Press Fit Connector Design and Assembly

A press fit connector is held in the board by the elastic deformation of a compliant pin in a plated hole. There is no solder, so the joint depends entirely on the mechanical interference between the pin and the barrel, and the reliability of the connection is set by three numbers: the pin geometry, the hole diameter and the plating thickness. Get those right and the joint is more reliable than a soldered one; get them wrong and the failure appears after a few thermal cycles.

How the Joint Is Made

The pin has a compliant section that is wider than the hole. As it is pressed in, the section deforms elastically and makes contact with the barrel over a defined area, which creates a gas tight connection with a low and stable resistance.

The deformation is elastic rather than plastic, which is what allows the pin to be removed and reinserted a limited number of times. A pin that has been plastically deformed will not make reliable contact on a second insertion.

The contact area is what carries the current. A compliant pin has several contact points around the barrel, and a properly formed joint has a contact resistance that is comparable to a soldered joint and far more consistent than a hand soldered one.

Pin Types and Geometry

The common compliant designs differ in the way they deform. An eye of the needle pin compresses as a closed loop, a C section pin deforms as an open spring and a slotted or a dual beam design spreads the compression over two or three separate contact zones.

The choice is made for the hole size and the insertion force. A design with a wide elastic range tolerates a larger variation in the hole diameter, while one with a narrow range holds a tighter contact but demands a tighter hole.

The pin material and its plating also matter. A phosphor bronze or a beryllium copper pin holds its spring properties better than a brass pin, and the plating determines the contact resistance and the resistance to corrosion at the interface.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/Quick-Turn-PCB-Fabrication-Viasion-1.png" alt="Press fit connector being inserted into a backplane” />

Hole Diameter and Tolerance

The hole diameter is the critical dimension. It has to be drilled to a tolerance tight enough that the pin falls within its elastic range on every hole, and the tolerance has to be held across the panel rather than on a sample.

The finished hole size after plating is what counts, not the drill size. The plating adds thickness, so the drill is oversized by twice the plating thickness, and a change in the plating process changes the finished hole even when the drill program is unchanged.

The usual specification quotes a finished hole diameter with a tolerance of a few hundredths of a millimetre. A hole that is too small raises the insertion force and can damage the pin or the barrel, while a hole that is too large gives a contact that is loose and a resistance that rises with thermal cycling.

Plating and Barrel Quality

The barrel has to survive the insertion without cracking. The plating thickness in a press fit hole is usually greater than in a standard plated through hole, because the barrel has to withstand the radial force of the pin.

Copper thickness aside, the plating quality matters as much as the quantity. A barrel with voids, with a thin section at the knee or with a nodule on the wall will fail under the insertion force, and the failure may not appear until the assembly is in service.

Where the hole is also being used as a via, the interaction between the two functions has to be considered. A hole that is filled for a press fit pin cannot also serve as a thermal or an electrical via unless the fill is conductive and the pin still makes contact.

Insertion and the Assembly Process

Insertion is carried out with a press that applies the force along the axis of the pin. An off axis force bends the pin, damages the barrel and produces a joint that looks correct but has a fraction of the intended contact area.

The insertion force per pin is modest, but a connector with several hundred pins produces a total force that the board and the support tooling have to carry. The board should be fully supported behind the connector during insertion, particularly on a thin backplane.

The insertion depth should be controlled rather than judged by eye. The pin has a defined seated position, and a press that stops short leaves the connector standing proud while one that over travels damages the board.

Inspection and Test

A visual check confirms that the pins are seated and that the connector body is flat, but it says nothing about the contact inside the hole. The electrical test is the meaningful measurement.

The contact resistance should be measured on a sample and compared with the specification, and the measurement should be repeated after thermal cycling on a qualification vehicle. A joint that meets the resistance requirement when new and fails after a few hundred cycles was under stressed from the start.

A cross section of a sample connector is the most informative check. It shows the deformation of the pin, the contact area against the barrel and the condition of the plating, and it is worth doing on the first article of each new connector or hole specification.

Rework and Repair

A press fit connector can be removed and replaced, which is one of its advantages. The pin should be extracted with a tool that pulls along the axis, and the hole should be inspected before a replacement is inserted.

The number of insertions that a hole can accept is limited. Each insertion works the plating and enlarges the hole slightly, and a hole that has been used several times may no longer hold the specified contact. The limit should come from the connector supplier and be recorded.

Where the hole has been damaged, the usual repair is a soldered connection or a replacement of the board. A press fit joint cannot be repaired by adding solder and then reinserting the pin, because the solder changes the hole geometry and the pin no longer has a defined interference.

Design Rules That Help

Keep the press fit holes on a grid that suits the connector, keep the plating specification with the hole callout and keep the insertion depth on the assembly drawing. Specify the hole diameter as a finished dimension with a tolerance.

Leave the space above the connector clear for the press tool, and provide support on the opposite side. Those two requirements are easy to overlook in a dense layout and they are the reason a connector cannot be assembled after the layout is released.

Practical Rules

Control the finished hole diameter, specify the plating for the insertion force, and support the board during insertion. Measure the contact resistance on a sample and after thermal cycling rather than relying on a visual check.

Record the hole specification and the insertion data with the build records and the defect history, and review the hole copper specification and the aspect ratio data when a new connector is introduced.

FAQ

What holds a press fit pin in place? Elastic deformation of the compliant section against the plated barrel. There is no solder, so the joint depends on the interference between the pin and the finished hole.

Why does the finished hole diameter matter more than the drill size? Plating adds thickness, so the drill is oversized by twice the plating thickness. A plating change alters the finished hole even when the drill program is unchanged.

Can a press fit hole be reused? A limited number of times. Each insertion works the plating and enlarges the hole slightly, so the supplier limit should be recorded and respected.