Conveyor Edge Clearance and Board Support on SMT Lines

Every board that travels down an SMT line is held by its edges. The conveyor rails, the chain, the tooling pins and the support blocks all touch the panel, and the space left for them has to be designed into the board before the panel is released. Getting that space wrong produces sag, component damage and placement errors that look like machine faults. This guide explains how gopcb specifies conveyor edge clearance and board support on its lines.

What the Conveyor Touches

The conveyor contacts three things: the two long edges that ride on the rails, the underside where support tooling presses up, and sometimes the top surface where a hold down device clamps. Each contact consumes space that the circuit cannot use. Ignoring that space leads to boards that cannot be transported, or that are transported with damage.

Contact pressure matters as well as area. A chain conveyor carries the panel on a narrow line of contact, which concentrates stress at the edge. Heavy panels and thin laminates are the most vulnerable, and the damage they sustain during transport often appears as a cracked edge or a lifted pad rather than as a clean break.

Edge Clearance Requirements for SMT Lines

Conveyor edge clearance is the strip along each long edge that must stay free of components, vias and exposed copper. A common requirement is three to five millimetres per side for components, measured from the panel edge inward, with additional space for fiducials and tooling holes. The exact figure depends on the machine and should be confirmed with the line before design release.

The clearance must also account for the rails themselves, which sit inside the board outline on some machines and outside on others. Where a design is tight, the fabricator can add a process edge that is removed after assembly. The rules around that approach are covered in our notes on board outline tolerance.

Board Support and Sag Control

Long boards sag under their own weight, and thin boards sag under the pressure of the squeegee or the placement head. Sag changes the board height at the moment of printing or placement, which shifts deposit volume and placement depth. Good board support holds the panel flat within a few hundredths of a millimetre across the working area.

Support blocks, magnetic pins and vacuum tables are the usual tools. Pins must be placed so they contact areas without components on the underside, and they must be positioned below the board centre where deflection is greatest. A support plan drawn from the panel layout, rather than improvised at the machine, keeps support repeatable between shifts.

Panel Design for Conveyor Handling

Panel design decides how well all of this works. Rails should run along the long dimension where possible, because a long panel on short rails is more likely to twist. Fiducials need clear space and consistent contrast, and tooling holes need to be positioned so that the panel locates the same way at every machine.

Breakaway tabs must not sit where the conveyor contacts the panel, and they must not fall inside the support pin areas. Designing tabs, rails and tooling holes at the same time avoids the situation where a correct panel cannot be transported because a tab has been placed under a chain. Our breakaway tab design notes cover the mechanical side of that trade-off.

Tooling Pins, Pallets and Vacuum Support

Tooling pins locate the panel precisely and take the wear that would otherwise reach the board. Pins should be matched to the tooling hole diameter with a small clearance, and they should be replaced when they show flats or burrs. A worn pin allows the panel to shift, which shows up as a placement offset that changes from panel to panel.

Pallets carry flexible or unusually shaped assemblies that cannot ride the rails directly. They add mass, so the conveyor must support the extra load, and they consume vertical space in the reflow oven. Vacuum support is the alternative for thin panels, pulling the board flat against a platen and holding it there through printing.

Reflow Conveyor and Thermal Effects

In the reflow oven the conveyor carries the panel through a temperature gradient, and both the panel and the conveyor expand. Mesh conveyors tolerate that expansion better than chain rails, but they support the board less evenly. The combination of heat and limited support is why warp measured after reflow often differs from warp measured before it.

Edge clearance interacts with thermal mass as well. A wide metal rail touching the board edge conducts heat away from the components nearest the edge, which can leave those joints cooler than the rest of the panel. Designs that place large parts at the extreme edge of a panel should be reviewed for this effect.

Handling at Printer, Placement and AOI

Each machine in the line imposes its own requirement. The printer needs a flat, well supported area for the squeegee stroke, the placement machine needs clearance for the head and nozzle, and the optical inspection system needs a stable, repeatable position so that its images align with the program.

Inspection systems are the most sensitive to positioning, because a shift of a fraction of a millimetre changes the image the software compares. Using the same tooling holes and the same rail position at every machine keeps the panel in the same frame of reference, which simplifies programming and reduces false calls.

Conveyor Maintenance and Inspection

Conveyor hardware wears. Chain links stretch, rails collect flux and dust, and width adjustment mechanisms drift out of parallel. A conveyor that is out of parallel transports panels at a slight angle, and the resulting skew appears as a placement error even though the machine itself is accurate.

Maintenance should include checking rail parallelism, cleaning the chain, verifying the width setting against the panel specification, and inspecting support pins for damage. The tooling holes that locate the panel deserve the same attention, as described in our tooling hole registration guide.

Problems That Show Up as Process Defects

Conveyor and support faults rarely present themselves as transport faults. They appear as insufficient paste on one side of the board, as placement offset that varies with panel position, or as solder defects concentrated along an edge. Because the symptoms look like process problems, the conveyor is often the last thing inspected.

When a defect pattern repeats along an edge or correlates with panel position, the handling system should be checked before the process is adjusted. Reviewing the defect map against the line layout, in the same spirit as the checks described in our production process flow notes, usually identifies the cause quickly.

FAQ

How much edge clearance does an SMT line need? Most lines ask for three to five millimetres of clear space along each long edge for components, plus additional area for fiducials and tooling holes. The exact figure depends on the conveyor and the support system, so it should be confirmed with the assembly house before the panel is designed.

Can a process edge be removed after assembly? Yes, and it is common on dense boards. The process edge is added to the panel, used for transport and support, then removed by routing or by breaking along a controlled line. The removal point must not cut into the finished outline.

Why does placement offset change from panel to panel? Often because the panel is not located the same way each time. Worn tooling pins, a conveyor that is out of parallel and inconsistent support pin placement all allow the board to shift. Checking the handling system is faster than recalibrating the placement machine.