PCB Layout Rules: Edge Clearance, Keep-Outs and Fiducial Marks

Component placement is the first physical decision made on a PCB, and it determines almost everything that follows: whether the board can pass through the assembly line, whether connectors survive repeated mating cycles, whether tall parts block the airflow that cools the enclosure, and whether the pick-and-place machine can find the board accurately enough to hit a 0.4 mm pitch pad. The layout rules in this article cover the mechanical and process-driven side of placement rather than the electrical side: the clearance between components and the board edge, the keep-out ring around pluggable connectors, the placement of heat-generating tall parts, the auxiliary rails and stamp holes used to carry a board through production equipment, and the design of the fiducial marks that optical placement systems use as their coordinate reference.

Highlights:

Keep components at least 5 mm from the board edge, keep SMD parts out of the 3 mm zone around pluggable connectors and edge connectors, and keep crystals and clock drivers at least 10 mm away from the edge.
When the 5 mm conveyor-edge keep-out cannot be satisfied, add an auxiliary rail: 5 mm wide for a standalone board and a minimum of 8 mm for a panelized board, separated with stamp holes.
Fiducials are 1 mm solid copper circles with a solder-mask opening twice the diameter and a circular or octagonal copper ring for protection, placed in an L pattern, at least 6 mm from the board edge, three or more per side.

Basic Placement Rules and Edge Clearance

Every component on the board should sit at some distance from the board outline. Unless a part has a mechanical requirement that dictates otherwise, maintain a clearance of at least 5 mm (197 mil) between the component and the board edge. This clearance protects the part from the routing and profiling operation, keeps the assembly fixture from touching it, and leaves room for the conveyor rails that grip the board.

For placement orientation, 0° and 90° are the recommended directions. Standardizing on two orthogonal orientations simplifies the placement program and reduces nozzle changes during PCB assembly. Equally important is leaving enough working space between components for whatever operation the part requires. An angled memory socket, for example, needs free space in front of its slot so that the module can be inserted and removed without colliding with neighbouring parts.

Figure 11 | Schematic diagram of oblique-insertion component layout

Tall and Short Components: Thermal Placement

Component height is a thermal variable, not just a mechanical one. Place tall components behind short ones relative to the airflow, and orient them along the direction of least wind resistance. A tall part placed upstream of a low part creates a wake that starves the low part of cooling air; a tall part placed downstream of a low part behaves as a downstream obstruction that the flow has already had a chance to work around. Following this order prevents the air duct from being blocked and keeps the thermal solution functioning as designed.

Figure 12 | Thermal design considerations during layout

Keep-Out Zone Around Pluggable Components and Edge Connectors

Connectors that are mated and unmated in the field transfer mechanical stress into the board. To keep that stress away from solder joints and brittle chip components, avoid placing SMD components within a 3 mm (120 mil) band around pluggable components and edge connectors. This band is a keep-out zone: the movement of the mating force and the flexing of the board around the connector can crack a nearby capacitor or lift a small resistor even if the connector itself survives. Treating the 3 mm band as reserved space is simpler than trying to reinforce every part that happens to fall inside it.

Figure 13 | Keep-out area for layout of pluggable components or board-edge connectors

Boards with Exposed Windows or Cutouts

Some boards require an opening in the solder mask or a cutout that leaves a region of the board exposed. On such boards, keep components at least 2 mm away from the edge of the window. The window region is subject to different surface treatment, additional handling, and often a mechanical interface above the board, so the 2 mm offset protects both the part and the process that forms the opening.

Placement of Critical Signal Components

Crystals, clock drivers, and other components that generate or distribute critical timing signals should be placed away from interface circuitry and away from the board edge. Keep them at least 10 mm from the edge. The board edge is where cables, connectors, and external noise sources couple into the design, and it is also the region most exposed to mechanical stress and handling. Moving the timing devices inboard isolates them from both the electrical aggressors and the mechanical hazards.

Component Bodies Must Stay Inside the Board Outline

Except for parts with a mechanical requirement to overhang the outline, no component body may extend beyond the PCB edge. The pad edge (or the component body itself) must be at least 5 mm from the conveyor edge. If a part overhangs the outline, it will foul the conveyor rail or the placement nozzle, and its pads will be exposed to the profiling cutter. The 5 mm dimension is measured from the same conveyor edge used for the general edge-clearance rule, so the two requirements reinforce each other rather than duplicating work.

Figure 14 | Component body must maintain a certain distance from the PCB edge

Auxiliary Edges and Stamp Holes

The 5 mm keep-out along the board edge is also the width the conveyor needs to grip the board. When a design cannot satisfy that requirement because components must sit close to the outline, the standard solution is to add an auxiliary edge: a strip of bare board attached to the functional board that the conveyor grips instead. The auxiliary edge is sacrificial material, broken off after assembly.

The width of the added strip depends on whether the board is panelized. For a PCB that is not part of a panel, the auxiliary edge should be 5 mm wide. For a panelized PCB, the auxiliary edge must be at least 8 mm wide. The larger figure accounts for the panel frame, the separation features, and the additional handling the panel sees before it is depanelized.

Stamp holes are the perforated line that separates the auxiliary edge from the functional board. They produce a clean break with minimal burr and avoid the need for a routed slot. If components have to be placed on the auxiliary edge itself or along a stamp-hole edge, and those parts would interfere with production line equipment, the auxiliary edge must be enlarged to provide the necessary clearance.

Figure 15 | Schematic diagram of adding castellated holes (stamp holes)

Fiducial Mark Placement

A fiducial mark is the optical reference point that the pick-and-place machine's vision system uses to establish the board's position and rotation before it places parts. Without fiducials, the machine relies on the board outline and the tooling holes, which are far less precise. Two classes of fiducial are used: board-level (global) fiducials that align the whole panel or board, and IC-level (local) fiducials that align a single fine-pitch package.

Board-Level Fiducials

Shape and size: a solid circle with a diameter of 1 mm.

Solder mask opening: a concentric circle with a diameter twice that of the fiducial itself. At the 2 mm diameter edge, a circular or octagonal copper ring is required as protection for the mark.

Figure 16 | Board fiducial mark

The general principles for board-level fiducials are:

Any board that passes through SMT equipment must have fiducials; a board that skips the SMT process does not need them.
Use at least three fiducials per side.
For single-sided assembly, fiducials are required only on the component side.
For double-sided assembly, place fiducials on both sides and keep the front and back positions essentially consistent.
Arrange the fiducials in an L shape along the board edge, with the marks spaced as far apart as possible. The centre of each fiducial must be more than 6 mm from the board edge. If that cannot be met, at least ensure the distance from the conveyor edge satisfies the requirement.
Do not place fiducials in areas where every layer of the PCB has been cleared out. The fiducial needs a solid plane layer underneath it as a reference.

IC-Level Fiducials

Local fiducials are required for QFP packages with a pin pitch of 0.4 mm or less, and for BGA, CSP and similar packages with a pin pitch of 0.8 mm or less. At these pitches, the alignment accuracy obtainable from board-level fiducials alone is not sufficient, so each fine-pitch device gets its own pair of reference marks nearby.

The IC fiducial uses the same 1 mm solid circle. Its solder mask opening is sized as for an ordinary pad, and no outer copper ring is needed.

Figure 17 | IC fiducial mark

Overall Layout Quality

Beyond the individual clearance rules, the finished layout should be both visually ordered and practically usable. A placement that satisfies every dimensional constraint but leaves no room for rework, test probing, or thermal relief is still a poor layout. Treating the rules above as the minimum, and reviewing the board as a whole for accessibility and appearance, produces a design that is as easy to build and service as it is to manufacture.

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