Many advanced system designs have criteria that require mounting an existing module onto a new PCBA. The best approach is to use a solder interface called castellations or a castellated edge. Semiplated copper indentations along the side of a PCBA provide both electrical and structural mounting support for a board-to-board interface. There are several applications for this type of interface. We will review what castellations are, why they get used, and several design considerations for castellated edges.
Connectors on rigid PCBAs do not use pins or solder balls. Instead, castellations bring the circuit nodes to the outside PCBA edges without any extra hardware. Copper traces intended for connection to the outside follow the board’s edge. A hole gets drilled through the PCB within the copper trace. Its other names include annular rings or plated holes.
When the board gets separated from the array, the PCBA will get cut along its edge through these openings, forming a partial copper cavity–a castellation–which is electrically connected within the module circuit. Using these copper castellations as solder connections, the module can attach to a larger system board.
Castellations provide electrical connections from one PCBA to another. Smaller boards often serve as modules that provide a specific function to the system. For instances such as FCC-certified RF radio electromagnetic frequencies that get tested to specific standards, this can be especially useful. In this way, each module can get tested for known operation before the final system assembly. Castellations also facilitate debugging during prototyping by bringing all module nodes into an accessible area. This facilitates an easy SMT process for mounting modules.
PCBA design for castellations for SMT modules and system boards requires some minor planning. With some foresight, most CAD design layout packages can accommodate castellated edge designs.
A normal via process applies for developing the castellated holes on the PCBA edge. Holes get drilled through copper plating traces that extend beyond where the board edge will be eventually cut. Once the edge is cut to separate the PCBA from the rest of the array, these vias will resemble semicircle copper cylinders that span the thickness of the PCBA. It is possible to create castellated holes using small cutouts and lead-in holes. However, the most common method is to cut a via within a copper trace in half.
Design attributes for castellated holes include:
PCBAs on which modules will be mounted must have a footprint that is large enough to accommodate soldering castellated edges. The pads should be the largest dimensions allowed by the layout tool since their purpose is not only for electrical connection to the system but also for the structural mounting of the module. The area beneath the module must be kept free of components and other through-hole mountings since the module must sit flush with the PCB. By using solder paste on the pads and solder masking where appropriate, the proper solder volume can be applied without the risk of cold or weak joints. Work with your CM partner during your project development to understand any other criteria needed for castellated mounting within your design for manufacturing (DFM) plan.
Integrated functional modules can be produced on a single assembled PCB using castellations that can be stacked on a larger board for a second assembly. The castellations provide both electrical and structural connections for the module as an SMT device. The SMT assembly process for a castellated module is like that of other surface mount devices. Visual and electrical final test inspection can be accomplished on all castellated nodes after mounting, unlike other types of packaging with hidden pins.
During the design process, ensure that large annular rings are used both on the surface as well as the inner layers to strengthen the structure of the interface. Work with your CM partner to establish the use of the ENIG surface finish for the best results. Finally, ensure that there is proper spacing for the break tabs that separate the PCBA from the array. This interface is critical to the success of the castellations.