Panelization: Preparation, Placement and Process Control

A panel is the format in which boards are fabricated and assembled. The panel carries several boards, a border that the machine can grip, tooling holes that locate it, fiducials that the placement machine uses to find it, and the tabs or score lines that will later release each board.

This article covers what the panel has to provide, how its size and features are chosen, and how the requirements are communicated.

What A Panel Has To Provide

The panel has to be rigid enough to be handled and transported, flat enough for the stencil to seal against it, and sized so that the assembly equipment can grip and index it. It also has to hold each board in the exact position the placement programme expects, which is the function of the tooling holes and the fiducials. Finally, it has to allow the boards to be released without damaging them.

Those requirements pull in different directions. A large panel gives a better machine utilisation and fewer handling steps; a small panel is flatter and easier to hold. A thick border is robust but wastes material; a thin one is fragile and can deflect under the conveyor. Panelization is therefore a design compromise, and one that is best made with the assembly house rather than in isolation, since the limits come from its equipment. The outline of each board inside the panel is defined under board outline and mounting design.

Panel Size And Machine Limits

The maximum panel size comes from the smallest machine in the line, which is often the stencil printer or the reflow oven rather than the placement machine. The minimum size comes from the conveyor rails, since a panel narrower than the rail spacing cannot be transported at all, and from the width of the border needed to hold it. Typical limits are expressed as a rectangle, and a panel that is inside the limit in one dimension and outside it in the other is still unusable.

Panel thickness matters as well, and it is set by the board itself plus any addition from the border. A very thin panel flexes between the rails and can be lifted by the stencil as it separates, which causes smearing. A very thick panel may exceed the machine’s clearance. The thickness and the material of the border are normally the same as the board, so the panel behaves uniformly through the oven. The way the panel moves through the assembly sequence is part of the PCBA development process.

Tooling Holes And Fiducials

Tooling holes locate the panel mechanically, both in the machine that grips it and in any fixture used later. They are normally placed in the border, outside the board outlines, and their position is held to a tighter tolerance than any other panel dimension, because everything else is referenced to them. The hole size is matched to the tooling pin, and the number and position of the holes are set by the equipment.

Fiducials are the optical marks the placement machine uses to find the panel and to correct for its position and rotation. They are placed in the border at diagonal positions, so that any error in the panel’s placement can be resolved into an offset and a rotation, and they are also placed on each board where the board pitch is large or where the machine supports local alignment. A fiducial is a small pad of copper with a defined mask clearance and a defined contrast against the surface, and it is useless if it is placed next to a feature that confuses the vision system. The way placement uses them is described under placement order and pad positioning.

Border, Spacing And Breakaway

The border is the strip that surrounds the boards. It carries the tooling holes and the fiducials, it provides the surface the conveyor grips, and it is discarded after assembly. Its width is set by the tooling and by the rails, and it has to be wide enough to hold everything without crowding. Where the panel is built from a number of boards with no room for a full border, a shared border between two boards can serve both.

Board to board spacing is the gap between adjacent boards, and it is set by the separation method rather than by the assembly process. A routed gap is wide enough for the router bit plus its clearance; a score line needs no gap at all but requires that the boards share an edge. The breakaway features that connect the boards to the panel and to each other are the tabs and the score lines, and their dimensions determine whether the board is held firmly during assembly and released cleanly afterwards.

Copper And Material Considerations

The copper on the border matters more than it appears. The plating current reaches the boards through the border, so the border must carry enough copper to plate every hole evenly, which means a plating bus that connects the board surfaces. The bus is normally a strip of copper on the border that is connected to the circuits, and it is cut when the boards are separated.

The border also affects the balance of the panel. A large border with little copper on it is a region of low copper density, and the resulting imbalance can cause the panel to warp during lamination and reflow. Adding thieving to the border, or arranging the boards so that the border copper is distributed evenly, reduces that effect. The same balance considerations apply at board level, and they are the reason that copper distribution is checked over the whole panel rather than per board.

Panel Drawing And Release

The panel drawing gives the panel outline, the position and orientation of each board, the tooling hole positions and sizes, the fiducial positions and sizes, the border width, the board to board spacing, and the separation method with its tab or score line dimensions. It also states which side the components are on, since that decides where the panel can be supported and where the cuts can be made.

The drawing is normally produced with the assistance of the assembly house, because the limits belong to its equipment. Where the same board is assembled at two different sites, the panel may have to be produced in two variants, and the tooling holes and fiducials are the features that differ between them. Keeping the panel drawing under revision control, with the assembly house’s approval recorded, prevents the situation in which a panel is quoted for one line and run on another.

FAQ

Who decides the panel size? The assembly house, within limits set by its equipment, and the decision is normally a joint one. The designer states the board size and the quantity, and the supplier proposes the arrangement.

How many fiducials are needed? At least three, or two placed diagonally, so that the vision system can resolve both offset and rotation. More are added where the panel is large or where the board pitch requires local alignment.

Can the border be used for anything else? It can carry test coupons for plating thickness and impedance, and it is the usual place for them. It cannot carry anything that will be needed after separation, because it is discarded.