Define the Depth Reference and Remaining Material in a PCB Cavity

A cavity depth of 0.5 mm is ambiguous unless the starting surface and finished condition are clear. Board-thickness variation, surface coatings and the manufacturing reference can change the material remaining beneath the cavity. Define the floor’s function and the critical remaining structure before discussing a machining tolerance.

State what the cavity must accomplish

Identify whether it provides component clearance, locates a part, exposes an intended layer or creates a controlled thin region. These objectives can require different manufacturing methods and inspection. A pocket for mechanical clearance is not automatically equivalent to a cavity that must stop on copper.

Würth Elektronik’s delivery specification distinguishes surface-related milling depth from depth referenced to the machine table. This illustrates why the reference matters. Agree on the applicable interpretation with the actual fabricator and express the required finished result clearly.

Dimension the floor and surrounding walls

Specify the opening, location, corner radii, depth reference and relevant tolerances. Identify whether soldermask, copper or another finish is included in the reference surface. Mark which wall and floor features are functional and which can follow normal process capability.

As an illustrative tolerance exercise, a board varying from 1.50 to 1.70 mm with a pocket varying from 0.45 to 0.55 mm could leave 0.95 to 1.25 mm beneath the pocket if both dimensions are independent and referenced as assumed. The calculation shows why controlling pocket depth alone may not control the remaining thickness closely enough.

Protect internal and adjacent features

Overlay the cavity on the approved stackup and each copper layer. Include vertical and lateral tolerances, tool geometry and any intended exposed metal. Confirm that unrelated conductors, vias and resin interfaces remain outside the affected region with the required allowance.

Review assembly operations inside the cavity. Component placement, dispensing, cleaning and inspection may need access beyond the final part envelope. A pocket that mechanically contains a component can still be unsuitable for the process used to install it.

Agree on inspection before ordering

Define how depth, floor condition and remaining material will be checked. Consider whether a surface measurement, section, representative coupon or another method supplies the needed evidence. Record any inaccessible feature that cannot be verified by a simple top-view image.

Retain the section drawing, stackup and supplier-confirmed process with the dimensional report. Revisit them after changing board thickness or internal construction. A cavity requirement should describe a verifiable three-dimensional feature, rather than leaving the fabricator to infer the intended floor from a two-dimensional outline and one nominal depth.