All About Castellated Mounting Holes
At Rush PCB Inc., we make all types of PCBs, including those with castellated holes. In this article, we will overview castellated holes, their history, applications, benefits, design attributes, challenges, and why they are required in modern electronic design.
Complex multi-layered PCBs use plated vias to allow an electrical connection of traces across layers. Non-SMD boards use PTH or plate through holes to mount leaded components. Some boards that must extend connections to external devices or boards use castellated holes. However, there is a difference between regular vias and PTH, as they facilitate easy and reliable connections between two PCBs.
Typically, castellated holes allow users to make reliable electrical connections between two boards through their edges. These are semicircular holes plated through on the edge of a PCB. Apart from providing electrical connections between boards, castellated holes also simplify the process of fitting small modules to fit on larger boards. Usually, the fabricating process of castellated holes consists of two stages: one, with the operator cutting the holes in half, and the other, with plating the cut holes with a conducting material like copper. Castellated holes in PCBs offer durability, robust connectivity, and an enhanced assembly process for complex PCB components. Typically, breakout boards, RF modules, and IoT boards make maximum use of castellated holes.
What are Castellated Holes
The name castellated is because the holes resemble turrets or castellations along the edge of a castle wall. Half of each plated hole lies on the edge of the PCB, while electrically, they individually connect to an internal layer. This way, the plated half-holes electrically connect the external region of the PCB to its internal layers.
The half-cylindrical pads are useful for direct vertical solder interconnection between boards stacked on top of each other. Castellated holes typically perform a function similar to that provided by edge connectors, but they do not take up so much space and allow much higher connection density. Castellated holes provide reliable connections suitable for modular designs and SMT components.
History of Castellated Holes
In the early days of PCB design, engineers created castellated holes when experimenting with various methods of interconnecting circuit boards. These days, designers can make castellated holes with greater precision and better reliability while making it easier to work with them. The process has also evolved drastically with advancements in PCB fabrication technology. Castellated holes offer a reliable interconnection technology to a range of applications involving IoT devices, RF modules, and breakout boards.
Benefits of Castellated Holes
Using castellated holes offers several advantages for PCB designers and users. The benefits include enhanced durability and improved connectivity. They simplify the assembly process by providing easy soldering capabilities between boards. Compared to other external and board-to-board connection methods, the advantages are:
Higher Density — Using castellated holes, designers can add more IO connections along the board edge than is possible with connectors. Staggering the holes can add to the density even further.
Space Saving — Using castellated holes rather than connectors saves a lot of space on each interconnected board. Vertical height is lower for interconnected stacked PCBs.
Lower Cost — Eliminating cables and or mating connectors between PCBs lowers the assembly cost.
Higher Reliability — Unlike connectors, castellated holes have no moving parts or wear-out mechanisms, increasing their reliability.
Higher Durability — Castellated holes can better withstand the stresses of repeated connection and disconnection.
Better Performance — Castellated holes perform better at high frequencies as compared to connectors or cables.
Simple Assembly — It is possible to stack PCBs and interconnect them by soldering without using additional components.
Higher Serviceability — It is possible to unsold individual PCBs from the stack for repair rather than replacing the entire assembly.
Better Design Flexibility — Designers can place castellated holes anywhere along the PCB edge according to convenience.
Easy Customization — The designer can size, and place castellated holes according to specific interconnection requirements.
Providing excellent interconnection between PCBs, castellated holes ensure there is minimum signal loss and optimum electrical performance. Not only does this robust interconnection significantly improve the overall reliability and performance of electronic devices, but it also makes them an effective option for designers wanting to maximize the functionality and efficiency of their products.
Further to improving the connectivity, the design of castellated holes allows them to withstand the stress of repeated connection and disconnection, making them more durable as compared to PTH or vias. IoT devices and modular electronic systems that require frequent updating and hardware replacement benefit from this increased durability.
However, simplification of the assembly process for PCBs is the most significant advantage of using castellated holes. They allow for easy alignment and interconnection between two PCBs, thereby reducing assembly complexity and improving product reliability. As castellated holes are half-circular, they eliminate the risk of damage or misalignment during PCB assembly, allowing easy alterations to the pin layout.
Castellated holes make it possible to solder one board to another, allowing smaller PCB modules to function as sub-circuits for larger PCBs. As the interconnection between the boards is tight, it improves the solder joint’s quality, reducing the chances of accumulation of dirt and dust. The above features make castellated holes a good choice for various applications.
Design Attributes for Castellated Holes
While designing castellated holes for PCBs, designers must consider the following:
Layer Connection — Typically, castellated holes connect the outermost layer to one of the inner layers. However, designers can also use them to interconnect any two adjacent layers.
Hole Size — The most common diameters designers use for castellated holes range from 0.3 mm to 0.75 mm, matching adjacent hole/via sizes.
