Panelisation and Tab Routing for Board Assembly
A panel exists so that the assembly line can handle many boards at once, and every decision about its size and its breakaway method has a cost somewhere. A panel that is too small wastes machine capacity, while a panel that is too large warps, sags or will not fit into a feeder.
The routing between the boards is where most of the practical difficulty lives. Tabs have to be strong enough to hold the boards through printing, placement and reflow, and weak enough to break cleanly afterwards without damaging the edges.
Panel Size and Machine Limits
The panel has to fit the smallest machine in the line as well as the largest, and the conveyor width, the stencil frame and the reflow oven opening all impose separate limits. A panel that fits the printer but not the oven is a common and expensive oversight.
Machine capacity per panel is the other half of the calculation. A large panel with many boards reduces the number of machine cycles per board, but it also means that one placement error affects more boards at once, so the yield consequences should be included in the choice.
Rail Width and Edge Clearance
The conveyor rails grip the panel along its edges, and no component may sit inside the strip that the rails occupy. This clearance is specified per machine and is usually between three and five millimetres along the two long edges.
Where a board is small enough that the edge clearance consumes a large fraction of the outline, the panel is built with a breakaway rail that is removed after assembly. The rail is a process feature rather than part of the product, and it should be identified as such on the fabrication drawing.
Tab Routing and Tab Count
Tabs are the bridges of material left between the board outline and the panel frame, and their width and number determine how much the board moves during assembly. A board held by two narrow tabs on one side will sag in the oven, and the sag is enough to shift a fine pitch placement.
The rule of thumb is that the tabs should prevent motion in all three axes, which usually means at least one tab per side on a rectangular board. Suspending a board from a single edge is only acceptable where the assembly is small and light and the process has been verified with that arrangement.
Mouse Bites and Perforated Tabs
A mouse bite is a row of small drilled holes across a tab, leaving a series of narrow necks that break when the board is separated. The pattern gives a clean break with little deformation, and the remaining bumps are small enough not to interfere with the edge.
The hole size, the web between them and the number of rows all matter. A web that is too wide requires force to break and damages the laminate, while a web that is too narrow fails in the machine before the assembly is complete. Both ends of that range are worth testing on the actual stack up.
V-Cut Scoring
A V-cut is a groove machined along the edge of the board on both sides, leaving a thin web that snaps when the board is bent. It gives a clean, straight edge and needs no tab space, which makes it efficient for products with straight sides and no edge components.
The limitation is geometry. A V-cut cannot follow a curved outline, it needs a straight run, and it leaves a bevel on the edge that some enclosures will not accept. The web thickness also has to be controlled, because a web that is too thick will tear the laminate instead of snapping.
Component Clearance at the Break Line
No component should sit so close to a break line that the separating force or the residue reaches it. A chip capacitor placed within a millimetre of a V-cut is at risk during depanelling, and a tall part beside a tab can be struck by the tooling.
Where the layout is tight, the clearance is added on the panel rather than the board, so the finished outline keeps its intended dimension while the process keeps its space. This is one of the details that should be reviewed alongside the other rules in design guidelines.
Depanelling Methods
Manual snapping with a fixture is the simplest method and the one that depends most on the operator. The same panel broken by two people can produce different edge quality, so the fixture and the technique should be documented where the edge is visible in the product.
Routing machines and laser depanelling give a repeatable result and are preferred where the edge is critical or where the board carries brittle parts. The trade is cycle time and capital, and the choice should follow the value of the assembly rather than the volume alone.
Fiducials and Panel Markings
Fiducials belong on the panel as well as on the board, because the placement machine locates the panel first and the board second. A panel without global fiducials forces the machine to use board level marks that may be missing on a small board.
Panel markings should also identify the product, the panel number and the direction, so that a partly assembled panel can be identified after a line stop. These markings are added to the fabrication data and are removed with the frame at depanelling.
Handling and Warpage
A panel with asymmetric copper distribution will bow during reflow, and a bowed panel can jam a conveyor or misplace components. Balancing the copper across the panel, adding thieving where necessary and choosing the tab positions to oppose the movement all reduce the risk.
Where the product is thin and the panel is large, the panel itself can be the source of a placement defect that the machine cannot compensate for. Panel design is therefore part of process capability rather than a fabrication detail, and it deserves the same review as the stencil or the profile.
Additional Considerations for This Build
Practical attention to panelisation pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating panelisation explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.
Process Control and Verification
On a design of this kind, panelisation is the item that decides how the rest of the board is arranged. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.
Related reading: our fabrication notes, board quality and design release notes cover the same ground.
A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.
Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.
A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.
Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.
FAQ
Should every board be panelised? Almost always, because handling individual boards through a printer and a placer is slower and less accurate, but a very large board may be processed as a single unit.
Are mouse bites better than V-cuts? They suit different shapes. Mouse bites follow any outline, while V-cuts are limited to straight edges but leave a cleaner line.
How wide should a tab be? Wide enough to restrain the board in the oven, which on a typical small board means three to five millimetres across the full thickness.
Who decides the panel layout? The assembler and the fabricator together, because machine limits and fabrication limits both apply and neither is visible from the other side.
