Introduction

Castellated holes, also known as half-etched holes, are specialized holes found in printed circuit boards (PCBs) that are used to extend the edge of the PCB. These half-circle indentations along the edge of a PCB board allow for direct external connections to internal layers and components. Castellated holes provide a simple and efficient way to increase the available space for connections on a PCB.

What are Castellated Holes?

Castellated holes are semi-circular voids formed by etching away half of the hole during PCB fabrication. This creates notches or castellations along the edge of the PCB. The result is that you can solder leads directly to internal layers of a multilayer board.

Castellated holes allow connections to internal layers of a PCB.

The half-etched areas create openings to inner copper layers that components can be soldered to. This provides access to internal routing layers without having to route connections to the outer edge. It enlarges the available space for traces, vias, and solder pads on the PCB perimeter.

Some key characteristics of castellated holes:

• Semi-circular shape along the edge of the PCB
• Formed by etching away half of a plated through-hole
• Exposes internal layers for connections
• Enlarges perimeter space for traces and pads

Benefits of Using Castellated Holes

There are several benefits to using castellated holes in a PCB design:

1. Increased Connection Points

Castellated holes essentially increase the available space around the edge of the board. This allows for more connections to internal layers without having to route everything to the outer perimeter. The semi-circle notches provide direct access to inner layer traces, pads, and vias.

More connection points means designers can add extra components, traces, and solder joints. This is especially useful for boards with high-density interconnects.

2. Better Signal Integrity

By providing direct access to internal layers, castellation avoids having long traces or vias routed to the outer edge. Shorter traces improve signal integrity by minimizing line impedance, crosstalk, and other transmission line effects.

Direct solder connections also avoid losses associated with vias between layers. The shortened paths keep signals cleaner with less reflection, ringing, and attenuation.

3. Compact Layout

Castellated holes minimize the need for trace routing so components can be placed closer together. This allows for a more compact PCB layout overall. Devices and connectors can be clustered along the board edge, saving space.

The reduced routing also results in less layer-to-layer crosstalk and interference for better performance. Dense boards benefit greatly from castellation.

4. Flexible Attachment Points

The notched holes provide attachment points for interconnects, test points, and debugging. Leaded components can be soldered directly to inner layers which is useful for prototyping and repairs.

Castellation allows flexibility in where connections are made. For example, edge mount connectors normally require perimeter pinouts but can now interface directly with internal layers.

5. Improved Manufacturability

PCBs with castellated holes are easier to assemble and solder. Components and connectors line up conveniently along the board edge for access. This avoids issues with limited interior space and improves manufacturability.

The exposed inner layers also enable test points for improved diagnostics and troubleshooting. Connections can be probed to verify designs.

6. Cascade and Stacking Options

Castellated boards can be cascaded and stacked vertically. The notched edges line up with each other allowing multiple PCBs to be interconnected along the sides.

Boards can be assembled in a staggered, overlapping fashion for 3D compaction. This is useful for very dense and complex electronics projects.

Applications and Examples

Castellated holes are especially useful in high density interconnect (HDI) applications where routing space is limited. Some examples include:

• Cell phones, tablets, and handheld electronics
• Wearable devices
• Internet of Things (IoT) products
• Medical devices
• Remote sensors
• Automotive electronics

Any small PCBs that need access to internal layers benefit from castellation. The holes are also used when edge connections are required, such as with plug-in modules, PCI cards, and memory modules.

Here are some specific applications:

Edge Connectors

Plug-in cards, modules, and memory sticks use castellated holes to create pads and fingers along the insertion edge. The notched holes connect to internal layers so no outer layer trace routing is needed. Common examples include PCIe cards, mini-SIM cards, and DIMM memory modules

A mini-SIM card uses castellated holes along the edge for the contact pads.

Stacking Connectors

Castellated boards allow vertical stacking with connections between layers. Female sockets along the edge accept male pin headers on other PCBs to form multi-board assemblies. The aligned holes create channels for the pins to mate within.

Castellated boards can stack together using male and female connectors.

Radio Modules

Wireless radios like WiFi, Bluetooth, and cellular modules require RF shielding. Castellated holes help create the shielding enclosure by exposing ground pins around the perimeter. The holes connect to ground layers which the shield attaches to.

Castellated holes help form RF shielding around a radio module.

Cascaded Boards

To save horizontal space, castellated boards can be interconnected in an overlapping cascaded arrangement. The notched edges allow soldered connections between layers of different boards for a dense 3D stack. This is useful when vertical space is available.

Castellated holes allow PCBs to cascade together in a dense, overlapping stack.

