Castellated PCB and Holes: Design, Manufacture, and Solder Applications

What are Castellated PCBs and Castellated Holes

Typically, PCBs (Printed Circuit Boards) are standalone circuits printed on a single PCB. However, in industrial settings, bigger PCBs are built from modular sub circuits and are assembled into the larger PCB in phases. These modular sub-circuits can be built utilizing Castellated PCB and are made using castellated holes. Castellated holes provide a way to solder multiple PCB modules together for electrical connection easily and seamlessly. 

The castellated holes for castellated PCBs are made at the corners of the PCB where the semi-plated holes are carved at the edge of the PCB board as shown in below:

PCB Edges and Castellated Holes
PCB Edges and Castellated Holes

 

Castellated holes are a handy solution when it comes to fitting small modules onto a large board. They bring the added benefit of allowing component pin layout reorganization, which helps minimize the chances of damage during installation. In this article, we will delve into a comprehensive overview of castellated PCBSs, castellated holes, and the design and manufacturing process of castellated PCBs, exploring their advantages, and applications in detail.

The Function of Castellated Holes

Castellated holes, also known as castellation’s, are unique holes that are created on the edges of printed circuit boards (PCBs). These holes resemble small indents or semi-plated cutouts along the board’s sides. These castellated holes can take on various forms such as half circles, fragmented portions of a circle, or other shapes, depending on how they are used in a particular application (as shown in the figure below). Castellated holes are specifically designed to make it easier to mount and assemble printed circuit boards (PCBs). These holes serve the purpose of allowing one PCB to be conveniently placed on top of another during the assembly process. By incorporating these holes along the edges of the board, they can be utilized to replicate specific sections of a PCB circuit. This replication feature enables the efficient production, testing, and soldering of smaller sub-circuits onto the main PCB, which already contains other parts of the circuit, as and when required (as shown in the figure below).

How Castellated Holes Are Made?

The castellation’s (castellated holes in PCB) are created by using a sharp router bit to cut through regular plated holes, leaving half of the hole intact. These castellation’s are commonly used for connecting one board to another, especially in signal circuits. They can be directly soldered to the edge of the corresponding holes on the main board, which helps save space and eliminates the need for any additional connectors. During manufacturing and assembly, castellation’s are designed with break tabs that connect the PCB to a larger panel. The number and position of these tabs depend on the size and shape of the board. Typically, castellation’s are used in rectangular boards where two out of the four edges have these special holes.

These castellated holes can also be used to replicate specific sections of a PCB circuit, making the assembly process of complex components with high pin density much simpler. To ensure reliable manufacturing and assembly, it is important to have castellated holes with good mechanical strength that prevent issues like delamination. Overall, castellated holes offer an efficient solution for designers aiming to maximize the efficiency and reliability of electronic devices.

The castellated PCBs are typically used for surface mount components and can be found in a wide array of electronic devices, ranging from smartphones to industrial control systems. 

Castellated PCB Design and Manufacturing

There are quite a few differences between simple PCB and castellated PCB design and Manufacturing Processes. The selected differences are listed in Table 1 below:

Table 1: Design and Manufacturing Differences between Simple and Castellated PCB

Parameters of Difference Simple PCB Castellated PCB
Designing Standard Rectangular or Square Design Notched or Castle Edges
Edges & Corners Finishing Straight and Smooth Finish Rough or Jagged Edges on the PCB
Soldering Capability components can be Soldered to both Sides of the PCB Components can be soldered on one side only 
Mounting Methods Screws, Clips, Adhesive Designed to mount the PCB onto another PCB
Breakout Requirements Further processing of PCB is not required Post-processing is required for breakout connections

 

When making the castellated PCB many design and manufacturing considerations are crucial. Following are some of these considerations: 

Utilize Specialized Drilling Equipment

The drilling equipment used for forming castellated holes should be specifically designed for this purpose. Specialized equipment helps ensure precise and accurate hole formation, resulting in consistent and reliable castellation quality.

Break Tabs: 

When designing the castellated PCB, the castellated holes must cover a limited part on the edge of the PCB board so that the rest of the space is available needed for the components soldering.

