Many hardware and layout engineers eventually encounter a design-for-manufacturability (DFM) feedback that says: "Adjust component placement. No components are allowed within 5 mm of the board edge on the first side, and within 4 mm on the second side." It can be frustrating when that keep-out erodes precious routing and placement area. However, this rule exists for concrete, production-critical reasons. Understanding the mechanism behind it helps avoid costly issues during assembly.
This article explains the typical 5 mm/4 mm edge keep-out requirement tied to SMT conveyor rails, why the values differ between the two sides, what happens when the rule is ignored, and how to design proactively to satisfy DFM without compromising functionality.
DFM Requirement: Edge Keep-Out Zones
The typical rule enforced on many SMT lines is as follows:
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First side (Solder Side, bottom side): The area within 5 mm of the conveyor rail edge (including any break-away rails) is a restricted zone.
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Second side (Component Side, top side): The area within 4 mm of the conveyor rail edge (including any break-away rails) is a restricted zone.
Prohibited within the restricted zone:
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Any SMT components and their pads (lands).
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Note: Through-hole (PTH) holes require case-by-case evaluation based on the specific product structure and assembly process.
Why 5 mm and 4 mm? The Role of SMT Conveyor Rails
The core reason for these keep-out zones is the way SMT production lines handle PCBs. Automated equipment transports boards using edge-conveyor rails that clamp and support the panel throughout printing, placement, inspection, and reflow. The rails, clamping mechanisms, and support tooling impose practical limits on how close components can safely sit to the board edge.
1. Why 5 mm on the First Side (Solder Side)?
On the first pass, the board enters the line with the solder side down. Edge rails grip and convey the panel through the printer, pick-and-place, and reflow oven.
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Physical interference risk: The conveyor rails or rail clamps close on the PCB edges to hold the panel. If tall components (for example, electrolytic capacitors, connectors) are placed too close to the edge on the solder side, the rails can collide with them during clamping or while entering equipment. This leads to cracked or displaced components, bent leads, broken solder joints, or even scrapped boards.
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Thermal deformation: During reflow, the PCB undergoes thermal expansion and slight warpage. A 5 mm buffer provides margin for edge deformation. Without it, edge pads and adjacent components may be stressed by rail contact, causing solder cracks, lifted parts, or intermittent connections as the board warps and relaxes through the thermal cycle.
In short, the first pass travels face-down. Clearance on both edges prevents the rail and clamp hardware from contacting or stressing components placed near the edge.
2. Why 4 mm on the Second Side (Component Side)?
On the second pass, many designs place components on the top side while the bottom side (already assembled) faces down. Although the rails primarily grip the opposite edge, edge clearance is still required on the top side for mechanical and process stability.
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Support and stability: The board must remain stable as it moves through the line. If components are placed too close to the top-side edge, the center of mass and support points can interact unfavorably. During high-speed transport, indexing, or slight changes in direction, the board can wobble or flex, increasing the chance of mechanical contact with the rail, support fingers, or guide walls. This degrades placement accuracy and can damage edge components.
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Sensor interaction: Many automated lines use edge-detection sensors to locate and index boards. A 4 mm clearance improves reliable sensor detection and reduces the risk of misreads that can jam the line or misalign the board relative to stencils and pick-and-place coordinates.
In short, top-side components placed too close to the edge increase the risk of instability, interference, and alignment errors during second-pass processing.
Consequences of Ignoring Edge Keep-Out Rules
Violating these constraints during design can lead to the following issues in manufacturing:
| Issue Type | Observed Effect |
|---|---|
| Soldering defects | Edge components experience uneven heating, resulting in tombstoning, weak joints, or solder bead spatter. |
| Clamping/transport defects | Components are deformed by rail pressure or collide with mechanical guides, causing shorts or opens. |
| Component damage | High-value devices such as BGAs or connectors impacted at the rail can be damaged beyond rework, scrapping the entire assembly. |
| Reduced throughput | Frequent board jams and stoppages require line intervention and retuning, significantly lowering productivity. |
Special Cases and Exceptions
There are situations where edge-keep-out rules must be interpreted with respect to the process and panel design. These should be resolved through a formal DFM review:
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Mouse-bites and break-away rails:
When a PCB includes a break-away rail (also called a process rail), the "board edge" for assembly purposes is the physical outer edge including the process rails. Keep-out distances are measured from the rail edge, not the final product edge after depanelization.
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PTH components:
The rule references case-by-case evaluation for through-hole features. PTH components are often wave-soldered or hand-soldered, and their interaction with the conveyor system differs from SMT. If leads are short, do not protrude beyond the edge, and the assembly method does not conflict with rail handling, some placements may be acceptable. Always align on this early with the factory to avoid surprises.
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Panelization:
For V-score (V-cut) or tab-router panels with bridges, the geometry of rails, webbing, and depanelization features complicates edge clearances. It is often prudent to enlarge the restricted zone to 8–10 mm in panel designs to protect both assembly and depanel quality, particularly around V-cuts and tab locations.
Design Recommendations for Engineers
To avoid late-stage rework and streamline both layout and manufacturing, incorporate the following practices early in the design:
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Define keep-out boundaries: In your PCB CAD tool (for example, Altium Designer or Allegro), define keep-out regions inside the board outline at 5 mm on the first side and 4 mm on the second side. Use the dedicated Keep-Out Layer or component placement constraints to prevent accidental violations.
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Place tall and large components centrally: Locate high-profile and bulky components toward the board's interior. Reserve the periphery for smaller passives, routing, and copper pours as needed. This minimizes the risk of rail interference and improves thermal uniformity at the edges.
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Add process rails: For boards without edge connectors, consider adding process rails of approximately 5 mm to support transport. Process rails can host fiducials, tooling holes, and test features without consuming the functional board area. They also improve handling and reduce the risk to near-edge components.
Conclusion
PCB design is not only about circuit connectivity—it must align with manufacturing realities. The 5 mm and 4 mm edge keep-out zones are not arbitrary; they are practical safeguards embedded in SMT conveyor and clamping mechanics. Adhering to these clearances protects yield, minimizes rework, and sustains line throughput. By incorporating keep-outs, planning component placement thoughtfully, and coordinating panelization and PTH details with the factory, engineers can meet performance goals without compromising manufacturability.