PCB Layout Considerations

Proper layout is important when working with castellated holes in a PCB design. Here are some key considerations:

• Placement – Position components to take advantage of the shortened connections to internal layers. Cluster related items near perimeter holes.
• Routing – Minimize trace lengths for better performance. Avoid necking traces between closely spaced holes.
• Annular rings – Include sufficient annular rings around pads for solderability. Account for larger rings on edge layers.
• Hole sizing – Match holes to lead diameters. Ensure adequate clearance between holes.
• Mask expansion – Expand solder masks away from holes to prevent shorts during reflow when solder wicks into holes.
• Plating – Plated edge margins improve solder adhesion. May require thicker plating in high vibration environments.
• Testing – Include test points at holes for probing inner layer signals during diagnostics.
• Chamfers – Include edge board chamfers to relieve mechanical stress around holes.
• Assembly – Use adhesive or guides to align boards and prevent shifting during cascade stacking.

PCB Fabrication Process

Creating castellated holes requires additional steps during PCB fabrication:

1. Drilling – First, traditional through-holes are drilled through the entire PCB stackup at desired locations. High accuracy is required.

2. Plating – Holes are plated with copper to form the pads and connection points to inner layers.

3. Layer imaging – Circuit patterns are imaged onto each inner layer core. This coats areas around holes to be retained.

4. Developing – The PCB panel is developed to expose the sacrificial half-hole areas. This selectively dissolves away uncoated photoresist.

5. Half-etching – The panel is etched to remove the now-exposed hole halves. The other halves remain coated and protected.

6. Stripping – Photoresist is stripped away, revealing the finished half-etched castellations.

7. Laminating – Individual cores are aligned and laminated together into a multilayer board.

8. Contour routing – The outer board profile is routed to shape with castellated edges.

This additional processing does add cost compared to standard PCB hole drilling. However, in interconnect-dense designs the benefits often justify the extra fabrication steps.

Design Rules and Limitations

There are some constraints when working with castellated holes:

• Minimum size – The semicircular voids have a smaller diameter than regular drilled holes, limiting how small they can be fabricated.
• Aspect ratio – The etched depth dictates the aspect ratio. Higher layer counts have deeper holes which impacts structural strength.
• Hole spacing – A minimum distance between holes is required to have enough web thickness. This limits hole density.
• Layer alignment – Tight registration control is needed when laminating layers to properly align holes.
• Routing constraints – Traces routed between holes require adequate spacing. Necking is a concern.
• Mechanical stress – Thermal cycling and vibration can cause cracks emanating from holes. Chamfers and edge plating help.
• Solder wicking – Excess solder may wick into holes during reflow, causing shorts. Solder mask expansion offsets this.
• Cleaning – Etching residues inside holes may require additional cleaning steps.

While castellated holes are extremely useful, they do have limitations. Working closely with your PCB manufacturer is recommended to establish proper design rules and achievable tolerances up front.

Frequently Asked Questions

Here are some common questions about castellated holes in PCBs:

Are castellated holes more expensive to fabricate?

Yes, the additional processing steps required to create the castellated holes do add some cost compared to standard through-hole PCBs. However, for dense interconnect designs the benefits usually justify the higher fabrication costs.

What are the minimum hole size and spacing?

The minimum castellation hole size is typically around 0.3 to 0.4 mm. Spacing between holes depends on the hole size but 0.2 to 0.25mm is common. Work with your PCB manufacturer to determine achievable tolerances.

How are components attached to castellated holes?

Leaded parts can be soldered directly into castellated holes. Leadless components are often reflow soldered to pads adjacent to the holes. Alternately, edge mount connectors mate with the holes to provide connections.

Can castellated holes crack during board flexing?

Yes, the points where etched holes transition to full copper thickness are stress concentration points. Excessive flexing can cause cracking over time. Use of corner chamfers, elastic solder masks, and thicker edge plating help increase resilience.

How are castellated boards stacked together?

There are a few techniques. Boards can be soldered together using pin headers or edge mount connectors. Adhesives can be used to bond boards, using guide pins for alignment. Enclosures provide compression strength once assembled.

Conclusion

Castellated holes provide a simple method to extend connections to internal PCB layers without excessive routing or layer transitions. The half-etched voids efficiently utilize the board perimeter, enabling dense component placement and compact designs. Although they cost more to fabricate, castellated holes bring significant interconnect benefits that justify their use for space constrained and high speed circuits. When designed and manufactured properly, boards with castellated edges offer flexibility, signal integrity, and reliability improvements over standard PCB hole drilling alone.