Panel Rails, Tooling Holes And Breakaway Tabs
A panel exists because most assembly equipment cannot handle a small, irregular board. The individual circuits are laid out on a frame that carries them through the printer, the placement machine, the reflow oven, and the test fixtures, and the frame is made of rails, tooling holes, and the tabs that join the boards to it. Every one of those features is a design decision, and each of them constrains where components and traces may be placed.
This article explains what the panel frame has to do, how wide the rails and the tabs should be, what the tooling holes are for, and which rules keep the frame compatible with the machines that will handle it.
What The Panel Has To Do
The frame has to hold the individual boards rigidly enough that a stencil can sit flat on them and a placement machine can align to them, and it has to survive every thermal excursion of the process without losing that rigidity. It also has to be compatible with the equipment: the rail fits the conveyor, the clamps of the printer and the placement machine grip the rail rather than the board, and the tooling holes register on the pins of the fixtures used for wave soldering, selective soldering, test, and conformal coating.
A panel that is too small is handled poorly by the conveyor, and one that is too large sags in the middle of the oven. The practical range is set by the smallest and largest panel the equipment accepts, and by the weight that the conveyor can carry. Where a board is small, several of them are combined on one panel, and the panel is then designed around the machine rather than around the product.
<img src="https://www.gopcba.com/wp-content/uploads/2024/09/2.jpg" alt="Panel rail with tooling holes beside the board array” />
Rail Width And Edge Clearance
The conveyor rail is the part of the panel that the equipment actually touches, so it has to be wide enough to be gripped without being crushed and wide enough to carry a tooling hole. A width of five millimetres is the common minimum and eight to ten millimetres is a comfortable working figure, with the extra width used for the tooling holes, the barcode, and the fiducials. The rail also has to be free of components over the full length of the clamp zone, which typically extends several millimetres in from the edge on both sides.
Board edge clearance follows from the same constraint. A component placed within a millimetre of the board edge will be close to the rail after depaneling, and it will be gripped, shadowed, or damaged by the fixture at some later step. The usual rule is to keep components between half a millimetre and a millimetre from a routed edge, and further from an edge that is scored, because the scoring wheel leaves a distorted band that can pass under a solder joint. Tall parts near the edge also obstruct the rail of the reflow oven and the fingers of the wave solder pallet.
Tooling Holes And Registration
A tooling hole is a plain hole, usually between two and four millimetres in diameter, drilled in the rail and used as a mechanical reference. It is normally non plated, with a nominal clearance around the fixture pin, and there are usually at least two of them placed diagonally so that the panel can only sit in one orientation. Their position is defined relative to the board pattern rather than to the panel edge, and the tolerance on that relationship is what determines how well the fixture and the artwork agree.
Tooling holes are used by more processes than most designers expect. The selective soldering fixture, the in circuit test fixture, the depanel router, and the conformal coating mask all register on them, and each fixture adds its own clearance requirement. Where a hole must serve several fixtures, it is worth making it slightly larger and locating it from the same datum as the fiducials, so that the mechanical and the optical references describe the same panel.
<img src="https://www.gopcba.com/wp-content/uploads/2026/09/52.webp" alt="Breakaway tab connecting a board to the panel frame” />
Breakaway Tabs And Routing
The tabs are the bridges that hold each board to the frame, and their width and number are set by the weight of the board and by the stress that the panel will see. A tab between three and five millimetres wide is typical, and a large or heavy board needs more of them so that it does not flex during handling. The tab can be routed with a slot on both sides, or it can be left thinner than the board by a scoring wheel so that it snaps, and the two methods leave different edge qualities.
Where a tab is perforated with a line of small holes, the remaining material between the holes sets how much force is needed to break it and how much burr is left behind. The holes are usually placed a little inside the board outline so that the break leaves a small tab remnant rather than a notch in the board, and the pitch is chosen so that the break is clean. Components and traces have to be kept clear of the tab area on both sides, and the router path has to stay outside the keepout zone, because a burr or a lifted pad at the tab is a common source of a field failure.
Consequences For The Assembly Process
Breaking a panel creates debris. Routing produces glass fibre dust that settles on the boards, scoring produces particles along the break line, and both are a contamination risk if the panel is broken after soldering. Breaking a panel also flexes the boards around the tab, and a flexed board can crack a ceramic capacitor or an area array joint, so the process usually specifies where the panel is supported during separation and in which direction the break is made.
The order of operations matters as well. A panel that is broken before reflow is easier to inspect but harder to handle, while a panel that is broken after reflow has to survive the stress without damaging joints. The decision is usually made with the assembly house, and it is documented on the panel drawing together with the tab locations and the separation method. The fabrication side of the same drawing is described under PCB design and fabrication, and the layout consequences under layout decisions that affect production.
Documentation And Verification
The panel drawing carries the rail width, the tab positions and widths, the tooling hole diameters and coordinates, the fiducials, and the panel identification. The identification matters for traceability, because the panel is the unit that moves through the line, and a date code or a serial number on the rail is easier to record than one on a small board. Where the board itself carries a marking, it should be placed so that it is still readable after the panel is separated.
Verification is a first article check. The panel is measured against the drawing, the tooling holes are test fitted to the fixture pins, the tabs are broken on a sample to see whether the break is clean, and the fiducials are confirmed to be readable with the marks on the frame rather than on the board. The rules for the board outline itself are described under board outline and mounting design.
FAQ
How wide should a panel rail be? Five millimetres is the practical minimum and eight to ten millimetres gives room for tooling holes, fiducials, and a barcode. The rail must also be free of components across the clamp zone of every machine that will hold it.
Are tooling holes plated? They are normally non plated, because the hole is a mechanical reference and plating adds thickness variation and a reflective ring that a fixture pin does not need.
Can the panel be broken before reflow? It can, and the choice is a trade between handling convenience and the risk of moving or damaging joints. Breaking before soldering avoids stressing joints, while breaking afterwards keeps the boards on a rigid frame through the process.
