Very often, small PCB modules such as Wi-Fi modules that solder onto a larger printed board much the same way as ICs do, have plated half-holes or castellated holes along the edges. Although it looks like a PTH cut through, PCB manufacturers use a high quality bespoke process to plate the castellation holes, and do not cut out the holes. This makes the holes clean, smooth and with stronger edges that have no sharp burrs or deformation.

Castellation features are useful for mounting a PCB on to another and soldering, or for inserting into specially designed edge connectors. Several features contribute to the complexity of a castellated hole. For instance, the critical design attributes are the hole size, the number of holes per board, single or multiple hole design, surface finish, and overlay pad design.

Accordingly, manufacturers support four types of castellated holes: 

Type I:Large drilled plated holes cut in half 

Type II:Smaller lead in hole cut in half, placed on the periphery of the larger drilled plated hole

Type III:Primary large drilled plated hole with a smaller feature hole, with a post processing tool cutting into the smaller hole. This feature accommodates a special connector.

Type IV: Primary drilled plated hole, tangent to the board profile, with a post processing tool cutting into the hole.

The size of the holes and their numbers per individual PCB allows using the plate break process at the final fabrication operation. This post plate processing allows efficient removal of burring at the hole profile interface. Type II holes may need an additional burr removal process depending on the dimensions at the interface of the two holes. Although castellation allows use of any surface finish, a decrease in hole size with an HASL finish is more difficult to process and affects the quality of the castellation.

General rules for designers to follow when putting in castellation holes are:

• Use the largest hole size possible.
• Use overlay pads on both sides, top and bottom, and make them as large as possible.
• If possible, place inner-layer pads for anchoring the barrel of the hole. This also helps reducing burrs.
• When not using the castellation for a connector device, allow a larger tolerance for the dimensions of the opening.

The plating process requires the plated edge to be on an outer edge of the panel. This is important during panelizing the design. For instance, a board with castellation holes on both the left and right edges can be panelized only on the top and bottom edges. This also means one cannot panelize a rectangular board with castellation holes on all its four sides.

Conclusion

It is necessary for designers to consult their fabricators to ensure they are capable of producing castellated boards. Some fabricators have a limitation on the type and size of holes they allow for castellation. For instance, some limit the minimum diameter of plated half-holes to 0.5 mm. Designers need to add a plated hole or via on the border of the board where they require the half hole. Half the via should be on the board and the rest outside the outline. For more information regarding if the application of castellated holes is suitable for your PCB design and function, please don’t hesitate to contact our team at sales@pcbglobal.com