Tooling Holes: Tooling Hole Placement For Panel Fixturing

Tooling holes are the mechanical reference that ties a bare board or a panel to every machine that touches it. They locate the panel in the drill, the plater, the router, the stencil printer and the placement machine. If they are misplaced or undersized, every downstream process inherits the error.

Their design is simple but unforgiving. Size, position, tolerance and the relationship to the circuit pattern all have to be defined together, and the features have to survive the whole process, including the chemicals and the heat.

What Tooling Holes Do

A tooling hole is a precision hole, usually 3.0 mm or 4.0 mm in diameter, that accepts a locating pin on a machine or a fixture. The pin and hole pair defines the position of the panel in two axes, and often a second hole, slotted or round, defines the rotation. Without that reference, each machine would have to find the pattern optically.

The advantage is repeatability. A machine that locates on pins sees the same origin for every panel in a lot, so its internal offsets stay valid. Optical location is slower and depends on the contrast of the targets, which changes with finish colour and with the presence of solder mask.

Size And Placement Tolerance

The hole diameter is chosen to match the standard pin sizes on the production equipment so that no adapter is needed. Three millimetre and four millimetre holes cover most lines, while some placement machines use 2 mm pins for small panels. The tolerance on the diameter is usually plus or minus 0.05 mm, tight enough to limit play but loose enough to drill reliably.

Placement tolerance for the hole centre is typically plus or minus 0.05 mm relative to the panel datum, and it must be measured on the finished board after plating and after routing. Hole position is affected by material movement during lamination, so the placement tolerance is normally looser than the drill tolerance quoted by the fabricator.

Placement Relative To The Circuit

Tooling holes are placed in the panel border, outside the array of circuits, where they cannot interfere with routing or with component clearance. A clearance of at least 3 mm between the hole edge and the nearest copper or routed edge keeps the pin away from the circuit and stops burrs from reaching it.

Where a hole must sit inside a board outline, for example on a large backplane, it is treated as a mechanical feature with its own keepout. The keepout should include the hole, the plating and a margin for the washer or standoff that will eventually sit on it, and it must be shown on the fabrication drawing.

Panel Fixturing Through The Process

Panel fixturing follows the panel from inner layer imaging through lamination, drilling, plating, solder mask and finally assembly. Some processes locate on pins, some on a conveyor edge and some on optical targets, so the panel needs features that satisfy all of them. A consistent datum scheme keeps the accumulated error small.

The most common scheme uses three holes: two round holes on one edge to fix position and rotation, and a third, often slotted, on the opposite edge to prevent over constraint. A slotted third hole absorbs the small dimensional changes that occur when the panel is heated in a reflow oven.

Relationship To Breakaway Tabs

Breakaway tabs connect the circuits to the panel border, and they carry the mechanical load until the panel is depanelised. Tooling holes are placed so that the tab material between the hole and the circuit is thick enough to survive handling. A hole placed too close to a tab weakens the tab and can let the circuit break free early.

The tab layout also determines the order in which the panel can be depanelised. V-score, routed tabs and perforated tabs behave differently, and the sequence should be documented so that the panel does not lose its reference before the last operation that needs it. The related outline rules are described under board outline and mounting design.

Verification And Common Errors

Verification of tooling hole position is done with the same coordinate measuring approach used for other mechanical features. The measurement is taken from the primary datum on a finished panel, and the result is recorded with the lot. A simple pin gauge check confirms the diameter but not the position.

The common errors are a hole that is oversized and lets the panel shift, a hole that is placed inside the circuit area, and a hole that is not carried through to the final panel because it was added only to the inner layer artwork. Each of these faults shows up as a registration problem at assembly rather than at fabrication, which makes them expensive. The way these mechanical features are documented is described under design guidelines for manufacturability.

Process Control and Verification

On a design of this kind, breakaway tabs is the item that decides how the rest of the board is arranged. 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. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. 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.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

Process Control and Verification

On a design of this kind, breakaway tabs is the item that decides how the rest of the board is arranged. 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. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

Process Control and Verification

On a design of this kind, breakaway tabs is the item that decides how the rest of the board is arranged. 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. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

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

How many tooling holes does a panel need? Three is the practical minimum: two to fix position and rotation, plus one that is slotted or oversized to avoid over constraint. More holes are acceptable but add cost and consume border area.

Should tooling holes be plated? Usually not. Plating adds thickness variation that changes the fit on a locating pin. Where a hole is also used as a mounting hole in the finished product it is plated to the customer drawing and given its own tolerance.

What causes a tooling hole to lose position after lamination? The panel shrinks and expands with the resin system during lamination, and the movement is not perfectly uniform. Measuring the hole on the finished panel and specifying a loose enough placement tolerance absorbs the effect.