Cavity‑embedded Cavity PCB for RF power amplifier housing

1. Definition and Types

A cavity is a "cutout" that extends from an outer layer to a specific internal layer. According to IPC-6012F, there are three primary types based on their metallization:
◈ Type 1: No metallization on the walls or the floor (bare substrate).
◈ Type 2: Metallization on either the floor or the walls (not both).
◈ Type 3: Metallization on both the floor and the walls (forming a fully shielded or conductive pocket).
Common Categories
◈ Top/Bottom Side Cavity: Open to the surface for mounting components (e.g., thinning a device).
◈ Buried Cavity: Completely enclosed within the internal layers to house embedded components.
◈ Step Cavity: A multi-level cavity with varying depths.

2. Technology Requirements

Manufacturing a cavity PCB is significantly more complex than a standard board. It requires specialized equipment and material handling:
◈ Material Choice: Often uses No-Flow or Low-Flow Prepreg. Standard prepreg flows too much under heat and pressure, which would fill the cavity with resin during lamination.
◈ Material Systems: Common materials include high-Tg FR4, Rogers (for RF), or polyimide.
◈ Machining Methods:
    ● Mechanical Milling: Controlled-depth CNC routing to scoop out material.
    ● Laser Ablation: Uses high-energy lasers to remove dielectric layers with extreme precision, often stopping once it hits a copper "stop" layer.
    ● Sequential Lamination: Pre-cutting windows in specific layers before they are stacked and bonded together.

3. Design Rules (DFM)

To ensure the board can be manufactured successfully, designers must follow specific Design for Manufacturing (DFM) guidelines:

4. Typical Stackup

The stackup of a Cavity PCB is designed to accommodate the depth of the pocket while maintaining structural integrity.
A common 4-Layer Cavity Stackup for a component recess might look like this:
◈ Layer 1 (Top): Signal/Power (This layer and L1-L2 prepreg are removed at the cavity location).
Prepreg: No-flow prepreg is used here to prevent resin bleed into the open pocket.
◈ Layer 2 (Inner): Ground Plane (Acts as the Cavity Floor; usually a solid copper area for thermal or mounting).
Core: Standard FR4 or High-Speed core.
◈ Layer 3 (Inner): Signal/Power.
Prepreg: Standard flow prepreg.
◈ Layer 4 (Bottom): Signal/Ground.
To help you visualize and plan your design, here is a detailed 6-layer stackup example using a cavity that extends from the Top Layer down to Layer 3.
This configuration is common for embedded RF modules or thermal heatsink attachments, where Layer 3 acts as the cavity floor.
◈ 6-Layer Cavity PCB Stackup Table
In this example, we assume a total board thickness of approximately 1.6mm.

◈ Critical Design Rule: The "Stop" Layer
The Cavity Floor (Layer 3 in the table above) must be designed with a solid copper pad that is slightly larger (usually +0.2mm per side) than the cavity opening itself.
    ● Why? In mechanical milling, the copper acts as a visual and physical indicator for the depth.
    ● Laser Ablation: In laser-cut cavities, the copper layer reflects the laser energy, preventing it from burning through to Layer 4.
◈ Comparison of Cavity Methods
Depending on your project's volume and budget, you will choose one of these two primary manufacturing flows:

◈ Pro-Tip for Assembly
If you are placing a component inside the cavity, ensure the Soldermask is removed from the cavity floor (unless it is for insulation). If the floor is for thermal grounding, use Gold (ENIG) or Silver plating to prevent oxidation within the pocket.
Cavities and Coin technologies are used for the boards used in following industries and applications.

At the cutting edge of tomorrow's technology

End devices are becoming smaller and smaller, and the space on the printed circuit boards is becoming more and more precious. For a long time now, it has no longer been sufficient to assemble the outer layers conventionally, and the inner layers (multi layers) are also being used.
Cavities make it possible to accommodate components in the inner layers and thus to fit increasingly complex assemblies in a smaller space.
By utilizing several layers, the space on the same area can be utilized to the maximum.
In order to place micro chips, heat sinks or resistors on the circuit boards, recesses, so-called cavities, are made in the circuit board by means of a laser in order to remove the desired material.
The laser first opens the copper surface and then removes the dielectric. Another laser pass cleans the exposed inner copper surface from resin residues.
We provide various package printed circuit boards and module PCBs with cavity structure exposing selected inner layer patterns (bonding pads, ground layers, etc.) on multi-layer PCB and build-up PCB.

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