Cavity Board Design Guide

A cavity board carries a recess in the laminate so that a component can sit below the surface, either to keep the overall height within a limit or to bring a sensor or an optical part closer to the outside face. The cavity is produced by controlled depth milling or by a stackup in which a layer is cut out before lamination, and either method puts the reliability of the remaining dielectric under the floor of the recess.

Why a Cavity Is Used

The first reason is component height. A package that is taller than the space available can be sunk into the board, which lowers the profile of the assembly without changing the components on it.

The second reason is proximity. An optical sensor, a microphone or an antenna has to be close to the outside surface, and a cavity brings it there without a separate housing or a flexible interconnect.

The third reason is thermal or mechanical. A die or a metal tab can be placed in a cavity so that it contacts a heat sink or a metal base through the board rather than through a stack of thermal interfaces.

Two Ways to Make One

The first method is to mill the cavity after lamination. The depth is set by a controlled depth milling operation, and the floor of the cavity is the copper and dielectric of an inner layer.

The second method is to build the cavity into the stackup, by cutting an opening in the prepreg or the core before the layers are pressed. The opening fills with resin during lamination, and the resulting geometry is set by the layup rather than by a machining operation.

Both methods can be combined with a stepped structure, in which a larger shallow recess contains a smaller deeper one. The stepped form is used where a component has more than one height or where a lid has to sit in the opening.

Depth Control

The depth of a milled cavity is the critical dimension. It has to be measured from the finished surface, and the measurement has to account for the variation in the laminated panel thickness, which is greater than the variation in a single sheet.

The floor of the cavity should be a copper layer rather than bare dielectric. Copper stops the milling in a way that a resin does not, and it also provides a plane under the component which is useful for shielding and for thermal spreading.

A tolerance should be stated for the depth, for the flatness of the floor and for the position of the cavity relative to the datum. A cavity that is deeper than specified leaves a thin dielectric under the component, and one that is too shallow holds the component above the surface.

Stackup Rules

The dielectric between the floor of the cavity and the next copper layer is what carries the mechanical and the electrical reliability. The thickness should be increased rather than minimised, because a cavity concentrates the stress where the laminate is thinnest.

The copper balance around the cavity should be maintained. A large cut out in a copper layer changes the resin flow during lamination and produces a thickness variation, and the balance can be restored with thieving around the opening.

The layer below the cavity floor should not carry a signal that is sensitive to a change in its reference. The cavity changes the dielectric above that layer, so the impedance of a trace that runs under the cavity will differ from the impedance of the same trace elsewhere.

Component and Assembly Considerations

The component has to be placed inside the cavity, which means the placement nozzle needs clearance and the vision system has to see the fiducials on a surface that may be at a different height. Those two requirements should be discussed with the assembly house before the design is released.

The paste printing over a cavity is difficult because the stencil cannot sit flat over an opening. The usual solution is a stepped stencil or a separate dispensing operation for the pads inside the recess.

Where the cavity is closed by a lid, the sealing method has to be compatible with the process. A lid that is soldered needs a solderable surface on the rim, and one that is glued needs a defined bond line and a cure that does not damage the components inside.

Fabrication and Yield

The deep milling operation is a source of yield loss. The depth control is subject to tool wear, to the panel thickness and to the flatness of the panel on the machine table, and the loss appears as cavities that are out of tolerance rather than as visible damage.

The cleaning after milling has to remove the debris from inside the recess. A cavity that retains dust offers a path for contamination and creates a problem for the conformal coating that will be applied later.

The finish has to reach the surfaces inside the cavity. An immersion finish reaches the recess as easily as the surface, while a finish that depends on a mechanical process may not, and the specification should say which surfaces have to be finished.

Reliability

The failure modes are related to the thin dielectric under the floor and to the thermal expansion of the different materials in the stack. A cavity that is close to a plated hole is at risk because the expansion of the hole and the thin section interact.

Thermal cycling is the test that reveals the weakness. A cavity floor that delaminates or a copper layer that cracks will show up after a number of cycles, and the sample should include the worst case combination of cavity depth and dielectric thickness.

The mechanical handling is also a consideration. A board with a cavity is more fragile than a solid one, and the handling and the packing should be considered as part of the design rather than as a production problem.

Design Flow

The cavity should be defined in the layout, with its outline on a mechanical layer and its depth on the fabrication drawing. The drawing should also show which layers form the floor and which surfaces must be finished.

A discussion with the fabricator before the design is released is worth more than a tolerance on the drawing. The fabricator can say which depth is achievable with the chosen stack, and the answer may change the stack rather than the cavity.

The assembly requirements should be settled at the same time, because the placement, the printing and the inspection of a component inside a recess all differ from a normal board, and each of them has a cost that should be known before the design is frozen.

Practical Rules

Design the cavity floor as a copper layer, keep the dielectric under it generous and state a depth tolerance. Confirm the achievable depth with the fabricator and the assembly method with the assembly house.

Record the cavity dimensions with the build records and the fabrication notes, and review the fabrication notes and the registration rules when a recess is added to a stack.

FAQ

How is a cavity made? By controlled depth milling after lamination, or by cutting an opening in a layer before pressing so that the cavity is formed by the layup.

Why should the cavity floor be copper? Copper stops the milling consistently and provides a plane under the component for shielding and thermal spreading.

What limits the dielectric under the floor? Reliability. A cavity concentrates stress where the laminate is thinnest, so the dielectric below the floor should be increased rather than minimised.