Centering:

The edge of the PCB board must align exactly with the center of the castellated hole so that soldering this PCB with a larger PCB is not misaligned.

Pad Spacing: 

Proper and adequate spacing between the pads allows for simpler modular connections which are the basis for the need for castellated PCB.

Some Size Configurations of Castellated PCB
Some Size Configurations of Castellated PCB

 

Size: 

The pads on the main board should be properly sized so that they can accept the solder fillet completely. This makes the castellated PCB the exact fit for a larger PCB.

How to Design Castellated Holes and Edges in a PCB For an SMD Module?

When designing a PCB to accommodate an SMD module with castellated holes and edges, careful attention is important to ensure proper functionality. Following are some of the steps that can be followed to design castellated holes and edges in a PCB for an SMD module:

Understand The Module Specifications

Begin by determining the size and shape of the SMD module. This information is essential in determining the appropriate placement and dimensions of the castellated holes.

Create The Module Footprint

Once you have the module’s size and shape, create a footprint for it. This includes incorporating castellated holes and edges into the design.

Position The Module on the PCB Layout

Place the SMD module onto the PCB layout, ensuring proper alignment with other components and the overall circuitry.

Establish Connections: 

Connect the module to the rest of the circuitry using appropriate traces and vias. This step ensures the electrical integration of the module with the rest of the PCB.

Thoroughly Check for Errors: 

Before finalizing the design, carefully inspect for any potential errors or issues. Verify that the castellated holes have the correct size and placement to avoid any manufacturing or functionality problems.

Generate Manufacturing Output: 

Once you are satisfied with the design and have resolved any identified errors, generate the manufacturing output files required for production. These files can then be released to the manufacturing workspace.

Utilizing Electronic Computer-Aided Design (ECAD) tools like Altium Designer can significantly simplify the process of incorporating castellated holes for modules into your PCB layout. By following the steps mentioned above, you can confidently design and integrate castellated holes and edges in a PCB to accommodate an SMD module.

How to Stack Castellated PCBs?

When it comes to stacking PCBs that utilize castellated holes, it’s important to meet specific conditions to ensure proper alignment and high-quality solder joints. Here are some recommended conditions to consider for stacking PCBs with castellated holes:

Establish An Optimal Gap Between Castellated Holes

Aim for a gap of approximately 0.60mm between the castellated holes. This gap allows for sufficient space to accommodate solder and helps prevent any interference or short circuits.

Ensure Adequately Sized Pads on the Main Board

The pads on the main board should be large enough to accept the entire solder fillet. This helps provide a robust and reliable solder joint, enhancing the overall connectivity and stability of the stacked PCBs.

Employ Plated Through-hole Castellated Holes

It is essential to have the castellated holes plated through the hole. This ensures excellent conductivity throughout the components of the board, promoting reliable electrical connections and minimizing potential signal loss.

By meeting these conditions, the intended PCB stack-up can be achieved, and the quality of the solder joints can be effectively validated and ensured. Castellated holes are particularly beneficial for small modules such as patch panels or Wi-Fi modules. They are well-suited for modular designs and can be seamlessly integrated with surface-mount technology (SMT) components, offering flexibility and convenience in PCB assembly and integration processes.

Castellated PCB Soldering and Assembly

When soldering castellated holes, specific techniques can be employed to ensure reliable and strong connections. Here’s a recommended flow of soldering techniques for castellated holes:

i. Preheat the PCB

Preheat the PCB to reduce thermal shock and prevent warping during soldering. This can be done using a preheating station or a hot plate.

ii. Use a Stencil

Apply solder paste to the castellated holes using a stencil. The stencil ensures precise and consistent application of solder paste, which is crucial for proper connectivity & reliable solder joints.

iii. Apply Flux

Use a flux pen to apply flux to the castellated holes. Flux helps remove any oxidation or contamination on the hole surfaces, ensuring proper wetting and adhesion of the solder. 

iv. Use a Soldering Jig

Securely hold the PCB and module in place during soldering using a soldering jig. The jig ensures proper alignment and prevents movement during the soldering process, resulting in precise and accurate solder joints.

v. Reflow Soldering

Use a reflow oven to heat the PCB and melt the solder paste. The reflow oven provides even heating, allowing the solder paste to melt uniformly and form strong and reliable solder joints. This controlled heating process ensures consistent and robust connections.

