Panel Size Selection for PCB Fabrication

The panel is the manufacturing unit. It is larger than the finished board and it carries the borders, the tooling features and the array of boards that will be separated at the end. Panel size selection is a balance between the amount of material that is used, the capability of the machines that will process the panel and the yield that the process can hold across the whole area.

The Panel and the Board

A board design is placed into an array, and several arrays are placed on a panel. The panel dimensions are chosen from the equipment rather than from the board, because every machine in the line has a maximum and often a minimum size that it can handle.

The material utilisation is the ratio of the total board area to the panel area. A high utilisation lowers the material cost per board, and it is the first number to look at when the array is planned.

The utilisation interacts with the yield. A panel that is packed to its maximum leaves less room for the process variation at the edges, and the loss at the edges can outweigh the material saved in the middle.

Machine Limits

Each process has a working window. The drilling machine has a table size and an accuracy envelope, the etcher has a conveyor width, the lamination press has a platen size and the surface finish line has a rack size.

The panel must fit all of them, and the narrowest machine sets the limit. A panel that fits the press but not the etcher will have to be processed on a different line, which may change the cost more than the material saving is worth.

A very small panel is also a problem. It is hard to handle, it wastes machine capacity in loading and unloading and it can be lost in a chemical bath. The minimum size matters as much as the maximum.

Borders and Tooling

The border is the area outside the arrays, and it carries the tooling holes, the fiducials, the test coupons and the identification. The width should be chosen from the tooling rather than from a habit.

Tooling holes are used to locate the panel in every machine and in the assembly line. Their position is a datum for the whole process, so the border must be wide enough to place them at the specified distance from the edge.

Coupons for impedance, for the material and for the plating are placed in the border or in a dedicated strip. The space for them should be planned with the array rather than added afterwards, because a coupon that has to be squeezed in changes the utilisation the array was designed around.

The Array and the Breakaway

The array carries several boards with a gap between them, and the gap is the material that will be removed during depaneling. The gap width is set by the router bit or the scoring wheel and by the tolerance of the separation.

The number of boards in an array is a compromise. A large array reduces the handling per board but increases the risk that a defect in one board takes the whole array with it after assembly.

The breakaway tab dimensions and the position of the tabs should be chosen so that the separation does not stress a component or a solder joint. A tab beside a fine pitch device can transmit a bend that cracks a joint.

Material Utilisation

Utilisation is calculated from the board area and the panel area, and it is usually improved by rotating the array, by using a mixed array and by choosing a panel size that divides evenly into the material sheet.

A mixed array combines boards of different sizes on one panel. It improves the utilisation but it complicates the traceability and the depaneling, and it is used where the products have similar volumes.

The material sheet size should also be considered. A panel that divides evenly into the standard sheet leaves no offcut, while a panel that is slightly too large produces an offcut that is too small to use.

Yield Across the Panel

The process is not uniform across a panel. The plating current density varies, the etchant concentration changes along the conveyor and the lamination pressure is different at the centre and the edge.

A panel that is designed at the maximum size will include areas where those variations are largest, and the boards in those areas will have a lower yield. The position of the boards on the panel should therefore be considered when a marginal design is being placed.

The board position should also be tracked. A defect that always appears in the same position points at the equipment rather than at the design, and the traceability of the panel position is what makes that visible.

Panel Size and Assembly

The assembly line has its own limits. The conveyor width, the placement area and the reflow oven all set a maximum panel size, and a panel that is too large for the assembler will have to be separated before assembly, which removes the benefit of processing several boards at once.

A panel that is too small for the assembly line can be handled in a carrier. The carrier adds cost and handling, and it should be considered in the comparison.

The panel orientation and the fiducial placement should suit both the fabrication and the assembly, because the two use the same datum and a change between them introduces an offset.

Cost and the Decision

The cost model should include the material utilisation, the process yield, the handling cost and the tooling cost. Material is often the largest single item, but a small improvement in utilisation that reduces the yield can raise the total.

The decision should be made with the fabricator rather than in isolation. The fabricator knows which panel sizes suit the equipment, where the coupons will go and what the typical yield is for a panel of that size.

A change of panel size after the tooling has been ordered is expensive. The stencil, the assembly fixtures and the test fixtures are all designed around the array, so the panel should be settled before any of them is released.

Practical Rules

Choose the panel for the equipment rather than for the board, plan the border and the coupons with the array, and check the utilisation and the yield together. Confirm the panel against the assembly line as well as the fabrication line.

Record the array and the panel data with the build records and the tooling cost structure, and review the breakaway tab design and the depaneling methods when the array is defined.

FAQ

What limits the panel size? The narrowest machine in the line. The press, the etcher, the drilling table and the assembly conveyor each have their own maximum, and the smallest of them decides.

How is material utilisation improved? By rotating the array, by mixing boards of different sizes and by choosing a panel that divides evenly into the material sheet without leaving an unusable offcut.

Why does a large array carry a risk? A defect in one board can affect the whole array after assembly, and the boards at the edge of a large panel sit where the process variation is greatest.