Press-Fit Connectors: Hole Diameter, Insertion Force and Reliability

A press-fit connection is made by deforming a compliant pin inside a plated hole until the two surfaces weld together under pressure. There is no alloy involved, so the reliability comes entirely from the hole, the plating and the force that drives the pin, and all three are process parameters rather than part properties.

How a Compliant Pin Works

The pin is formed with a compliant section that compresses elastically as it enters the hole and then springs back against the barrel wall. The contact pressure is high enough to break through the surface films on both surfaces and to cold weld the tin to the copper at several points around the circumference.

That gas-tight contact is what makes the connection reliable without solder, and it depends on the elastic range of the compliant pin being matched to the hole it enters. A pin that is over-compressed takes a permanent set and loses contact pressure, while one that is under-compressed never develops enough pressure to weld. The elastic range belongs to the copper alloy the pin is made from, which is why a compliant pin is specified as a system together with the hole rather than as a component that will fit any plated opening of the right nominal size.

Hole Diameter and Its Tolerance

The hole diameter is the single most important board parameter, and it is specified as a finished hole after plating with a tolerance of about plus or minus 0.05 mm. The drill size is derived from that figure by subtracting twice the plating thickness, which makes the copper thickness part of the hole specification rather than a separate item.

Because the tolerance is tight and the plating varies across a panel, the hole is verified on the finished board by pin gauge or by optical measurement rather than from the drill programme. A hole that is 0.05 mm small can make insertion impossible, and one that is large leaves the pin without contact pressure. Where a panel carries several connectors from one build, the measurement is repeated on the first and last panel of the lot, because plating thickness drifts along the plating line rather than across a single panel.

Plating Thickness and Barrel Strength

The barrel sees the radial force of the pin, so the copper has to be thick enough to carry it without cracking. A minimum of 25 micrometres of copper is the usual requirement, and thinner plating in a press-fit hole is a common cause of a barrel that fractures during insertion rather than during thermal cycling. A cracked barrel often passes a continuity check because the pin is still touching it, and the break appears later as an open once thermal cycling has moved the two faces apart.

The surface finish on the barrel also matters, and a tin finish of 0.3 to 0.5 micrometres over the copper is what forms the cold weld. Our plating thickness notes describe how the copper and the finish are measured on the same section.

Insertion Force and Press Equipment

The insertion force for a single pin is typically between 10 and 40 newtons, so a 100 way connector needs a press capable of several kilonewtons and a tool that applies it evenly across the body. The force is measured during insertion and recorded as a curve, not as a pass or fail.

The curve is where the useful information sits, because a pin that is bent, a hole that is missing or a connector that is not seated all produce a distinctive shape. Setting a force window around the curve turns the press into an inspection station, and it catches defects that would otherwise be found at electrical test.

Board Thickness and Barrel Length

A thick backplane means a long barrel and a long pin, and the pin has to reach through the board with enough length above it for the compliant section to sit in the hole rather than in the air. Pin length is chosen from the board thickness range, and a connector specified for one thickness cannot simply be used on another.

The barrel length also affects the force, because a longer barrel is more flexible and deforms further under the pin. On a stack of boards the outer ones see a different force from the middle, so the press programme is set from the worst case rather than from an average. The press tool is shimmed from the board surface rather than from the machine table, so that the travel is measured from the face the tool actually touches.

The Eye of the Needle Section

The eye of the needle is the pierced section in the middle of the compliant part, and its dimensions set the stiffness of the pin. A larger eye makes the pin more compliant and lowers the insertion force, while a smaller one raises the force and the contact pressure.

The geometry is fixed by the connector supplier, so the design freedom on the board side is the hole diameter and the plating. That is why the hole tolerance is quoted in the connector datasheet as a range, and why a hole outside it is a board defect rather than a connector problem.

Rework and Re-insertion

A pressed pin can be removed, and the hole can accept a new pin, but the number of cycles is limited by the damage the barrel accumulates. The first insertion work hardens the copper and enlarges the hole slightly, so the force on a second insertion is lower and the contact pressure with it.

Most suppliers allow one, or at most two, re-insertions in the same hole and specify a larger pin or a repair eyelet beyond that. The limit is written into the process instruction, because a hole that has been used three times looks identical to one that has been used once. Removal tools are matched to the pin geometry as well, since a tool that grips the wrong part of the compliant section pulls the pin without releasing it and damages the barrel on the way out.

Reliability and Testing

The qualification tests are thermal cycling, vibration and a low level contact resistance measurement, and the resistance is the figure that shows whether the cold weld is intact. A joint that looks correct but has a high resistance is one where the pin and the barrel never developed enough pressure.

The tests are run on boards built with the production hole and plating, because the result depends on both. Our hole copper notes describe how the barrel is sectioned after cycling to look for cracks that the resistance measurement has not yet detected. The resistance itself is measured with a four wire method at a defined current, since a two wire reading includes the probes and the leads and hides the few milliohms that carry the information.

Documentation and Process Control

The drawing should carry the finished hole diameter with its tolerance, the copper thickness and the finish, and the press programme should record the force window per connector. Those four items are the whole specification, and a change to any of them invalidates the qualification.

The first article checks the hole with a pin gauge, measures the force curve and inspects a sectioned joint, and the results are kept with the build record. Our production process flow notes place that check after plating and before the connector is pressed, which is the only point at which the hole can still be corrected.

FAQ

Can a press-fit connector be installed by hand? It can be seated with a hand tool, but the force is not measured and the result is not verified. The press curve is the only practical way to know that every pin in a large connector developed the contact pressure it needs.

Does the hole need a tighter tolerance than a soldered hole? Yes. A soldered joint tolerates a larger clearance because the alloy fills it, while a compliant pin has to deform by a controlled amount, so the hole range quoted in the datasheet is usually narrower than a standard drilled tolerance.

Why does a second insertion feel easier? Because the barrel has been enlarged and work hardened by the first pin, so the same pin develops less contact pressure. That is the reason for the re-insertion limit rather than a mechanical preference.