Array Spacing and Breakaway Tab Design in PCB Panels

Array spacing and breakaway tabs look like details on a fabrication drawing, but they decide how the panel behaves on the assembly line and how clean the board edge is when the array is separated. Space that is too small leaves no room for the nozzle, the support pin or the router bit, while tabs that are too few let the board move during handling.

Both values are chosen once, usually at the panel design stage, and then inherited by every process downstream. Getting them right costs nothing, while correcting them later means a new stencil, a new assembly program and a new panel drawing.

Why Array Spacing Matters

Spacing between individual boards is set by the widest tool that has to pass between them, not by the board outline. A placement nozzle, a dispense needle, a coating spray and a router bit all need clearance, and the largest of them sets the minimum.

The space also carries the tooling features the line needs: fiducials, tooling holes and test access. Where spacing is tight, those features are pushed onto the rail, which then has to be wide enough to carry them without losing the stiffness the rail was added for.

Rail Width and Handling Edge

The rail is the edge of the panel that the conveyor grips, and its width is set by the conveyor specification rather than by the product. Widths between 3 and 5 mm are common, with a wider rail where the panel is thin or where the rail must also carry fiducials.

Rail stiffness matters as much as width. A rail that flexes will change the reference for the optical system and the placement machine, so a thin panel benefits from a wider rail even when the conveyor would accept a narrower one, as discussed in the panel array layout guidance.

Breakaway Tab Count and Position

Tabs hold the boards in the array during assembly and are broken or routed away at the end. The number and position of tabs decide how much the board can move during placement and how much stress the separation step applies to the edge.

Fewer tabs reduce the routing work but allow more movement, while more tabs stiffen the array and increase the stress when the board is separated. Tabs are normally placed so that they are symmetric, and they should avoid areas where components or tracks run close to the edge.

Spacing for Nozzles and Tooling

The nozzle that places a component needs to reach the outermost pad without the machine body touching the array beside it. That reach is a machine specification, and the panel design should be checked against it before the layout is released.

Support pins on the underside need clearance too. A pin that cannot be placed because of the array geometry leaves the board unsupported at exactly the area where placement force is applied, which is one of the reasons support pin placement should be part of the datum and fiducial review rather than a machine set-up decision.

Separation Methods and Edge Quality

Boards leave the array by one of three routes: hand snapping along a scored line, a punch or die, or a router that cuts the tab away. The method decides the edge the customer receives, and it should be chosen with the finished outline tolerance in mind rather than with the assembly line alone. A method that leaves burrs on the outline will show up later as a fit problem in the enclosure.

Routing leaves a machined edge with a radius set by the bit diameter, while snapping leaves a rougher edge with micro-cracks that extend a short distance into the laminate. Where the edge carries a connector or sits against a metal chassis, the smoother routed edge is usually worth the extra cost.

Router Versus V-Groove Separation

A router follows the outline at a programmed path and removes the material completely, which suits arrays with complex outlines, internal cut-outs and mixed panel sizes. A V-groove scores both faces and leaves a controlled web that breaks along the score, which is faster and cheaper but limited to straight lines that run the full length of the panel.

The web left by a V-groove is the parameter that decides how the break behaves. A web between 0.25 mm and 0.35 mm on a 1.6 mm board breaks cleanly with a small burr, while a thicker web tears the laminate and a thinner one risks separating in transit or on the conveyor. The web depth is set by the blade, so it has to be checked as a first article dimension.

Stress and Board Edge Damage

Breaking a tab applies a bending moment to the edge of the board, and the strain that reaches the laminate depends on how close the nearest track or plated hole sits to the break line. As a working rule, copper should stay at least 0.5 mm from the break line and plated holes further away still, with the distance measured from the finished outline rather than from the tab.

Where the design cannot respect that distance, the tab should be routed rather than snapped, because routing removes material progressively while snapping applies the whole load in a single event. Cracks that start at an edge rarely stop there, and they usually appear later as a lifted pad, a cracked via barrel or a failed thermal cycle test.

Panel Size and Handling Limits

Panel size is constrained from two directions: the machine envelope sets the maximum, and handling sets the practical maximum. A panel that is too large to be lifted without flexing will be flexed, and the flexure damages the boards at the corners of the array first.

A practical range for a conveyorised line is a panel between 100 mm and 460 mm along the transport direction, with a mass one operator can move with two hands at waist height. Panels outside that range are usually split into two arrays rather than forced through the line, and the split is recorded on the panel drawing.

Acceptance Criteria and Documentation

The array drawing should state the rail width with its tolerance, the board-to-board gap, the tab count per board, the tab dimensions, the separation method and the maximum burr allowed at the broken edge. Each of those is a value the shop can measure, so each can be accepted or rejected without an argument about intent.

The panel record should carry the array revision, the router program revision and the separation method used for the lot, so that a complaint about an edge can be traced to the process that produced it. Where the same array is built for several customers, the drawing set should identify which variant was shipped.

FAQ

How much rail width does a panel need? Between 3 and 5 mm suits most conveyors, with the wider figure used for thin panels or where the rail also carries fiducials and tooling holes.

Should breakaway tabs be routed or snapped? Routed tabs give a cleaner edge and less stress, while snapping is acceptable where the break line stays at least 0.5 mm clear of copper and plated holes.

What gap should be left between boards in an array? The gap is set by the widest tool that has to pass between the boards, typically 2 mm or more so that a router bit or a nozzle can reach the outline without touching the neighbouring board.