Controlled Depth Milling: Cavity Depth Tolerance in Rigid PCBs

Controlled depth milling removes material from a rigid board without cutting through it, leaving a floor of laminate at a defined depth. It is used for component cavities, for pockets that recess a connector or a shield, for blind slots that stop short of the opposite face, and for the stepped areas that let a metal coin or a heat spreader sit flush with the surface. The process is judged on two numbers: how accurately the floor is placed in the z direction, and how much material remains beneath it for the board to keep its mechanical and electrical integrity.

Where Controlled Depth Milling Is Used

The most common application is a cavity that allows a tall component to sit below the surface so that the assembly stays within a height limit. The same operation produces pockets for potting, recesses for an embedded heat spreader and stepped edges where a connector has to sit flush with the outline.

A second group of applications is electrical rather than mechanical. Milling away part of the dielectric is one way to reach a controlled impedance target or to open a window onto an inner layer for a later process. In these cases the floor depth is a functional dimension and not merely a clearance, so the tolerance is set by the electrical requirement rather than by the height of the part.

Z-Axis Control and Depth Setting

Depth is set by the machine z-axis relative to a reference, and everything depends on how that reference is established. The reliable method is to touch off on the actual surface of the panel at the cavity position, or on a probe point beside it, rather than to assume a nominal thickness. Laminate thickness varies across a panel and between panels, so a program written against a nominal value can be out of tolerance before the first cut.

Once the surface reference is taken the cut depth is the programmed value, but the machine also has to hold it. Spindle nose runout, tool holder seating, thermal growth during a long program and backlash in the z-axis all contribute. A machine that holds 0.02 mm over a short program may drift to 0.05 mm over a full panel unless the program re-references periodically.

Cavity Depth Tolerance and Residual Web Thickness

The tolerance on depth is normally written as the cavity floor position, with a value that reflects both the machining capability and the purpose of the pocket. For a clearance cavity under a component, plus or minus 0.1 mm is common; for a pocket that locates a heat spreader, plus or minus 0.05 mm is more typical.

Residual web thickness is the material that remains between the floor of the cavity and the nearest copper feature below it. It is usually specified separately, because it is what prevents a cavity from becoming a breach. Web thickness of 0.15 mm to 0.25 mm is a workable minimum for standard FR-4 at typical cavity sizes, and the value has to be checked against the layer stack rather than against the total board thickness.

Tool Geometry and Cut Parameters

A flat-bottom cutter leaves a floor that is parallel to the surface, while a ball nose leaves a scalloped floor that is unsuitable for a locating surface. Corner radius, flute count and coating all affect how cleanly the edge of the cavity is cut. For a small cavity in a dense area a cutter two to three times the depth of cut in diameter gives the stiffness needed to hold depth.

Feed and speed are set so that the cutter shears the laminate rather than rubbing it. A feed that is too light burns the resin and leaves a raised, resin-rich floor; a feed that is too heavy deflects the tool and produces a floor that is deeper in the middle of the cut than at the edges.

Spoil Board and Fixture Flatness

The panel is supported during the cut by a spoil board or a machined fixture, and its flatness is transferred directly into the floor of the cavity. A fixture that is not flat produces a cavity whose depth varies across its own area, which is the most common cause of a depth chart that looks like a map of the fixture.

Vacuum holding is preferred over mechanical clamping for thin panels because it supports the whole area rather than a few points. Where mechanical clamping is used, the clamps should be placed outside the cavity region, and the fixture should be checked with a depth gauge before each lot.

Depth Verification Without Damaging the Part

Depth verification on the finished part is done with a dial indicator, a depth micrometer or an optical gauge, referenced to the original surface beside the cavity rather than to the fixture. Where the cavity is small, a step gauge that spans the cavity and the surrounding surface gives a repeatable reading without touching the floor.

Cross-sectioning gives the most complete picture, because it shows the floor, the web and any copper below in one view, but it destroys the sample. The practical approach is to measure depth non-destructively on a sample of cavities per panel and to section one coupon per lot, so that the non-destructive readings can be tied back to a known section.

Panel Handling and Contamination

Milling produces dust, and dust in a cavity is a defect. Debris that is not removed before the next process can be trapped under a component or under a heat spreader, and the resulting void is only found at final inspection. Extraction at the cutter, followed by a cleaning step, is more effective than relying on a single post-process clean.

Handling also matters because the cavity reduces local stiffness. A panel with a large pocket flexes more than a solid one, so boards should be supported during transport and during subsequent assembly. Cracks that appear near a cavity corner during assembly are often caused by flexing that started at the milling station.

Design Rules for Cavities and Slots

The floor should not be specified thicker than it needs to be, and the cavity should not be placed directly over a copper plane unless the web thickness has been calculated with that plane in the stack. Where a cavity is combined with a step for a stepped stencil, the step depth and the cavity depth are two separate dimensions and should be dimensioned separately on the drawing.

Corner radii should match the cutter that will be used, since a sharp internal corner cannot be milled without an undercut. Where the cavity meets the board outline, a wall of at least 0.3 mm should remain so that the edge is not weakened and so that the router that cuts the outline has material to support the cut.

Process Records and First Article

The first article should record the surface reference that was taken, the programmed depth, the measured depth at several points in the cavity, and the residual web thickness from a section. Those four values together describe whether the setup matches the drawing, and they are the only evidence that the cavity is correct rather than merely present.

In production the record should also carry the tool number and its accumulated cutting time, the fixture identification and the date of the last flatness check. When a depth problem appears, these fields are what make it possible to decide whether the cause was the tool, the fixture or the reference, and to correct it before the next panel is cut.

FAQ

How deep can a controlled depth milling cavity be? It is limited by the residual web thickness required below the floor rather than by the cutter. With a web of 0.15 mm to 0.25 mm below the cavity, a typical six-layer board can carry a cavity that removes most of two or three layers.

Why does cavity depth vary across a single pocket? Almost always because the fixture or spoil board is not flat, so the panel is not uniformly supported. Checking the fixture with a depth gauge before the lot is faster than adjusting the program.

Is depth verification on the finished board enough? It should be combined with a section. Non-destructive readings confirm the floor position, but only a section shows the residual web thickness and any copper beneath it.