Castellation – Almax

[ Overview ]

What Is Castellation?

Permanent, low-profile board-to-board connections without a separate connector.

Castellation is an interconnect method formed along the edge of a printed circuit board. Plated holes are positioned at the board perimeter and then routed through, leaving semicircular plated features—often called castellated holes, castellations, or castellated pads.

These exposed, solderable edges align with matching lands on a host PCB. During assembly, the module is placed on the host board and soldered in position. The resulting joints carry electrical signals or power while also helping secure the module mechanically.

Within ALMAX Keypads & Interfaces, Castellation belongs to the Interconnects group. It is a board-level connection method, not a complete keypad, circuit family, or electronics system by itself.

Permanent, low-profile board-to-board connections without a separate connector

This page focuses on Castellation as a solderable PCB-edge interconnect for mounting electronic modules directly to a host circuit board; related products and technologies are mentioned only where they help explain construction choices, integration, alternatives, or system fit.

[ System Fit ]

Where It Fits in the Product System

A castellated module normally sits directly on a larger PCB or rigid electronic assembly. The connection is usually hidden inside the finished product, beneath an enclosure, control panel, keypad, display, or other user-facing interface.

ALMAX may integrate castellated modules into

Smart keypad and interface controllers
LED or display-support modules
Sensor and signal-conditioning boards

Compact communications or processing modules
Product-specific electronics located behind an HMI
Rigid or rigid-flex assemblies that require a compact module connection

The host board provides matching copper lands, routing, mechanical clearance, and access for assembly and inspection. The module may also need programming pads, test points, thermal clearance, or additional support depending on the design.

[ How It Works ]

How Castellation Works

Plated holes are designed along the module PCB edge.

The board outline is routed through those holes, creating plated semicircular terminations.

Each termination aligns with a corresponding solder land on the host PCB.

Solder paste is deposited on the host board, and the module is positioned like a surface-mount component.

Reflow creates solder joints that provide the intended electrical paths and help retain the module.

The finished result is a direct module-to-board connection with no separate mating connector. Its success depends on coordinated land geometry, board fabrication, surface finish, placement accuracy, solder volume, reflow conditions, and inspection criteria.

[ Variations ]

Common Types and Variations

Standard edge castellations

Best for compact signal or low-power modules mounted along a straight board edge.

Multi-edge castellation

Used when connections or mechanical retention are needed on more than one side of the module.

Rigid PCB modules

Common where the module carries controllers, drivers, sensors, or communication electronics.

Rigid-flex module construction

Used when a soldered rigid section must transition into a flexible circuit or constrained assembly.

Reinforced layouts

May add suitable copper features, vias, or mechanical support where the application places greater stress on the soldered module.

Castellation with secondary support

Adhesive or underfill may be considered when the mechanical or environmental requirements justify added support.

[ Applications ]

Typical Applications

Compact industrial control modules
Medical-device interface electronics
Smart keypads with local control or communication functions
LED, indicator, or display driver modules

Sensor interface and signal-conditioning boards
Wireless or identification modules integrated into larger electronics
Aerospace, transportation, and defense assemblies where packaging efficiency and secure internal connections matter
Sealed devices that do not require the module to be unplugged during normal service

Castellation is most relevant when the electronics can be treated as a permanent submodule. If regular removal, field replacement, or repeated mating is required, a separable connector may be more appropriate.

[ Key Features ]

Key Features

Solderable plated terminations formed at the PCB edge
Direct module-to-host-board mounting
Low connection height compared with many connector systems
Electrical and mechanical attachment through the same solder joints
Compatibility with standard surface-mount placement and reflow workflows when properly designed

Visible edge joints that can support inspection
Flexible pin assignment for signal, power, ground, or control paths
Product-specific pad pitch, module outline, and connection count
Potential integration with rigid, rigid-flex, and compact electronics architectures

[ Benefits ]

Customer Benefits

Benefits include:

Compact packaging

Removing a separate connector can reduce Z-height and free space inside the device.

Simplified sourcing

The connection function is built into the PCB edges and host-board lands rather than added as another mating component.

Production-ready modularity

A tested electronics function can be designed as a repeatable module for placement on a larger board.

Reliable permanent connection

Soldered joints avoid fretting at removable contact interfaces and suit applications that do not need frequent disconnection.

Efficient assembly

Properly designed modules can fit established pick-and-place and reflow processes.

Integrated development

ALMAX can coordinate the module, host-board interface, mechanical fit, test access, and surrounding HMI architecture as one product-specific solution.