By following these soldering techniques in a systematic flow, you can achieve strong and reliable solder joints between the castellated holes and the corresponding pads on the other PCB. This ensures proper connectivity and durability, making them well-suited for applications such as IoT hardware, RF modules, and breakout boards.

Board-to-Board Soldering Capabilities of Castellated PCB

Board-to-board soldering capabilities are crucial when integrating multiple PCBs into a single assembly. Castellated holes offer an excellent solution for mounting one PCB on top of another during assembly. Following are some of the benefits of using castellated holes for board-to-board soldering:

Easy Soldering and Excellent Connectivity

Castellated holes are designed for easy soldering and provide excellent connectivity between the PCBs. They offer convenient access points for soldering, ensuring reliable electrical connections.

Suitability For Small Modules and Durability

Castellated holes are particularly suitable for small modules like patch panels or Wi-Fi modules. They are more durable compared to traditional vias or plated through holes.

Support for modular designs and SMT components

Castellated holes are highly beneficial for modular designs, allowing for the easy integration of different PCB modules. They also work well with surface-mount technology (SMT) components, providing flexibility in assembly processes.

Wireless PCB-to-PCB connections

Castellated holes enable wire-less PCB-to-PCB connections, making them ideal for applications such as IoT hardware and RF modules. They facilitate efficient communication and transmission between interconnected PCBs.

Proper alignment and solder joint quality

When mounting one PCB on top of another using castellated holes, ensuring proper alignment and solder joint quality is crucial. Proper alignment guarantees accurate connections, while reliable solder joints ensure long-lasting and robust interconnections.

Benefits and Applications of Castellated PCBs

Incorporating castellated holes into PCB design presents numerous advantages, such as improved connectivity, increased durability, simplified assembly processes, and the ability to perform board-to-board soldering. These holes possess properties that make them highly practical, including their low profile and compact nature.

The incorporation of castellated holes in PCB design offers a multitude of benefits. Here’s the human-written explanation of each benefit:

Space-saving

Castellated holes occupy less space on the PCB compared to traditional through-hole vias. This space-saving feature allows for more compact PCB designs, enabling greater flexibility in product dimensions and overall miniaturization.

Improved Thermal Management

Castellated holes can be utilized to enhance thermal management in PCBs. By enabling efficient dissipation of heat through the PCB, these holes contribute to improved cooling and overall thermal performance, especially in applications with high heat generation.

Reduced Signal Interference

Castellated holes can help reduce signal interference by creating a ground plane around the signal traces. This shielding effect minimizes electromagnetic interference (EMI) and enhances signal integrity, leading to improved overall system performance.

Cost Effectiveness

Castellated holes offer a cost-effective solution for board-to-board connections. Their efficient design and manufacturing process contribute to reduced manufacturing costs while maintaining product reliability. This cost-effectiveness is especially valuable in large-scale production scenarios.

Effectively, the use of castellated holes in PCB design brings numerous benefits, including space-saving capabilities, wireless connectivity options, improved thermal management, reduced signal interference, and cost-effectiveness. These advantages make castellated holes a valuable component in various applications, enabling enhanced functionality and performance in PCB designs.

Panel Requirements for Castellated PCB Assembly

When designing a panel for castellated PCB assembly, it is important to consider various requirements to achieve efficient and cost-effective manufacturing. Here are the explanations of the panel requirements for castellated PCB assembly:

Fiducials

Place at least two local fiducials diagonally on the outer edges of the surface-mount components. Fiducials assist with precise alignment during assembly, ensuring accurate placement of the components on the PCB.

Tooling Holes

Incorporate tooling holes at the edges of the panel. These holes serve as reference points for proper alignment during the assembly process. They enable accurate positioning of the panel and ensure consistent alignment of the castellated PCBs.

