board elements should enter into or cross. This area may be or contain copper and can be of any shape.
In most cases, keepouts are used to set aside certain board areas far enough from other elements to prevent or minimize EMI. However, they are also used to provide spacing for fanout tracing for surface mount components; such as processors or FPGAs that are often the primary component of PCB evaluation and development boards. Some common types of keepouts are listed below.
Types of PCB Keepout
Probably, the most common type of keepout is setting aside a copper area around a board-mounted or connected antenna to prevent EMI affecting the transmitted or received signal fidelity. The keepout may also contain the antenna traces to other circuitry.
Setting aside space for fanout around a component, especially an EM radiator, is also common. This is especially true for microprocessors, FPGAs, AFEs and other moderate to high pin count components that typically are utilized in an SMD package.
- Board edge clearance area
Board edge clearance is important for manufacturing. Specifically, during PCB assembly where the panels are depanelized into single boards. To accomplish this there must be adequate clearance for routing or scoring.
At times it can be advantageous to define a keepout area around traces. This is sometimes used for coplanar grounding of transmission lines to achieve controlled impedance.
- Drill hole
Many boards are mounted by screws or bolts. In these cases, it can be helpful to define spacing around the drill hole for bolt heads, etc. Inadequate spacing can affect assembly, interrupt circuit operation and even result in board damage. For vias, following your CM’s DFM rules is usually sufficient.
Depending upon the connector type and placement, there may be two keepout considerations for your board design: the connector-board footprint and panelization. Often connector or headers footprint layouts do not include space for external wiring or cables to connect. In these cases, a keepout is valuable to ensure that your circuit can actually operate as intended.
Another good usage of a keepout is to provide space for flipping or moving a horizontally mounted switch.
The list above gives some common types and uses for PCB keepouts. However, there may be other situations where you find it necessary to define a keepout area. For example, if your design employs a component; such as an operational amplifier where there is a large impedance mismatch from input to output, the circuit may be susceptible to feedback current leakage and it may be necessary to provide protection in the form of a PCB guard ring. Although not classified as a keepout, a guard ring does serve as a physical barrier against external components and traces, as well as preventing internal currents from leaving the area. Now, we are ready to see how to ensure that keepouts fulfill their function.
Keeping Out of the Keepouts
PCB keepouts are only effective if they actually fulfill their objective. That is to provide isolation from any and all transgression by external elements for a certain area(s) of your board. Accomplishing this goal requires that you follow good keepout usage guidelines as listed below.
PCB Keepout Guidelines
- Determine why the keepout is needed
- Based on usage, determine how much space is required
- Use silkscreen marking to identify the keepout area
- Ensure your design file includes keepout information
PCB keepouts can be invaluable assets to your board design that ensure it performs as intended. By following the guidelines above and making good use of them you can avoid placement conflicts and aid the reliability of your PCBA once deployed.
|Tempo's Custom PCB Manufacturing Service
At Tempo Automation, we will make sure that your design objectives are reflected accurately in your finished boards; including any keepouts.
And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.
If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on PCB keepouts or how to make sure they are correctly implemented on your boards, contact us.