[ Materials & Construction ]

Materials, Construction,
and Technology Options

Castellated modules are commonly based on rigid PCB materials such as FR-4. Rigid-flex constructions may use polyimide flex sections where the electronics must bend, route, or connect beyond the rigid module area.

The connection structure may include

Copper-plated edge features
A solderable surface finish selected for the assembly process
Solder-mask clearances and dams suited to the pad layout
Host-board lands matched to the module geometry

Ground or power connections distributed according to electrical needs
Alignment or mechanical-reference features
Test and programming points where required
Optional adhesive, underfill, or structural support for demanding applications

Material and finish choices should be verified against soldering, storage, environmental, regulatory, and reliability requirements. Printed-silver PET circuits are not direct substitutes for solderable PCB castellations; soldered module edges require a construction compatible with the selected soldering process.

[ Design Considerations ]

Design and Integration Considerations

[ Performance ]

Performance and
Durability Factors

Long-term behavior depends on the complete design rather than the castellated feature alone. Key influences include PCB material, copper and plating quality, surface finish, pad geometry, solder-joint design, reflow control, module mass, host-board stiffness, operating temperature, vibration, shock, and exposure to moisture or chemicals.

Depending on the design, materials and reinforcement can be selected for improved resistance to thermal cycling and mechanical loading. Sealing or conformal protection may help in exposed environments, but the coating, enclosure, and cleaning process must remain compatible with the electrical and inspection requirements.

ALMAX can define prototype and production checks around the application, including dimensional inspection, continuity testing, functional testing, solder-joint review, and environmental or mechanical validation when required. Exact performance limits should be established through project-specific requirements and testing rather than assumed from the interconnect type.

[ When to Choose ]

When to Choose Castellation

Choose Castellation when…

Choose Castellation when:
A PCB module should be permanently soldered to a host board
Low profile and compact packaging are important
A separate connector would add unnecessary height, cost, or complexity
The module can be assembled through a controlled SMT process
The product benefits from a modular electronics function without a removable connection

Consider alternatives when…

Consider alternatives when:
Slim Stacking may be better when two boards must remain separable in a very compact stack.
ZIF may be better for a thin flexible tail that must be inserted into a board connector without soldering the flex end.
Crimped terminations may be better when a robust, polarized, serviceable harness connection is required.
Cables may be better when the connection must bridge distance or tolerate routing movement.
Connectors may be better when repeated mating, field replacement, or standardized interfaces are priorities.

[ Related ]

Related Products and System Components

These are adjacent options and system components, not all fully covered on this page. Use the dedicated page when the customer is specifically looking for another connection method, complete assembly, input, output, or electronics architecture.

[ New Here? ]

Simple First-Time
Customer Summary

If you are new to Castellation, think of it as a row of solderable half-holes along a PCB edge that lets an electronics module mount directly onto another circuit board. It is commonly used when a permanent, compact module connection is preferred over a separate connector. The most important things to consider are pad geometry, assembly process, mechanical loading, and test or service requirements.

FAQ’s

Your questions, answered.

What is Castellation used for?

Castellation is used to mount a PCB module directly onto a host circuit board. The edge terminations carry electrical connections and contribute to the module's mechanical attachment.

How does a castellated PCB module work?

Semicircular plated features on the module align with solder lands on the host PCB. Reflowed solder joins the two boards so the module functions like a surface-mount component.

What is the difference between Castellation and a board-to-board connector?

Castellation creates a permanent soldered connection without a separate mating connector. A board-to-board connector is usually separable, which can help with servicing or replacement but adds components, mating height, and alignment requirements.

Can ALMAX customize a castellated module?

Yes. ALMAX can develop product-specific module geometry, connection count, pin assignment, host-board interface, electronics integration, mechanical fit, test access, and production requirements. The final scope depends on the customer's device and performance needs.

What materials are used for castellated modules?

Rigid PCB materials such as FR-4 are common. Rigid-flex constructions may incorporate polyimide where a flexible section is needed. Copper plating, a solderable surface finish, solder mask, and compatible host-board lands form the connection system.

Are castellated connections suitable for harsh environments?

They can be considered for demanding products because the module is soldered rather than repeatedly mated. Suitability still depends on the full design, including shock, vibration, temperature cycling, moisture, chemicals, enclosure protection, materials, and validation testing.

Can a castellated module be replaced?

Rework may be possible with suitable equipment and access, but Castellation is primarily intended as a permanent connection. If routine replacement is expected, a separable connector should be evaluated.

What information does ALMAX need to start?

Useful inputs include the module function, schematic or pin requirements, host-board layout constraints, available space, module height, signal and power needs, environmental conditions, expected production process, test strategy, and service requirements.