Blind Slot PCB vs Through Slot: Design and Manufacturing

Two Slots, Two Processes

A slot is a routed opening in a board that is longer than it is wide, and the word covers two very different features. A through slot is cut all the way through the panel, so the router or the laser passes completely through the material and both faces show the same opening. A blind slot is cut to a controlled depth and stops inside the board, so it appears on one face only. The distinction is not cosmetic. A through slot is a standard routing operation with a well understood tolerance, while a blind slot requires depth control, a defined bottom surface and a manufacturing process that can hold the depth without cutting deeper than intended on a thin laminate.

Why a Blind Slot Is Used

A blind slot is normally specified when something has to sit partly inside the board. It can provide clearance for a connector body, a battery or a shield can so that the overall stack height is reduced without cutting a hole that would be visible or that would break the mechanical continuity of the board. It can act as a light pipe channel or a waveguide, create a defined cavity for potting, or provide a pocket in which a metal insert or a stiffener sits flush with the surface. In each case the requirement is depth, and depth is what makes the feature expensive, because the router has to stop at a controlled Z position across the panel while the panel thickness itself varies within a tolerance.

Through Slot Capability and Design Rules

A through slot is cut with a standard router bit, so its minimum width is set by the smallest available cutter, commonly 0.8 mm and occasionally smaller. The ends of the slot are rounded to the tool radius unless the slot is long enough for the router to approach from both ends, and the finished length is measured between the end radii rather than across the full opening. Position tolerance for a routed slot is typically around plus or minus 0.1 to 0.2 mm, and it must be checked against the copper clearance so that the cut does not expose or short a trace. A typical requirement is 0.3 to 0.5 mm of copper free margin from the slot edge. Longer slots need a slightly larger clearance to allow for tool wander, and a slot close to another slot or to the board edge can force the router into a fragile section of material. Plated slots are a separate case: the barrel has to be plated after drilling, and the plating coverage and thickness in a slot are harder to control than in a round hole.

Blind Slot Capability and Design Rules

A blind slot has to be machined with a depth controlled router pass or with a milling machine that follows a programmed Z axis, and the achievable tolerance on depth is usually in the region of plus or minus 0.1 mm on a rigid board, loosening as the board gets thinner or as the depth tolerance has to be held over a large area. The bottom surface will show tool marks and a witness of the cutter path, so a blind slot that functions as a sealing surface or a waveguide floor may need a finishing allowance. The corners at the bottom of the pocket will be radiused by the tool, not square. Because the remaining material under the slot is thin, it is easily damaged during subsequent handling and during reflow, and it also affects the thermal and mechanical stiffness of the region, which matters under a connector or under a heavy component. Blind slots should never overlap a via, a plated hole or an inner layer plane, and they should be kept away from board edges by enough material to survive the routing operation. If the slot has to be plated or to have a defined finish on the bottom, that requirement should be raised with the fabricator before the geometry is frozen, because not every shop can apply a controlled finish to a milled internal surface.

Tolerances and Inspection

Inspection of a blind slot requires more than a visual check, because the depth is not visible from the outside. The usual methods are a depth gauge or a dial indicator, an optical profilometer where a precise floor profile matters, and a cross section on a first article coupon, which shows the actual remaining thickness and the condition of the copper underneath. For a through slot the check is dimensional and two dimensional; for a blind slot it is three dimensional. That difference is the reason the two features carry different price and lead time consequences, even when they look similar on a drawing.

Cost and Process Selection

A through slot adds a short routing operation to a step that is already happening when the board profile is cut, so it is nearly free in a shape that is already being routed. A blind slot adds a separate depth controlled operation, often on a different machine, with a longer cycle time and a programming charge, and it may require a second setup if the part has to be flipped. The price difference is therefore significant at prototype quantity and narrows only slightly in volume. Where the design can tolerate a through slot with a bonded cover or a stepped counterbore, that is normally the cheaper choice; where the depth is essential because the feature is part of the mechanical function, the blind slot is worth the premium.

Which to Choose

Choose a through slot when the opening is simply clearance, when the part sits all the way through the board, when airflow or drainage is wanted, or when the slot is really a connector cutout. Choose a blind slot when height reduction is the goal, when the board has to remain electrically or mechanically continuous on the far side, when the slot forms a channel or a cavity, or when an insert has to sit flush. In both cases keep the copper clear of the cut by the specified margin, define the end radii and the tolerance in the drawing, and state whether the slot is plated. For a blind slot, also state the remaining thickness under the floor rather than only the depth, because the remaining material is what actually has to survive the rest of the process.

FAQ

What is the difference between a blind slot and a through slot? A through slot is cut completely through the board and appears on both faces. A blind slot is machined to a controlled depth and appears on one face only.

How accurate is the depth of a blind slot? Around plus or minus 0.1 mm is typical on a rigid board, and it loosens as the board gets thinner or the depth has to be held over a large area.

Can the bottom of a blind slot be flat and square? No. The cutter leaves radiused corners at the floor and visible tool marks, so a sealing surface or a waveguide floor may need a finishing allowance.

How close can copper be to a slot? Keep at least 0.3 to 0.5 mm of copper free margin from the slot edge, and more where the slot is long or close to the board edge.

Is a blind slot much more expensive? Yes at prototype quantity, because it needs a separate depth controlled operation and often a second setup. Through slots are usually absorbed into the profile routing step.

Conclusion

The slot question is really a depth question. A through slot is a routing operation with a two dimensional tolerance, and a blind slot is a milling operation with a three dimensional one, including the thickness that remains underneath. Decide which function the opening serves, define the remaining thickness as well as the depth, keep the copper clear, and confirm with the fabricator that the depth and the floor finish you need are within capability. The machining limits are part of PCB capabilities, the clearance and keepout rules belong in PCB design and layout, and the routing sequence is described in PCB manufacturing. A prototype PCB assembly build with the real insert or connector proves the fit before volume in 2026.