Array Penalization

Consider implementing array penalization for high-volume board assembly. This involves producing smaller circuit boards in bulk and joining them together to form a single array. Array penalization optimizes the manufacturing process for efficiency and productivity.

Clearance

Provide sufficient clearance in the PCB design to accommodate various assembly shops and PCB construction technologies. The adequate clearance allows for smooth assembly processes and accommodates any specific requirements or restrictions of the chosen manufacturing method.

Robustness

Enhance the robustness of the PCB design, including stronger panels and more robust board finishes. Robust design choices ensure efficient and cost-effective manufacturing, minimizing the risk of damage or defects during the assembly process.

By following these panel requirements, the castellated PCB can be designed to facilitate efficient and cost-effective assembly. These considerations ensure proper alignment, connectivity, and durability in various applications, promoting successful production and reliable performance.

Other Considerations in Castellated PCB

When designing a PCB with castellated holes, several factors require careful consideration to ensure optimal performance. Here are some tips for designing castellated PCBs:

Component Placement

Place components away from the castellated holes to avoid interference during assembly. This helps ensure smooth integration and prevents any potential issues that may arise from component proximity to the castellated holes.

Trace Routing

Route traces away from the castellated holes to minimize signal interference. Proper trace routing helps maintain signal integrity and reduces the risk of crosstalk or other electrical disturbances.

Via Placement

Position vias away from the castellated holes to avoid interference during assembly. This ensures that vias do not hinder the mounting or soldering process and allows for a smoother integration of the PCBs.

Solder Mask

Apply a solder mask to protect the PCB from solder splatters and prevent short circuits. The solder mask provides an additional layer of insulation, safeguarding the PCB from accidental solder contact and ensuring reliable and safe operation.

By following these tips, you can design a castellated PCB with proper alignment, connectivity, and durability. This ensures reliable electrical connections in various applications, promoting seamless functionality and performance.

Misalignment In Castellated PCB Module and Prevention

When using castellated pcb to build larger pcb, appropriate handling of the issues that may occur due to misalignment is crucial. This may cause the overall pcb to fail miserably, or even worse cause untraceable issues during operation. To remedy and stay clear of these issues, following steps can be taken:

Please take the following steps to address potential misalignment and soldering issues related to the module and castellated holes on PCB: 

  1. First, carefully check for any misalignment in the module. Pay close attention to any shifting that might have occurred during assembly, especially around the castellation’s on the module bridge pads on PCB.
  2. Ensure precise alignment and positioning of the castellated holes to facilitate proper soldering.
  3. Adhere to specific design attributes when dealing with castellated holes. This includes using a minimum hole diameter of 1.0 mm and placing the castellated holes on either the top or bottom of the board’s edge.
  4. Whenever possible, try to utilize the largest available size for the castellated holes. This will aid in the mounting process and prevent the use of vertical edges (left and right).
  5. Check for solder bridging, a common issue in castellated hole assembly where excess solder creates unintended electrical connections between.

If you do encounter any problems, here are the recommended fixes:

  1. If misalignment is identified, carefully reposition the module to ensure proper alignment with the castellations on the module bridge pads on the PCB.
  2. For solder bridging issues, you can use a solder wick to remove excess solder and then reflow the joint.

By following these troubleshooting steps and implementing the appropriate fixes, you can avoid common alignment issues that may otherwise compromise the overall quality and reliability of the final product.

Conclusion

The article explores the advantages of incorporating castellated holes in PCB design. Castellated holes are specially shaped indentations, partially plated, and positioned along the edges of PCB boards. They serve as mounting points for stacking one PCB board onto another during assembly. The article delves into important considerations for designing castellated holes, including recommended specifications. It also covers the conditions necessary for effectively stacking PCBs that feature castellated holes, as well as soldering techniques and alignment troubleshooting specific to these holes. Additionally, the article discusses the panel requirements for PCB assembly and castellated PCB assembly. By utilizing castellated holes, notable benefits such as enhanced connectivity, improved durability, simplified assembly processes, and the capability for board-to-board soldering can be realized.

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