Hole Spacing — Typical pitch for castellated holes is between 0.8 mm and 1 mm.
Pad Size — The diameter of the annular ring around the castellated hole is about 0.25 mm to 0.5 mm larger than the hole diameter.
Edge Margin — Typically, designers prefer to leave a 1 mm copper-free margin from the board edge.
Number of Rows — Depending on the application, designers may use two parallel rows of staggered castellated holes.
Corner Shaping — Designers must avoid extending castellated holes into board corners, as this can cause cracking.
Rigidity — It is preferable to keep board material of more than 0.2 mm between adjacent castellated holes.
Plating Thickness — We recommend thicker plating of 20-25 microns in castellated holes, as this provides a better connection.
Castellated Holes in ECAD Software
Although your CAD software may not have a standard list of design objects for castellated holes, placing an array of castellated holes on a module is fairly simple. Designers must approach this keeping the fabrication process in view, which may offer some difficulties in assembly. Typically, the basic objects available in ECAD software tools are enough to place castellated holes in a PCB.
Designers use castellated holes in small modules as an alternative to pin-header mounting. This allows mating small modules to the main PCB with SMD soldering. Rather than a pin header, the castellated holes on the module will provide an adequately strong soldered connection between the module and the main PCB. To make this work the right way, designers must design two arrays:
An array of castellated holes along the edge of the small module
An array of pads on the surface of the main board for holding the smaller module
Castellated Hole Array on Small Module
There are two ways to place castellated holes. In the first method, designers can place them as half-holes along the edges along with an attached pad. In the second method, they can use plated through holes just within the half-cut hole along the board edge. In both cases, the designer can use regular pads in the PCB layout tools to define the array of castellated holes. They can line up the pads of the castellated holes with through holes along the edge of the board, subsequently placing another pad right up to the edge of the board. This pad will provide the additional copper necessary.
Of course, the designer is free to not use the second array of holes, as these are optional. However, adding the second array has the advantage of accepting a pin header, if necessary, such as in a prototype.
Pad Array on the Main Board
The designer must follow the standard land pattern design guidelines for SMD components when placing the pad array on the main board. While soldering, there is no difference between castellated holes and other through-hole features. However, it is possible to solder castellated holes to a pad, just as is done for a through-hole component. Typically, after soldering, some leftover solder fillets will be visible along the wall of the hole.
IPC Class 3 Compliance Considerations
For applications requiring a higher reliability PCB, such as for IPC Class 3 compliance, designers need to consider a few important points when creating castellated holes in a PCB. The design of the pads on the main board must be large enough to accept the entire solder fillet after soldering. We recommend expanding the solder mask a little around the pads to prevent bridging by the molten solder.
Additionally, if the designer uses the second row of holes in the array, they must place a solder mask dam between the holes. They must do this on the pad for the smaller board, as this is where the operator will apply the solder paste. This dam will prevent solder from wicking into the second hole.
Fabricating PCBs with Castellated Holes
To ensure high quality and yield, the designer must discuss with the fabricator to allow them to modify the Gerber. They will indicate the drill hits and the required information for plating along the board edge. Using castellated holes may need consideration of the following limitations:
Special drilling/routing fabrication equipment may be required.
Accommodating half-holes may require a larger minimum board thickness (above 1 mm).
Maintaining adequate edge webbing strength may require limiting the number of castellated holes possible.
Maintaining mechanical robustness may require edge reinforcements, chamfers, or thickened regions where castellated holes will appear.
Inspection and repair of soldered castellated holes can be challenging.
Temperature cycling may stress connections due to thermal expansion mismatch.
Castellated Hole Manufacturing Challenges
Several manufacturing issues may impact the overall quality and reliability of castellated holes. Some of them are:
Position and Alignment
For limiting the risk of damage during assembly and for proper electrical connection, the castellated holes must be accurately aligned and positioned for both the smaller board and the main board. Designers must check the alignment and positioning of castellated holes to verify they meet the tolerance levels specified.
Subjecting the PCB to high temperatures during the soldering process can cause changes in the PCB geometry, typically known as board warping. This may occur due to the presence of castellated holes. To minimize board warping, designers must use board materials with low CTE or coefficient of thermal expansion and implement appropriate thermal management strategies.
Extra solder creating an unwanted electrical connection between adjacent holes is a frequent issue in the assembly of boards with castellated holes. Designers can prevent this by properly spacing the holes and employing suitable solder mask designs. Moreover, the assembly process must include appropriate soldering equipment and techniques, along with suitable quality control measures.
According to Rush PCB Inc., using castellated holes results in innovative, effective, and reliable electronic products. The benefits of castellated holes in the form of indentations and plated half-holes along the edges of a PCB make them an essential tool for designers. However, castellated holes need careful attention to design considerations and the associated fabrication challenges.