Introduction
Printed circuit boards (PCBs) contain plated through holes and vias to allow mounting of electronic components and electrically connecting traces across layers. Castellated holes are a special type of plated through hole that provide a unique way to extend PCB connections to external devices or boards.
This article will provide a comprehensive overview of castellated holes, discussing their characteristics, design considerations, fabrication process, applications and benefits compared to other interconnect methods.
What are Castellated Holes?
Castellated holes are plated half-holes located on the edge of a PCB. They extend from the outer layer to just one inner layer, unlike regular through holes that span the entire board. The name comes from their resemblance to turrets or castellations along the edge of a castle wall.
As seen above, half of each plated hole protrudes outside the PCB edge, while the other half extends inwards to connect to an internal layer. This provides a means to electrically connect the external region and internal board layers via the metal plating in the castellated hole walls.
The exposed cylindrical pads can be used for direct vertical interconnection between boards stacked on top of each other. Castellations perform a similar function to edge connectors but allow higher density connectivity.
Castellated Hole Design
Here are some key considerations when designing castellated holes in a PCB:
Layer Connection
- Castellations normally connect the outer layer to the first inner layer. But they can also link any two other adjacent layers.
Hole Size
- Typical diameters are 0.3mm to 0.75mm to match adjacent via/hole sizes.
Hole Spacing
- Center-to-center distance between castellations is usually 0.8mm to 1mm.
Pad Size
- Annular ring pad diameter around each hole is slightly larger than hole.
Number of Rows
- Often, two parallel rows of staggered castellations are used.
Edge Margins
- A 1mm or more copper-free margin from board edge is typical.
Corner Shaping
- Castellations should avoid extending into board corners to prevent cracking.
Mechanical Stability
- Web of board material between holes should exceed 0.2mm for rigidity.
PCB Fabrication Process
Castellated holes are fabricated along with other PCB holes:
1. Drilling
- During CAM, a compressed drill file is generated with normal and castellated holes marked.
- The PCB manufacturer machines all holes in one pass using a special bit.
2. Plating
- Electroless copper is deposited on the walls of all through holes and castellations.
- This forms the conductive path between layers/externals pads.
3. Routing
- A V-shaped notch is cut into the edge where castellations are located.
- This exposes the plated half-hole while isolating them from the board edge.
4. Outer Layer Finish
- Standard processes are used to pattern copper and apply solder mask, legend, etc.
- Castellated pads are covered with solder mask for insulation.
This fabrication process requires precise drilling and routing but otherwise follows standard PCB manufacturing techniques.
Applications of Castellated Holes
Castellated holes enable several unique PCB assembly possibilities:
Stacking Boards Vertically
- Boards can be stacked and interconnected without sockets or headers.
- Aligned castellated pads on each board edge provide direct vertical connections.
Edge Mounting Components
- Components can be mounted vertically or horizontally along the PCB edge.
- Leads are soldered directly into exposed castellated pads.
Testing Points
- Provides easy test access to internal signals from board edge.
- Can probe castellated pads for debugging and troubleshooting.
Board-to-Board Cableless Interconnect
- Enables high density board-to-board connections without cables.
- Aligned pads on matched castellated edges are joined to interconnect boards.
Improved Thermal Performance
- Exposed pads along edge act like cooling fins to improve heat dissipation.
Interposer Linkage
- Can solder boards to both sides of a central castellated interposer board.
Probe Testing
- Allows probing board from edge for in-circuit testing during manufacturing.
LGA (Land Grid Array) Sockets
- Some LGA sockets have castellated edges to engage with PCB.
These examples demonstrate the versatility of castellated holes for innovative PCB construction and integration techniques.
Benefits of Castellated Holes
Castellated holes offer several advantages compared to other board-to-board and external connection methods:
Higher Density
- Castellations allow more I/O connections along a board edge than possible with connectors. Hundreds of high-density signals can be accessed through rows of staggered holes.
Lower Cost
- Eliminates need for mating connectors or cables between PCBs or for test points.
Space Savings
- No connectors consume space on the surface or increase stack height between vertically joined PCBs.
Design Flexibility
- Castellated holes can be placed anywhere along the edge for routing convenience.
Reliability
- Robust plated holes with no moving parts or wear-out mechanisms.
Assembly Simplicity
- PCBs can be directly stacked and soldered together without additional components.
Performance
- Allows high-frequency signals with lower noise compared to cables or connectors.
Serviceability
- Individual PCBs can be unstacked for repair instead of replacing entire assembly.
Customization
- Holes can be sized and placed according to specific connection requirements.
Comparison to Other Interconnects
Here is how castellated holes compare to some other PCB edge connection methods:
| Parameter | Castellations | Edge Connectors | PCB Edge Contacts |
|---|---|---|---|
| Density | Very High | Moderate | High |
| Cost | Low | High | Moderate |
| Design Effort | Moderate | Low | High |
| Signal Frequency | High | Moderate | High |
| Flexibility | Moderate | Low | High |
| Durability | High | Moderate | Low |
| Repairability | High | Low | Moderate |
So castellated holes provide a good combination of density, performance, durability, cost and design flexibility compared to other interconnect options.
Challenges of Castellated Holes
Some limitations to consider when planning to use castellated holes:
- Special drill/route fabrication equipment required. Not all PCB manufacturers support this capability initially.
- Larger minimum board thickness required (1mm+) to accommodate half-holes.
- Limited number of rows possible to maintain sufficient edge webbing strength.
- May need edge reinforcements like chamfers or thickened region for mechanical robustness.
- Inspection and repair of soldered castellations is challenging.
- Thermal expansion mismatch stresses connections during temperature cycling.
With careful design and assembly, these concerns can usually be addressed to utilize castellated holes effectively.
Conclusion
Castellated holes provide a versatile technique to extend PCB layer connections to the external edge for inter-board linking, stacking, probing or component mounting. They enable dense vertical interconnects between rigid boards without cables or connectors. With growing applications in 5G communication, network systems, consumer products and other high-speed vertical edge coupling requirements, usage of castellated holes will continue increasing in PCB designs.
Frequently Asked Questions
Can castellated holes be used on flex PCBs?
No, castellated holes are not recommended for flex PCBs as the rigid protruding pads would restrict bending and worsen flex life performance. They are only used on rigid boards.
What are the typical dimensions for a castellated hole?
Common diameters range from 0.3 to 0.75 mm. Pad diameters are ~0.1mm larger than hole. Hole length into board is usually over 1mm. Edge margins are above 1mm.
Can components be soldered horizontally onto castellated pads?
Yes, castellations allow mounting components like resistors or capacitors horizontally along the PCB edge. Their leads are soldered onto the exposed cylindrical pads.
How are castellated holes specified in PCB design?
Castellated holes are indicated on an additional fabrication layer in the CAM/Gerber data. The compressed drill file also marks them as half-holes.
How many rows of staggered castellations are typical?
Most designs utilize one or two rows of staggered castellated holes. More rows are possible but reduce edge material thickness affecting mechanical stability.
