Compliant Pin Joints: Hole Size, Insertion Force and Reliability

A compliant pin joint is made by pushing a shaped pin into a plated hole, where the pin deforms elastically and the resulting spring force holds it against the barrel wall. Nothing is soldered, nothing is heated, and the connection depends entirely on geometry, so the hole and the pin have to be specified together as one system rather than as two separate items.

How a Press Fit Joint Works

The pin is wider than the hole at its widest point, so inserting it compresses the compliant section and stores energy in the metal. That stored energy presses the pin against the barrel with a force that is maintained over the life of the product, which is what makes the joint gas tight and reliable. The pin is designed so that its elastic limit is never exceeded, which is what allows the joint to be pressed without permanent damage to either part.

The joint carries current through the contact area between the pin and the plated barrel, and it depends on the plating being intact and the hole being round. A scratch through the plating, or a hole that is oval after drilling, removes the contact pressure locally and creates a hot spot under load.

Compliant Pin Designs

Several geometries exist, including the eye of the needle, the solid pin with a slotted end, and the action pin with a shaped beam. Each has a different insertion force, a different retention force and a different tolerance to hole variation, and the connector manufacturer normally specifies which one to use.

The eye of the needle is the most common because its behaviour is well understood and its insertion force is moderate. The slot closes as the pin enters, which limits the stress in the barrel and allows a slightly wider hole tolerance than a solid design would permit. Solid pins are reserved for cases where the pin count is low and the retention force has to be large.

Hole Size, Tolerance and Drilling

The finished hole size is the parameter the whole joint depends on, and it has to include the plating thickness as well as the drilled diameter. A hole that is at the top of the tolerance gives a low insertion force and a weak joint, while one at the bottom may not accept the pin at all.

The tolerance is therefore tighter than for a normal via, and it has to be held across the panel rather than on average. Drill wear, entry and exit material and the plating distribution all shift the finished size, so the drill programme is usually maintained with a specific hole size control. Our hole copper notes describe the plating side.

Insertion Force and Assembly Tooling

Insertion force rises with pin count, so a connector with several hundred pins needs a press with enough force and a tool that applies it evenly. A hand tool used on a large connector bends the housing and loads the pins unevenly, which damages the ends of the row. A press fitted with a load cell and a controlled speed gives the operator a number to work to instead of a judgement to make.

The force should be measured rather than judged by feel. A force that is too low means a weak joint somewhere in the row, and one that is too high means the hole is undersized or the plating is too thick, either of which risks cracking the barrel. Our plating thickness notes cover the barrel build.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/1768208893-quick-turn-pcb-assembly.webp" alt="Sectioned barrel of a press fit hole in a backplane” />

Plating and Barrel Integrity

The barrel has to survive the insertion without cracking, which means the copper must be ductile, continuous and thick enough. A barrel with a thin wall or a crack at the knee of the hole will fail after a few thermal cycles even if it passed the initial press.

Aspect ratio also matters, because a long small hole is harder to plate evenly than a short one. Where the board is thick and the pin count is high, the plating process has to be qualified on a coupon rather than assumed. Our aspect ratio guide describes where the limits lie.

Backplanes and High Pin Count Connectors

Backplanes are the classic application, because a large board with many connectors and thousands of pins can be assembled without a soldering operation for the connector itself. That removes a whole thermal process from a board that is difficult to heat evenly.

It also means the board carries the mechanical load of every insertion cycle in the field. A backplane has to tolerate repeated mating and unmating, and the pin retention and the barrel integrity are what decide whether the joint survives a decade of service. Backplanes also carry heavy power, so the joints that matter most are often those closest to the supply.

Inspection and Rework Limits

Inspection after pressing is mostly a check that nothing obvious has gone wrong: the connector sits flat, no pins are bent or pushed through, and the board around the holes has not cracked. Sectioning a sample is the only way to see the barrel condition and the contact area.

Rework is possible but limited. A pin can be extracted and replaced with a slightly larger compliant design, and the hole can only take a small number of press cycles before the plating is damaged beyond use. That limit should be recorded rather than discovered. Counting the press cycles applied to each hole makes the limit auditable and stops the same hole being reworked until it fails.

Reliability Under Thermal Cycling

The joint is a mechanical contact, so it responds to thermal cycling differently from a soldered joint. Differential expansion between the pin, the barrel and the surrounding laminate changes the contact pressure, and a joint with marginal pressure at room temperature can loosen at temperature.

The failure mode is a slow increase in contact resistance rather than an immediate open, which makes it difficult to detect at final test. A thermal cycling qualification with resistance monitoring is the only reliable way to demonstrate that the design has sufficient margin.

Specifying the Requirement on the Drawing

The drawing should give the finished hole size with its tolerance, the plating thickness in the barrel, the pin part number and the required insertion and retention forces. It should also state the number of press cycles permitted per hole and the acceptance criteria for the finished joint.

That level of detail prevents a supplier from optimising one parameter at the expense of another. Our fabrication notes checklist lists the items worth confirming before a press fit panel is released to production.

FAQ

Can a press fit joint be soldered as well? It is not recommended. Filling the barrel with solder changes the compliance of the joint and can crack the plating as it solidifies, and the solder adds nothing to a correctly formed connection.

Why is the hole tolerance so tight? Because the retention force comes from elastic deformation rather than from a fastener. A hole a few hundredths of a millimetre too large removes the interference that the joint depends on.

How does gopcb control press fit holes? We control the finished hole size including plating, maintain the drill programme for the specific tool, verify barrel integrity by section, and supply the insertion force data measured on a sample connector from the customer bill of materials.