Process Edge Design for PCB Panels: Width, V-Cut, and Stamp Holes

Panels do not go through the assembly line by themselves. They are carried, clamped, and registered by the equipment, and the strip of board reserved for that purpose is the process edge. Its width, its integrity, and what is allowed to sit on it determine whether the panel can be handled at all, and whether the components and test points near the edge survive the SMT assembly line. Getting the process edge wrong does not produce an electrical fault; it produces a production stoppage or a mechanical defect that is difficult to trace.

The process edge is part of the panel array design rather than the circuit design, which is why it is often overlooked until the panel is being tooled. The rules are simple but they are mandatory, and they should be applied when the board outline is first drawn.

What the Process Edge Has to Do

Three functions have to be satisfied simultaneously. The edge carries the panel through the conveyor, so it must present a continuous, smooth surface to the transport rollers or rails. It provides a region for the tooling that clamps and registers the panel, so it must be mechanically strong and free of features that could interfere with a clamp. And it keeps the handling equipment away from the functional area of the board, so that neither the conveyor nor the clamping mechanism touches a component or a plated feature.

Because these functions are mechanical, the edge is normally solid copper-free laminate, or copper covered by solder mask with no exposed pads. The width is chosen to suit the widest transport requirement in the process, which in practice means choosing the width required by the assembly house rather than the minimum that appears in a standard.

Minimum Width and Conveyor Contact

A typical requirement is a process edge of at least 5 mm, and this should be treated as a floor rather than a target. Where the panel is heavy, where the conveyor uses rails that contact the edge closely, or where the assembly house specifies a wider requirement, the edge is widened accordingly.

The edge must also be uniform. A notch, a break, or a change in thickness along the conveyor contact area will cause the panel to jam, and the mechanical stress of a jam can damage the panel, the machine, or both. Where a board outline includes a cutout that reaches the panel edge, the panel array has to be arranged so that an intact strip remains along the conveyor path.

The edge material matters less than its integrity. Bare laminate, copper clad laminate, and mask-covered copper all work, provided the surface is flat and continuous, and provided no opening in the mask exposes a conductor that a clamp could short.

Separation: V-Cut or Stamp Hole

Individual boards are separated from the process edge either by a V-cut or by a row of perforations known as a stamp hole pattern. The V-cut is a scored groove along the separation line, and it is the more common choice because it is quick to produce and produces a clean edge on a rectangular board.

The V-cut has requirements of its own. The scoring wheel removes material and creates a stress concentration, so copper must be kept away from the score line by a distance that depends on board thickness, ranging from about half a millimetre on thin boards to 0.8 mm or more on a 1.6 mm board. The remaining material at the score is a fraction of the board thickness, so the panel separates predictably but should not be flexed before separation.

Stamp holes are used where the separation line is not straight, where the board has an irregular outline, or where the material cannot be scored. The perforations are small non-plated holes placed close together, and the board is broken along the row after assembly. The pattern leaves a slightly rough edge, and the holes consume a small amount of board area, which must be accounted for when the functional outline is defined.

Keep-Outs on the Process Edge

Two keep-outs are absolute. Pads and plated through holes must not be placed on the process edge, because the clamping and transport equipment can damage them or short them together, and solder can bridge across an exposed feature during wave soldering. Test points must also be kept off the edge, since the same equipment can damage a probe pad and a damaged pad invalidates the test coverage it was meant to provide.

Copper conductors may cross the edge in some processes, but only if they are covered by solder mask. The reason is protection rather than electrical performance: an exposed trace on the edge will be scratched by the conveyor, and a scratched trace that later corrodes is a reliability failure that will not appear until the product is in service. Where the process requires a bare copper edge for grounding or shielding, that requirement should be confirmed with the assembly house rather than assumed.

A related constraint is component clearance. Components close to the separation line can be damaged when the panel is depaneled, and their solder joints can crack from the bending force. The distance from the break line to the nearest component has to be defined as part of the panel design, not left to the placement.

Large Boards and Parallel Edges

Boards above roughly 80 square millimetres should have a pair of parallel process edges, one on each side in the direction of travel, with clear space above and below them so that no component body enters the tooling region. Boards that are long in one dimension usually need support on both long edges so that the panel does not sag during reflow, when the laminate softens.

Where the board is small, several copies are assembled into a single panel so that every board in the array has the required edges. This is the normal solution, and it also improves the utilization of the panel. The array layout, the spacing between boards, and the position of the tooling and fiducials are all part of the same drawing.

Panel Design Checklist

Before a panel is released, five items should be confirmed. The process edge width meets the assembly house requirement and is uniform along the conveyor path. No pads, plated holes, or test points fall on the edge. Copper on the edge, where present, is covered by mask. The separation method and the clearance from the break line to the nearest component are defined. And the panel includes tooling features and fiducials in positions the placement machine can use.

The panel drawing should also state the board thickness, the number of up-cuts, and the separation method, because these determine how the panel is produced and how it is depaneled. A panel drawing that leaves any of these undefined will be tooled to the fabricator’s assumption. Related outline constraints are described in board outline and mounting design and in PCB slot and edge routing rules.

FAQ

Can the process edge be narrower than 5 mm? Sometimes, if the assembly house confirms that its transport and clamping equipment can handle it. It should never be reduced by assumption, because the consequence is a jam or a handling defect rather than a warning message. The width is a process requirement, not a design preference.

Which separation method should be preferred? V-cut for straight edges on rectangular boards, because it is fast and produces a clean edge. Stamp holes are used where the outline is irregular or where scoring is not possible, accepting a slightly rougher edge and the area consumed by the perforations.

Why must copper on the process edge be covered with solder mask? Because the conveyor contacts the edge mechanically. An exposed conductor will be scratched, and a scratched conductor can corrode or break in service. The mask provides mechanical protection, and it also prevents solder from bridging across features during wave soldering. Placement and clearance rules that support panel design are covered in placement order and pad positioning.