Hundreds of years ago, sailors worried that on long voyages from home they risked inadvertently sailing over the edge of the world. Without factual information to rely on, not knowing what may lay beyond the edge was a terrifying thought. Even nautical charts from medieval times sometimes depict the unexplored edges of the map where potential dangers were thought to exist with the warning: “here be dragons.”
Today we know we’re safe from such fears. There haven’t been any sightings of dragons for a while now, and if you do sail due east or west, you’ll eventually run into another continent. But there is one edge that still commands a lot of respect, and that is the edge of a printed circuit board. All kidding aside, if PCB edge clearances aren’t respected by their designers, there could be a lot of problems ahead when it comes time to manufacture the board. Let’s take a look at some of the potential problems if PCB component to edge clearances are not observed, and what you as a PCB designer can do to ensure error-free manufacturing.
Problems Resulting from an Inadequate PCB Component-to-Edge Clearance
To use the maximum amount of available space on a circuit board, designers will often try to place components as close to the edge as possible. There can be many problems however that arise from doing this:
- Component damage: Circuit boards are often fabricated and assembled in apanel along with multiple instances of the board to lower production costs. These boards will have to be broken out of the panel at the end of the manufacturing process, and components that are too close to the edge may be damaged when the board is depanalized. This kind of damage can be intermittent and very difficult to find and debug.
- Conductors: When components are placed close to the edge, their trace routing will also be too close. The flexing of the board during depanelization can not only affect the components but the traces that are connecting to them.
- Automated processes: Components that are too close to the edge may interfere with the conveyor belts of automated assembly equipment such as wave solder or solder reflow machines.
- Component height: The taller a component is, the more it can potentially interfere with the assembly equipment. Components such as large electrolytic capacitors should be set further back from the edge of the board than other components.
- Edge connectors: These components will have to be placed on the edge of the board, but care must be taken to position them correctly according to their specifications. Without a precise location, they may not work correctly with their intended cabling, or they could end up having problems with the soldering process.
- Circuit board test: Test fixtures need enough room around the perimeter of the board for the fixture to make an airtight seal for the vacuum drawdown of the board. If components are placed too close to the edge, it makes this operation more difficult, requiring alterations to the test fixture.
To avoid these problems, here are some general guidelines for component-to-edge placement clearances.
Edge Clearances to Consider for Optimum Spacing
The general guideline for component clearances around the edge of a printed circuit board is 0.100 inches. This will give an adequate amount of room for the test fixture and most assembly operations. Often this number is reduced by the manufacturer depending on the needs of the design. There are some areas of the board where specific clearances must be adhered to:
- Panel V-grooves: For circuit board panels that will be scored with a V-grove for breakout, the parts must maintain a minimum of 0.075 inches to the edge of the board. This will give the cutting process enough room without damaging the components. For those parts that are taller, the minimum clearance grows to 0.125 inches to give those components a safe distance to the cutter.
- Panel breakout tabs: For circuit boards that will be separated from panels using a breakout tab, components next to a tab must maintain a distance of 0.125 inches to the edge of the board. For taller components that minimum clearance grows to 0.250 to protect the components during depanelization.
Another edge clearance to remember is the copper to the edge of the board. For those components with larger connection areas, their solder joints may fracture with depanelization as well and must be held further back from the edge than other parts.
There are a lot of edge clearances to consider depending on which components are being used and what the intended depanelization scheme is. Thankfully your PCB contract manufacturer can help you with these clearances while your design is still in layout.
How Your PCB Contract Manufacturer can Help with Design Questions
The CM that you are working with has a lot of experience manufacturing PCB designs, and they will know precisely what component to board edge clearances are required for your design. At VSE we have a full staff of engineers who routinely fine-tune designs to improve both their functionality and their manufacturability. We can help you with your design, ensuring that your component to board edge clearances are what they need to be for the highest quality and yields in circuit board manufacturing.
