Press-Fit Connectors: Hole Tolerance and Barrel Plating

A press-fit joint is made without solder, and that is precisely why it demands more of the board than a soldered joint does. The connection depends on the hole wall deforming against the pin and staying in contact for the life of the product, so the finished hole diameter, the plating thickness and the insertion force are all part of one specification. This article covers the geometry, the drilling and plating controls, and the measurements that show whether a batch of backplanes will hold its connectors.

How a Compliant Pin Holds

A compliant pin relies on elastic and plastic deformation. Its compliant section is wider than the finished hole, so insertion compresses the pin and forces the barrel and the pin together with enough pressure to produce a gastight contact. The joint is mechanical and it does not depend on solder at any point.

The contact depends on the two materials remaining in contact for the life of the product, which means the hole wall has to deform rather than fracture and the pin has to stay within its elastic range. That balance is set by hole diameter, plating and insertion force, all of which are specified together.

Finished Hole Size and Tolerance

The press-fit hole is specified as a finished diameter, measured after plating, and the hole tolerance is only a few hundredths of a millimetre wide. A typical requirement for a one millimetre pin might be a finished hole between 0.94 and 0.99 mm, with the plating contributing 25 micrometres of copper on the wall.

Because the drilling happens before plating, the fabricator works back from the finished dimension to the drill size and then holds the plating thickness tolerance to preserve it. That is why press-fit boards carry a note requiring the finished hole to be measured, and why the measurement is taken in the panel rather than on a coupon.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/1742563552959.png" alt="Press-fit connector being inserted into a backplane PCB” />

The finished diameter is checked with a pin gauge or optically, and both methods should be calibrated against the same reference so that the supplier and the assembler are not measuring two different things.

Plating Thickness and Barrel Quality

A press-fit barrel normally carries 25 to 40 micrometres of copper, thicker than the 20 to 25 micrometres usual for a soldered through-hole. The heavier wall resists the deformation of insertion and provides a reservoir of material for the gastight contact. The structure behind this is described in our hole copper guide.

Quality matters more than average thickness. A barrel with a thin or voided section cracks where the pin expands it, and the crack is invisible from the surface. Coupon microsections and a check for plating voids at the knee of the barrel belong in the qualification, not only in the first article.

Insertion Force and Its Measurement

The insertion force for a single pin typically falls between 20 and 80 newtons, and it has to stay inside that band. Too low and the contact is not gastight, too high and the pin or the barrel is damaged. The force is measured on a sample of production boards with the actual connector and a load cell.

Retention force is measured after insertion, and it is the figure that predicts field reliability. A pin that pulls out below the manufacturer minimum has already lost the interference that makes the joint work, even though it looks correctly seated.

Hole Wall Preparation and Drilling

Because the wall carries the load, drilling has to leave a clean barrel without epoxy smear and without a rough surface. Smear is removed by desmear or a permanganate process, and the copper is then deposited onto a wall the plating can adhere to.

The hole also has to be perpendicular to the board surface. A hole drilled at an angle reduces the effective wall length on one side, and the pin compresses that side further, changing both insertion force and retention. Drill registration and machine calibration are the controls that keep it inside specification.

Panel Thickness and Connector Length

A connector’s compliant section is designed for a range of board thicknesses, and using it outside that range changes the insertion force and the retention. A connector intended for 1.6 mm inserted into a 2.4 mm backplane does not engage the full compliant zone, and the joint is weaker than the design intends.

Board thickness tolerance therefore deserves the same care as hole size. Lamination thickness varies by a few percent, and on a thick backplane that variation is large enough to matter, so the stack-up states the finished thickness with a tolerance rather than a nominal figure.

Thickness is measured on the finished panel at several points, because press thickness varies across a large backplane and a thin corner can fall outside the connector window.

Assembly Force and Board Support

Press-fit insertion is often done with a flat rock or a press that drives every pin at once, and the force can reach several tonnes on a large connector. The board has to be supported across its whole area during insertion, or local deflection damages the barrel and the vias around it.

Support tooling is part of the process rather than an accessory, and it should be designed with the connector layout rather than after it. Backplanes pressed flat on a table are more likely to sustain this damage than ones pressed in a fixture with a backing plate.

Inspection and Verification

Optical inspection shows whether the pins are seated to the correct depth, and that depth is measured against the shoulder position rather than judged by eye. X-ray is used where a pin is hidden, and barrel deformation is confirmed on a microsection from a sample board in the qualification batch. Radiographic methods are covered in our X-ray and AOI guide.

Resistance measurement between pin and pad completes the verification, taken on a sample of joints rather than a single one. A resistance above the distribution of the others indicates a joint that is mechanically seated but electrically marginal, which is exactly the failure press-fit is meant to avoid.

Documentation for the Fabricator

The fabrication drawing should state the finished hole diameter and tolerance, the minimum copper thickness in the barrel, the hole wall quality requirement and the board thickness range. The assembly drawing then states the insertion force window and the retention force minimum for each connector type.

Where one hole pattern serves more than one connector type, the drawing should identify which connector goes where, because different pins have different compliant zones and therefore different hole requirements. A single hole call-out across a mixed layout is a common source of insertion problems.

FAQ

Can a press-fit pin be replaced? Once, if the barrel is undamaged. Repeated insertion enlarges the hole and reduces the interference, so the repair limit is normally written as one replacement per hole.

Is HASL suitable for press-fit holes? No. The alloy narrows the hole and its thickness is unpredictable, so press-fit boards are finished with a thin chemical finish or with a plating the pin can compress.

How is the finished hole measured? With a pin gauge or optically on the finished panel, because the drill size is only an intermediate dimension and plating changes it by tens of micrometres.