When it comes to PCB design, placement is everything. Although some component locations are determined by enclosure design or fixed due to interconnection with other boards or devices, most locations are determined by the designer. To ensure design functionality, component placement decisions need to address thermal, radiation and signal integrity issues. Furthermore, your design needs to take potential manufacturability and reliability issues into consideration.
By considering PCB board edge clearance requirements for components, connectors, drill holes, traces and panelization during design, you can improve manufacturability and reliability while reducing turnaround times.
Board Edge Clearance for PCB Components and Connectors
It is easiest to manufacture rectangular boards, scoring and separating them with a ‘pizza wheel’ cutter. All components and connectors need to be at least 0.04” away from the board edge, with resistors and capacitors even further from the edge. Because so many designs are irregularly shaped and dense, an alternative process is to route the board. The rules for board edge clearance depend on the board detachment process. These rules can get complex, but a mixture of scoring and routing can optimize the design.
The scoring process causes board flex, which leads to cracked components. Capacitors are especially sensitive, as they are brittle and lack toughness. Damage due to cracked capacitors is not always obvious. The circuit appears to work, but over time the cracked capacitor absorbs moisture and shorts out.
- Resistors: The recommended minimum edge clearance is 0.05”.
- Capacitors: The nearest point of the capacitor to the board edge should be no closer than 0.119”.
- Edge Connectors: If possible, use alternatives, such as connectors placed away from the board edge. If edge connectors are required, a good practice is tab routing or a combination of scored and tab routing.
Positioning Drill Holes and Traces
It is also important to keep drill holes away from the board edge, as drills too close to the board edge can cause cracks or breaks in the PCB. To prevent this, keep drilled holes at least 0.010” away from the board edge for both plated and unplated holes.
In addition to minimum edge clearance, take into account other PCB drill rules. Also, keep copper traces and planes at least 0.010” from the PCB edge.
PCB Panelization Guidelines
PCBs are manufactured products that are typically made in panels to expedite the process and minimize scrap, thereby reducing costs. This is true even for prototype runs. Either scoring or routing is used to facilitate board separation post-assembly. During scoring, a V-shaped groove is created between adjacent boards. The boards are then separated along the groove.
Scoring can only be done in straight lines while routing offers more flexibility in defining the board edge for separation. The downside is that instead of a straight edge, breakaways shaped like “mouse bites” are created where the boards separate.
DFM for HDI Printed Circuit Boards
The rules for board edge clearance depend on the panelization option used to separate the boards.
1. Keep components and traces at least the recommended copper keepout distance from center of scoring groove.
2. Adjust copper keepout according to board thickness.
Determining and implementing PCB board edge clearance requirements should be an essential part of your design process. These rules extend to every aspect of PCB manufacture, including components, connectors, drill holes and traces. Remember that panelization also requires board edge clearance adherence. Incorporating these DFM measures early in the design process will not only reduce unnecessary loss of material and development time, but can also improve your PCB manufacturing turnaround time.
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