Edge Clearance: Preparation, Placement and Process Control

Every surface mount line has a set of dimensional limits that are not written on the board drawing but are enforced every time the panel enters a machine. Understanding those limits explains why a design that is electrically perfect can still be difficult to build.

The Conveyor and Its Edges

A placement machine and a reflow oven hold the panel by its two long edges, and the width of the rail is fixed by the machine. That means the panel must be a rectangle within the machine’s width range, and the two edges that the rail grips must be clear of components.

The clearance is usually specified as a few millimetres on each side, measured from the panel edge to the nearest component body or solder joint. A component placed inside that band will be crushed or dislodged as the panel travels, and the damage may not be visible on the board itself. Our carrier notes describe how a panel that cannot meet the requirement is supported instead.

Rail Width and Panel Width

Machines have a minimum and a maximum rail width, and a panel outside that range cannot be processed at all. The practical consequence is that the panelisation must produce a width within the machine’s range, which sometimes forces a change in the arrangement of the boards on the panel.

The height of the assembly also matters, since a tall component on the underside will foul the rail. Where the design has a tall part on the bottom side, the panelisation must place the boards so that the part does not pass over the rail, or a carrier must be used.

Fiducials and Their Placement

A machine needs at least two fiducials to establish position and rotation, and three or four to correct for local distortion. They are placed on the panel border, and on the board itself where the board will also be assembled separately after depanelisation.

The fiducial is a copper dot with a clear area around it, and the contrast between the dot and its surroundings is what the camera detects. The clear area must be free of traces, mask features and silkscreen, and its size is proportional to the dot. Our design release checklist lists the dimensions.

Board Edge Clearance for Routing and Scoring

The separation method imposes its own clearance. A routed gap needs the router to pass without touching the component, and a score line requires a straight edge with clear space on both sides so that the breaking tool can support the material.

Components placed close to the break line are at risk from the bending that precedes the break, and the risk rises with the size of the component and with the stiffness of the joint. A keep-out band along the break line is the standard answer. Our breakaway tab notes describe the alternatives.

Tooling Holes and Their Function

Tooling holes serve the fabrication equipment rather than the assembly equipment, but their placement is constrained by both. They locate the panel during imaging, drilling and profiling, and they must not be obstructed by components or by the panel border features.

Where the assembly line also uses tooling holes, the diameter and position have to suit both sets of equipment. Confirming that early avoids a panel that has to be redesigned for the assembly line after it has been proven on the fabrication line.

Panel Warpage and Machine Handling

A warped panel will not sit flat on the conveyor, and a placement machine that assumes a flat surface will place components at the wrong height. The consequence is a poor paste transfer and, in the worst case, a nozzle that contacts the board.

Warpage limits are therefore tighter for assembly than for delivery alone, and the limit should be quoted in the assembly terms rather than inherited from the fabrication specification.

Marking Readability

A data matrix code printed on the mask has to be read by a camera at some point in the line, and the reading depends on the code’s contrast, size and quiet zone. A code that is legible to a human eye may still be unreadable to the machine if the cell size is too small for the camera’s field of view.

The reader’s specification gives the minimum cell size for a given distance, and that number should drive the code’s dimensions rather than the other way around. Our traceability marking notes describe the encoding options.

Getting the Limits Agreed Early

The most efficient approach is to obtain the assembly house’s capability list before the panelisation is fixed. It contains the rail width range, the maximum panel size, the minimum edge clearance, the fiducial specification and the maximum component height on each side.

With that list, the panel can be designed once. Without it, the panel is designed against assumptions that are discovered to be wrong at the first production run, and the correction is a new set of fabrication data and a new set of tooling. Our cost reduction notes describe where that cost lands.

Process Control and Verification

Reviewing the design before the data is released is far cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

FAQ

How much edge clearance does a panel need? Enough for the rail to grip without touching a component, which is usually a few millimetres on each of the two long edges. The exact figure comes from the machine, not from a general rule, and it should be confirmed with the assembly house before the panel is designed.

Can a non-rectangular panel be assembled? It can, but it needs a carrier that presents a rectangular outline to the machine. The carrier also provides the support that an irregular panel usually lacks, so the two requirements are often satisfied by the same tooling.

What does gopcb confirm about panel handling? We confirm the panel dimensions against the assembly equipment, the edge clearance, the position and size of the fiducials and tooling holes, the maximum component height on each side and the warpage limit. Where a panel cannot meet the requirements we propose an alternative arrangement before the data is released.